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RVing message boards => Tech Talk => Topic started by: Heli_av8tor on August 29, 2017, 09:24:58 AM

Title: Diagnosing Poor MPG
Post by: Heli_av8tor on August 29, 2017, 09:24:58 AM
I've been following this thread (and other similar) with interest.
My coach is getting between 5 and 5-1/2 mpg. I'm thinking based on others that this is at least 20% low. My cruise speed has varied from 55 to 68 with most around 60.
The coach weighs about 22,500 loaded and the toad is 3400. There is 48,000 miles on the coach.
I don't try to accelerate like a race car and anticipate stops and speed reductions so as to keep braking to a minimum.

Assuming it should do better, what should I look for? Does any of the OBD info give a clue?

Tom
Title: Re: Diagnosing Poor MPG
Post by: Charlie 5320 on August 29, 2017, 10:59:12 AM
I've been following this thread (and other similar) with interest.
My coach is getting between 5 and 5-1/2 mpg. I'm thinking based on others that this is at least 20% low. My cruise speed has varied from 55 to 68 with most around 60.
The coach weighs about 22,500 loaded and the toad is 3400. There is 48,000 miles on the coach.
I don't try to accelerate like a race car and anticipate stops and speed reductions so as to keep braking to a minimum.

Assuming it should do better, what should I look for? Does any of the OBD info give a clue?

Tom
Air filter, clean the maf sensor, and have it scanned to see if the 02 sensors are switching. They should switch very quickly, if they switch but are slow to react, may be a cause of a rich condition. My 8.1 doesn't get the fuel economy my old 7.4s did, but it's got a LOT more power. On my last 1200 mile trip my 19600 lb coach got 7.5 to 7.6 mpg running the speed limit most of the time. 36000 miles on the coach with original plugs and wires, it does have a K&N air filter the PO owner installed. I'm going to replace it with a Wix filter before the next trip to see if either one makes a difference.
Title: Re: Diagnosing Poor MPG
Post by: Heli_av8tor on August 29, 2017, 12:17:33 PM
The air filter is new.
I have not cleaned the MAF sensor.
I'm not sure what you mean by the O2 sensors not switching quickly.
The engine is operating in "closed loop". And the short term fuel trims are very responsive with any changes in load or throttle. Both banks are virtually identical in values and response.

Tom
Title: Re: Diagnosing Poor MPG
Post by: kdbgoat on August 29, 2017, 12:28:28 PM
O2 sensors operate at a high frequency. If the frequency is too low, it will cause the system to operate incorrectly. It used to be a common problem at some shops as they would check them with an ohm meter to be either open or closed, instead of using a proper scanner to check the frequency. Sometimes it was the scanners fault as that is all the scanners would read. 
Title: Re: Diagnosing Poor MPG
Post by: SeilerBird on August 29, 2017, 12:29:09 PM
I've been following this thread (and other similar) with interest.
My coach is getting between 5 and 5-1/2 mpg. I'm thinking based on others that this is at least 20% low. My cruise speed has varied from 55 to 68 with most around 60.
The coach weighs about 22,500 loaded and the toad is 3400. There is 48,000 miles on the coach.
I don't try to accelerate like a race car and anticipate stops and speed reductions so as to keep braking to a minimum.

Assuming it should do better, what should I look for? Does any of the OBD info give a clue?

Tom
You really should start a new thread to get some answers. This thread is almost dead.
Title: Re: Diagnosing Poor MPG
Post by: Charlie 5320 on August 29, 2017, 03:51:50 PM
The air filter is new.
I have not cleaned the MAF sensor.
I'm not sure what you mean by the O2 sensors not switching quickly.
The engine is operating in "closed loop". And the short term fuel trims are very responsive with any changes in load or throttle. Both banks are virtually identical in values and response.

Tom
A good 02 sensor will move around with no input from the throttle. With my Mac scanner I can see the voltage change constantly with no throttle movement. Cheaper scanners may not show this, I don't know. I had a 7.4 a few years ago that would occasionally have a check engine light, caused by a lazy 02 sensor. Fuel mileage picked up after it was changed, but it didn't appear to be fouled like most are, when they trigger the light.
Title: Re: Diagnosing Poor MPG
Post by: Heli_av8tor on August 29, 2017, 09:19:45 PM
You are right, it should be a new thread and I've done that.

See "Diagnosing Poor MPG"

Thanks to all for your thoughts. Please continue in the new thread.

Tom
Title: Re: Diagnosing Poor MPG
Post by: Heli_av8tor on August 29, 2017, 09:23:24 PM
I started this question in another thread and decided it should be its own topic.

My coach is getting between 5 and 5-1/2 mpg. I'm thinking based on others that this is at least 20% low. My cruise speed has varied from 55 to 68 with most around 60.
The coach weighs about 22,500 loaded and the toad is 3400. There is 48,000 miles on the coach.
I don't try to accelerate like a race car and I anticipate stops and speed reductions so as to keep braking to a minimum.

Assuming it should do better, what should I look for? Does any of the OBD info give a clue?

Tom
Title: Re: Diagnosing Poor MPG
Post by: Tom on August 29, 2017, 09:58:01 PM
Quote
I started this question in another thread and decided it should be its own topic

I merged the two topics so all the replies are in one place, rather than having two parallel topics.
Title: Re: Diagnosing Poor MPG
Post by: WILDEBILL308 on August 29, 2017, 10:03:10 PM
I started this question in another thread and decided it should be its own topic.

My coach is getting between 5 and 5-1/2 mpg. I'm thinking based on others that this is at least 20% low. My cruise speed has varied from 55 to 68 with most around 60.
The coach weighs about 22,500 loaded and the toad is 3400. There is 48,000 miles on the coach.
I don't try to accelerate like a race car and I anticipate stops and speed reductions so as to keep braking to a minimum.

Assuming it should do better, what should I look for? Does any of the OBD info give a clue?

Tom
Tom, With the weight you are pulling that isn't that bad. I think there are some things you could try. It depends on how much work you can do yourself. Do you have a stock exhaust? What have you tried sofar?
Bill
Title: Re: Diagnosing Poor MPG
Post by: Heli_av8tor on August 29, 2017, 10:41:41 PM
Thank you Tom.

My Exhaust is stock: Dual 3" through CC and Muffler. Then Y-pipe to single 4" tailpipe.

To clarify, the Short Term Fuel Trim is constantly and rapidly changing.

I'm using a WIFI OBD to my iPad for a scanner.

I've been so busy with other repairs that I haven't tried anything other than looking at the data from the OBD and trying to spot something there. And trying to figure out what to try.

I realize the 5.0 to 5.5 is not totally in the toilet, but it does seem 20% lower than others are "claiming".

Tom

Oh, my new tailpipe is clean after a 120 mile trip.
Title: Re: Diagnosing Poor MPG
Post by: Arch Hoagland on August 30, 2017, 12:09:01 AM
I get 7.1 averaged over 87,000 miles, hand calculated. I weigh in at about 26,000 lb with the toad so you are low.

My speeds are similar to yours.

How are you calculating you gas mileage...by hand or by readout?

Over how many miles is your mpg calculated?  It can vary trip by trip depending on where you go. wind, terrain, etc.

You really need to clean your MAF as it helps determine the fuel to air mixture.

Have you replaced your plugs and wires?
Title: Re: Diagnosing Poor MPG
Post by: Bill N on August 30, 2017, 06:51:45 AM
Don't forget to consider how much the generator is used.  On very hot days and with 3 animals in the coach (and two humans) we do run the generator on the highway to keep the whole coach cool and that would affect your overall gas mileage as the gen is dipping out of the same tank.  Just a thought.

Bill
Title: Re: Diagnosing Poor MPG
Post by: boatbuilder on August 30, 2017, 07:17:41 AM
O2 sensors will rarely cause the check engine light to come on.  Many times it is easier to just change them than spend the time to try and diagnose them.  I know many mechanics just change them every time they change spark plugs just as a preventative measure.
Title: Re: Diagnosing Poor MPG
Post by: Heli_av8tor on August 30, 2017, 07:54:13 AM
I calculate MPG the old fashion way, well, I do use a handheld calculator lol. My mileage was calculated over a 1800 mile trip (East from Iowa, across Illinois, into Indiana, to UP of Michigan, south through Wisconsin, back to Indiana, then to Iowa. The generator was used about 8 hours tops on this trip so would account for maybe 0.1 mpg. Engine A/C was run continuously. We did experience some mean cross-winds, but seldom had much headwind component to them.

I've not changed plugs or wires. Should it need plugs at 48,000?

I'm reluctant to just start throwing money at possible causes but it may come down to that.

I have a trip to Colorado in less than two weeks and may not have time to do much before we leave.

Thanks,
Tom
Title: Re: Diagnosing Poor MPG
Post by: Heli_av8tor on August 30, 2017, 09:14:23 AM
I'm starting to price O2 sensors and seeing a range from the $80's to over $200.
Any suggestions?
Title: Re: Diagnosing Poor MPG
Post by: Gary RV_Wizard on August 30, 2017, 09:43:33 AM
I would say the 5.0-5.5 mpg IS indeed "totally in the toilet" if that is highway mileage. That rig should easily get 7 mpg, even with a heavy foot. The one we had was a hair under 22,000 lbs, plus a 3500 lb toad, and we averaged 7.6-8.1 mpg over 50k miles.

I'm inclined to be skeptical about lazy O2 sensors, but it's conceivable I guess. Generally, though, if the sensor is working well enough for the ECM to get into closed loop mode, it should be ok.  But I confess I don't see how it could be operating closed loop and get such poor mileage unless the engine is always running near WOT, i.e under heavy load.  Is the tranny getting into OD?  No brake drag or anything like that? What are you seeing for engine rpms when cruising at highway speeds?   I think your '04 has only the 5-speed Allison, right?

I seem to recall the 8.1L has two O2 sensors, one in each bank? Can anyone verify or disprove that?
Title: Re: Diagnosing Poor MPG
Post by: boatbuilder on August 30, 2017, 09:59:26 AM
Just due to age, I would pull out a couple spark plugs and see what they look like.  It would probably be worth it just to change at least the plugs and the maybe O2 sensors.
Title: Re: Diagnosing Poor MPG
Post by: Heli_av8tor on August 30, 2017, 12:07:12 PM
It has an O2 sensor on each side in front of the CC. both banks report very similar fuel trims and seem to be constantly changing. I too really have trouble thinking they are at fault. It is definitely in closed loop.

It has the Allison 5-speed and gets to OD around 55 to 60 depending on load. RPM's around 2200 at 60 (from memory).

I'm not positive that the brakes are not dragging, but I have seen no indications. Will look more closely at that possibility on my next drive. The calipers were replaced just before I got the coach.

Tom
Title: Re: Diagnosing Poor MPG
Post by: Gary RV_Wizard on August 30, 2017, 08:39:44 PM
It's conceivable one or more plugs are fouled at 48k, but usually they are good for 100k or better. Worth a check, though, because the condition of the plug is a good barometer of how well the air/fuel mixture is being controlled.

You seem to already have a good understanding of how the closed loop ECM system works, so I'm not sure we can help a lot more without getting in there beside you with meters and visual inspections..
Title: Re: Diagnosing Poor MPG
Post by: Heli_av8tor on August 30, 2017, 09:20:12 PM
Just finished replacing the exhaust Y pipe so can now devote my efforts to the mpg problem.

Tomorrow I'll start with cleaning the MAF sensor and verifying that the dealer actually replaced the air filter as claimed.

I'll also pull and inspect the plugs.

Will report my findings.

Tom
Title: Re: Diagnosing Poor MPG
Post by: Arch Hoagland on August 30, 2017, 09:54:17 PM
"Just finished replacing the exhaust Y pipe so can now devote my efforts to the mpg problem. "

My 2004 W22 has dual exhausts not tied together. Did you go to dual exhaust or just put in a new Y pipe? 

Dual exhausts will help with power and MPG. 

I'll respectfully disagree with Gary about the plugs on a GM going 100,000 miles. Mine have been replaced twice in 87,000 miles as well as the wires.
Title: Re: Diagnosing Poor MPG
Post by: Heli_av8tor on August 31, 2017, 06:46:24 AM
I replaced the Y pipe. The inlet of this pipe is two 3" converging into a 4" outlet.
May have been better to have kept them separate but I didn't do that.

Back in my motorcycle racing days we would test spark plugs by tossing them into the air. If they stayed up there they were good. If they come down - replace them. I'll apply the same test on the MH plugs.
If I can get my compression tester in I'll check while the plugs are out.
Title: Re: Diagnosing Poor MPG
Post by: Gary RV_Wizard on August 31, 2017, 09:24:49 AM
Quote
I'll respectfully disagree with Gary about the plugs on a GM going 100,000 miles. Mine have been replaced twice in 87,000 miles as well as the wires.

LOL! No need to be "respectful" about something that is largely a matter of opinion!  ;)

I'll stand by my previous comments, though. If the ECM is managing the closed loop system as it is designed to do, the plugs should last 100k miles.  If the air/fuel mixture is maintained at the proper stoichiometric value, the plug should show no signs of damage or wear.  No fouling or burning evident and not even much physical electrode loss. I'll grant, though, that plugs will show more deterioration in an engine that does mostly stop and go driving, or rarely run long enough to get up to full operating temperature. The fuel mix gets somewhat rich during accelerations or deceleration and that hastens plug wear.
Title: Re: Diagnosing Poor MPG
Post by: Heli_av8tor on August 31, 2017, 07:56:34 PM
I cleaned the MAF sensor today. There was no visible contamination. The air cleaner was not new as claimed by the selling dealer. The paper part looked very good. However, the foam pre-filter (that fit over the paper like a bonnet) had some blockage (10%?). I tried washing it with soap and water and it started crumbling. A replacement will be here Saturday.

I also ordered plugs and wires. Amazon had a great deal on ACDelco 41-101 Professional Iridium Spark Plugs. Best I can research this should be a good plug for this engine. I didn't pull any plug wires as they didn't act like they were going to release and I needed to drive it to my storage lot.

Wires, plugs, and filter should be here Saturday. I'm still on the fence with the O2 sensors...

Tom
Title: Re: Diagnosing Poor MPG
Post by: WILDEBILL308 on August 31, 2017, 09:19:36 PM
It has an O2 sensor on each side in front of the CC. both banks report very similar fuel trims and seem to be constantly changing. I too really have trouble thinking they are at fault. It is definitely in closed loop.

It has the Allison 5-speed and gets to OD around 55 to 60 depending on load. RPM's around 2200 at 60 (from memory).

I'm not positive that the brakes are not dragging, but I have seen no indications. Will look more closely at that possibility on my next drive. The calipers were replaced just before I got the coach.

Tom
There are a number of small things that might help your milage. Simple things like using a full synthetic oil like Mobil 1, Free flowing exhaust. Yes I would jack it and see if the wheels spin without undue drage. One big thing DO NOT use a K&N type filter as they let to much dust to get past.
Are the tires the correct size as specked for your coach? there should be a data plate.
I picked up a 6.6% increase in GPS speed over what the speedometer shows. Have you checked what your actual/GPS speed is?
You might look at one of the aftermarket tuners like Bully Dog, https://www.bullydog.com/
or Banks, http://shop.bankspower.com/
Lots to look at and research.
Tried to post this yesterday but found it haden't.
Bill
Title: Re: Diagnosing Poor MPG
Post by: rls7201 on September 01, 2017, 07:21:18 PM
 On Oemy's Workhorse web site is a list of O2 sensors that will work on your coach. Lots of good Workhorse stuff there.
http://www.oemys-performance.com/whparts.htm (http://www.oemys-performance.com/whparts.htm)

W8000524       O2 Sensor   OEM
12572706       O2Sensor   AC Delco/RockAuto
213-1161      O2 Sensor   AC Delco
15282       O2 Sensor   Bosch
5S4403      O2 Sensor   Airtex/Wells
234-4669      O2 Sensor   Denso
21549      O2 Sensor   NGK
250-24491   O2 Sensor   Walker
Title: Re: Diagnosing Poor MPG
Post by: DearMissMermaid on September 01, 2017, 09:44:44 PM
Gas records.

Are you keeping accurate gas records?

Since I bought my rig, I have logged in every gas purchase and the odometer readings. I always mark my purchase with whether or not I filled the tank full. That way I can go back and calculate the actual gas mileage. I transferred the log to  an ongoing spreadsheet and worked out the costs and miles per gallon.

Most folks when estimating their miles per gallon,  just get it plain wrong. You really need to keep detailed ongoing records to get an accurate picture. In my log book I also mark down everywhere I have spent the night. So the entire time I've owned my rig, I know where I went, where I camped.

That information has been fun and very helpful, as I sometimes do repeat visits and similar trips to get back to places I enjoyed so much, I wanted to go back again.

Now in my case, I started out keeping up with generator use and gave up on that, too much work. So, my gas mileage does vary because the generator drains off the same tank. So that throws a monkey wrench in the works. I have an hour meter on my generator, but I am not convinced it works correctly.

So the summers I workamped, my miles per gallon was much higher than the summers I traveled. This is because when I travel in the summer, I sometimes have to use the generator on hot humid days in addition to the dash AC. When I was workamping all summer, I only used the generator once or twice a month to exercise it.

Driving around mountains made my miles per gallon go down, driving around flat Florida, my miles per gallon goes up again.

Sometimes the gas mileage went haywire during an emergency. For instance I had emergency surgery while my RV and dog were parked in the hospital lot running the generator nonstop. Then it was 5 miles to the campground. So 24 hours of generator plus 5 miles driving reduced the 55 gallon tank  about a fourth which is about 13 gallons. Noting that in my log book helps me remember when I get around to compiling my spreadsheet.

It would be easy to think something was bad wrong that between those fuel fills I got such dismal miles per  gallon, but I was careening around medical facilities with the generator running full tilt boogie.

Headwinds and tailwinds can affect gas mileage too as well as whether you are towing or not towing.

To budget for trips I figure a worst case scenario with only 6 miles per gallon and factor in  the highest cost per gallon for gas purchases. That way I always come in under budget on the trip (for gas) when all is said and done.  (Except for the trip that landed me in medical hell.)

I generally get about 8.5 miles per gallon AVERAGE in my Class C including the generator usage (that is after compiling several years of keeping records)  and I often do travel with full water tank and full gas tank. I don't like to let my gas get below a half tank. If an emergency comes up, I want gas for the generator. The only time my rig has needed road side assistance was on overly hot humid days, so having that gas available to fire up the generator was life saving.

If you don't have good accurate ongoing records, then it's possible you don't have a problem at all.

By the time you factor in generator, towing, winds, mountains and so on. 5.5 mpg may be about right for your rig.
Title: Re: Diagnosing Poor MPG
Post by: Heli_av8tor on September 04, 2017, 10:01:16 AM
OK, first to answer some of the questions posed.
The GPS speed matches the dash speedo. I have not checked it against the odometer.
Thanks for the Oemy's reference. It has been a help.
The tires are new Michelin's and are the correct size. (MICHELIN XRV 235/80R-22.5)
I too keep detailed logs including generator use.

When I cleaned the MAF sensor I found some corrosion on one terminal of the 4 pin connector. I cleaned both plug and socket and applied a thin coating of DeOxIt.
I'm hoping this is the "smoking gun".

I found the plugs to be the light to medium tan color. A couple had more buildup on the ground electrode where it meets the plug body than others. Not terrible but as cheap as they have become I'm glad I replaced them.
Some of the plug wires were impossible to get off without damage. Many of the plug boots under the metal shield were dried out and brittle. A new set is installed.
Based on the plug inspection I decided not to do a compression check as I didn't know if cranking without plugs and starting would throw any codes.

I did a 30 mile test drive last night. The up and down shifting seemed more appropriate (like I would do if a manual tranny) than before. The engine just seemed overall smoother. The rolling average mpg on the trip computer went from 5.1 to 7.1 mpg. some indication that it may be getting better mpg but not definitive. The test route had less residential than before and the wind was fairly calm.

Looks like we will be taking our Colorado trip without replacing the O2 sensors. This trip will tell me if I've made any real improvement. Will report results.

Thanks to all who helped me.

Tom
Title: Re: Diagnosing Poor MPG
Post by: Gary RV_Wizard on September 04, 2017, 10:09:55 AM
Incorrect readings from the MAF sensor will certainly cause the ECM to miscalculate, so that corrosion could indeed be a "smoking gun". The engine constantly runs rich (or lean) if the MAF values are inaccurate.  Hope that solves the mpg problem for you.

I just read an article that says a common source of inaccurate MAF readings is from an over-oiled aftermarket air filter. Maybe that K&N filter was the root cause? 

https://www.yourmechanic.com/article/symptoms-of-a-bad-or-failing-mass-airflow-sensor
Title: Re: Diagnosing Poor MPG
Post by: Heli_av8tor on September 04, 2017, 11:55:55 AM
Gary, the filter was paper with a K&N type foam bonnet on the front side. Wasn't any evidence of that it had been oiled. I too had read that oil contaminated MAF sensors.
It now has the ACDelco filter with the foam glued in place.

Keeping my fingers crossed that the problem is solved. It may help offset these hurricane gas prices :)
Title: Re: Diagnosing Poor MPG
Post by: Arch Hoagland on September 04, 2017, 02:16:43 PM
Two things I noticed after cleaning my MAF besides the no more CEL.

1. When sitting on level ground with my foot off the brake the coach should creep forward. Wasn't doing that with dirty MAF.

2. When climbing a mountain and transmission shifts down to lower gear you should start gaining speed just a bit with the 8.1. It didn't do that with a dirty MAF.

After cleaning I started gaining speed after a downshift on mountains and started creeping forward again with foot off brake.

Shop that cleaned my MAF said to never use a K&N filter due to over oiling. 
Title: Re: Diagnosing Poor MPG
Post by: Bill N on September 04, 2017, 02:20:02 PM
Two things I noticed after cleaning my MAF besides the no more CEL.

1. When sitting on level ground with my foot off the brake the coach should creep forward. Wasn't doing that with dirty MAF.

2. When climbing a mountain and transmission shifts down to lower gear you should start gaining speed just a bit with the 8.1. It didn't do that with a dirty MAF.

After cleaning I started gaining speed after a downshift on mountains and started creeping forward again with foot off brake.

Shop that cleaned my MAF said to never use a K&N filter due to over oiling.

Arch, may I ask you for the location of the MAF.  I do not yet have any of the indications you describe but with the same engine I suspect that I may enounter them some day.  Is this a major job or one an older guy with a bad back could possibly do? Thanks.

Bill
Title: Re: Diagnosing Poor MPG
Post by: WILDEBILL308 on September 04, 2017, 05:04:20 PM
Good luck on your trip.
Bill
Title: Re: Diagnosing Poor MPG
Post by: Charlie 5320 on September 04, 2017, 05:49:03 PM
Arch, may I ask you for the location of the MAF.  I do not yet have any of the indications you describe but with the same engine I suspect that I may enounter them some day.  Is this a major job or one an older guy with a bad back could possibly do? Thanks.

Bill
The MAF sensor is located in the right fender well right by the air filter housing. My coach has some material blocking or directing the airflow so I'll have to cut some zip ties to get mine out. Waiting on the new air filter before doing this. By turning the wheels all the way to the right gives much more room. My coach has a K&N filter that I'm replacing with a WIX.
Title: Re: Diagnosing Poor MPG
Post by: Heli_av8tor on September 04, 2017, 06:42:32 PM
Mine didn't have the baffle restricting access. I run the front jacks all the way down along with turning the wheel to the right. This gave plenty of room to sit in the wheel well to get to the MAF. I used blocks under the axle in case of a jack failure.
 Might depend on how bad your back is. It can be stressful as you can't fully stand up.
Title: Re: Diagnosing Poor MPG
Post by: Heli_av8tor on September 04, 2017, 08:26:49 PM
My CEL has not come on nor have there been any pending codes.

Good to hear that my transmission acting better is likely not just my imagination!

Tom
Title: Re: Diagnosing Poor MPG
Post by: Bill N on September 04, 2017, 09:13:00 PM
Thanks for the tips guys.  I'll take a look and see if I can get mine out to clean.  I had the filter changed out the last time it was in the shop and the guy showed me where that was and, like you said, he turned the wheel to the right to get access. Never thought of running up the jacks.


Bill
Title: Re: Diagnosing Poor MPG
Post by: Heli_av8tor on September 04, 2017, 10:09:09 PM
Bill,
Just don't bet your life on hydraulics if you use the jacks. Always use jack stands, wood blocks or something that will support the coach if a hose bursts.

Tom
Title: Re: Diagnosing Poor MPG
Post by: WILDEBILL308 on September 04, 2017, 10:32:24 PM
On your trip I would try a good gas additive, like Lucas 0r seafoam.
Bill
Title: Re: Diagnosing Poor MPG
Post by: John Stephens on September 04, 2017, 11:28:04 PM
A couple of thoughts...

The Vortec 8.1 is notorious for melting plug wires on one side because of the way they are routed on the W-24 chassis, so you might have the same problem with the W-22. Unless the original plugs were iridium, they probably will not last longer than 35-45,000 miles, even if your owner's manual says to go longer. You could have an O2 sensor problem, but you might also have a knock sensor problem that will give you the same rich mixture by having the plugs fire at the wrong time. Although, if your knock sensors go bad, your check engine light should be on.

At 34,000 miles, I installed new plugs, wires, fuel filter, air filter and knock sensors (light was on.) Essentially, as complete a tuneup as I could think of. It made a small difference in my mileage, maybe .5 mpg.

What made the biggest difference was not running my cab a/c this vacation. I got better mileage towing my car in mountains (8.4 mpg average) than I used to get going without toad on flat ground (7.5 mpg.) Using the coach a/c and keeping the generator running saved me gas and still gave us plenty of cool air.
Title: Re: Diagnosing Poor MPG
Post by: Arch Hoagland on September 05, 2017, 12:12:30 AM
Arch, may I ask you for the location of the MAF.  I do not yet have any of the indications you describe but with the same engine I suspect that I may enounter them some day.  Is this a major job or one an older guy with a bad back could possibly do? Thanks.

Bill

Bill...I raise the front of my coach up as far as possible. I then place two floor jacks under the front frame. Then I turn the wheels so I can get in the drivers side wheel wheel. The air filter is located there. It has three clips on the top, no others.
Remove the cover to change the air filter. You may have to pry the cover out as the bottom sticks a bit. The MAF is located after the air filter, about one foot. It has several wires coming out of it.

Just before my last trip I didn't remove the MAF to clean it, I just emptied one full can of MAF cleaner into the air line with the air filter removed while the engine was running. I've done it the same way on three other vehicles and it seemed to work good. 
Title: Re: Diagnosing Poor MPG
Post by: Bill N on September 05, 2017, 08:57:07 AM
Lots of good videos on YouTube on cleaning MAFs. Didn't realize it was that simple.  Will get a can of MAF spray and go to it.  Thanks for all the help.
Title: Re: Diagnosing Poor MPG
Post by: Heli_av8tor on September 07, 2017, 08:04:15 PM
Here are pictures of the two worse plugs. They are ACDelco 41-983 and have 49K miles.

I decided to also replace the O2 sensors before the trip. They will go in tomorrow.

The first leg of our trip is this Sunday so it won't be long until I know if my efforts have paid off.

Thanks again to everyone who offered their thoughts.

Tom
Title: Re: Diagnosing Poor MPG
Post by: Gary RV_Wizard on September 07, 2017, 08:13:22 PM
Not good looking plus at all. Definitely want to replace them, but the question why are they fouled like that in the first place? Hope your MAF fix eliminates the root cause...
Title: Re: Diagnosing Poor MPG
Post by: Charlie 5320 on September 07, 2017, 08:52:38 PM
I would defiantly replace the 02 sensors with plugs that looked like that. It has been running rich for a very long time. What did the rest of them look like? You may even have a problem with some injectors. Have you ever had a misfire code set? Do you remember what cylinders those plugs came from? If so I'd be checking them again after your trip. If they are darker than the rest, then it could have some bad injectors or coil pacs. Something is telling the ecm to dump a lot more fuel than necessary. When you get this thing figured out you will feel like you have a different coach. 
Title: Re: Diagnosing Poor MPG
Post by: Heli_av8tor on September 07, 2017, 09:35:17 PM
Here's a shot of all plugs. The picture is lower resolution than I'd like due to forum restrictions.

The foreground is the left (drivers side) bank. The right bank is in the rear. The front is on the left.

There is slightly more deposit buildup on the right bank plugs. Other than that I don't see any significant difference.

There have been no codes generated while I've owned the coach.

Tom
Title: Re: Diagnosing Poor MPG
Post by: Charlie 5320 on September 08, 2017, 07:59:06 AM
A couple of those plugs have quite a bit of deposits too, but all are on the rich side. All the plugs have a sooted base. The pic of the first two would alarm me, there is something else going on with those two cylinders, that I would fallow up on. I would use some type of fuel injector cleaner on the next tank of fuel mixed on the strong side. You may have some dirty injectors, that may be leaking when they are supposed to be closed. I like Lucas products, but you run what you want. Seafoam  I hear is good too, but I've never used it.
Title: Re: Diagnosing Poor MPG
Post by: Heli_av8tor on September 08, 2017, 10:23:11 AM
I'd wondered about dirty injectors, but hadn't considered them leaking fuel when they are supposed to be closed.

Have you tried the Techron Fuel System treatment? After reading this thread I think I will give it a try.

http://www.irv2.com/forums/f22/8-1-l-vortec-misfire-and-ping-solution-non-plug-wire-issue-114150.html

Tom

Title: Re: Diagnosing Poor MPG
Post by: Charlie 5320 on September 08, 2017, 02:13:34 PM
I'd wondered about dirty injectors, but hadn't considered them leaking fuel when they are supposed to be closed.

Have you tried the Techron Fuel System treatment? After reading this thread I think I will give it a try.

http://www.irv2.com/forums/f22/8-1-l-vortec-misfire-and-ping-solution-non-plug-wire-issue-114150.html

Tom
Yes I have, I use it in my Corvette a couple times a year because it sets so much. That's going to get expensive to treat a fuel tank in your MH. But I don't have any idea what a sea foam treatment would cost either.  Guess it don't really matter huh? Got to get it fixed. The injectors can leak quite a bit of fuel running at the pressures we run. Then every time you shut the engine down they will leak until the pressure drops.

My coach runs very well, but I'm going to pull the plugs and change them. I'm sure they are original, and they really don't coast that much. When I had the 7.4 engines in my last 3 coachs, they were hard on dist. caps and rotors. I changed them every other year. Wasn't just the 7.4s either. I had a 93 K Blazer that went thru caps and rotors and it was a 5.7, but the parts were the same.
Title: Re: Diagnosing Poor MPG
Post by: Gary RV_Wizard on September 08, 2017, 03:56:18 PM
In most gas coaches, it's not the cost of the plugs that is the deterrent. Rather, it is the lack of accessibility.

Unlike days of yore, fouled plugs are a symptom rather than the cause of an issue. An engine with closed loop fuel injection isn't running poorly because the plugs are fouled; rather, the poorly running engine caused the plugs to get full of crud. When the ECM is managing things properly, the only deterioration of the plugs should be a very slow erosion of the electrode tip. No baked on crud, no oily residue, and no burned spots, and the ceramic should be a creamy tan.

By all means check the plugs at 50k or whatever miles, but if you find a plug problem, start looking for the root cause.
Title: Re: Diagnosing Poor MPG
Post by: Heli_av8tor on October 05, 2017, 08:30:03 PM
Ok, I think my efforts have paid off, but not to the degree I'd hoped. We just got home from our 21 night, 2723 mile trip from Illinois to Colorado and Utah. We ran the 5.5 KW generator 19.7 hours. We used 441.3 gallons of gas computing to 6.17 MPG. If I assume the genny burns .6 gph the MH engine got 6.34 MPG. Not great but better than the 5 to 5-1/2 we got on our trip around Michigan's UP.

I ran the genny in lieu of the dash air for much of the trip across Iowa and Nebraska. The winds on the trip were not kind and almost always had a headwind component. The best we saw was a direct crosswind or very slightly shifted towards a tailwind. I'd hoped we would see a tailwind on the trip home but it wasn't to be. (Kind of my luck flying cross country in Cessna's, too.)

Our route took us over three major passes with long pulls and descents. The first was I-70 going west out of Denver. The second was Monarch Pass, and the third Wolf Creek Pass. The first two had me down to 1st gear running 25 mph (up and down). It really hurt that after burning all that gas to gain altitude I had to waste the stored energy by using low gears to come down safely. There were many other smaller pulls.

I don't have the coach loaded weight but suspect I'm bumping up to max. We pulled our Honda CR-V toad. All things considered I'm likely close to the best I can expect.

The engine ran great. I did have a "Check Transmission Temp" warning 3/4's of the way down Monarch Pass. It went out shortly after getting out of 1st gear. I found one of the two electric cooling fans locked up. I replaced it at Gunnison and haven't seen the warning since.

Thanks to all who made suggestions.

Tom

Title: Re: Diagnosing Poor MPG
Post by: ArdraF on October 06, 2017, 06:26:38 PM
Headwinds really cream your mpg so I'd say between that, using the generator, and where you went over high passes, that's probably not too bad all things considered.

ArdraF
Title: Re: Diagnosing Poor MPG
Post by: RedandSilver on October 06, 2017, 08:18:10 PM
Quote
Ok, I think my efforts have paid off, but not to the degree I'd hoped. We just got home from our 21 night, 2723 mile trip from Illinois to Colorado and Utah. We ran the 5.5 KW generator 19.7 hours. We used 441.3 gallons of gas computing to 6.17 MPG. If I assume the genny burns .6 gph the MH engine got 6.34 MPG. Not great but better than the 5 to 5-1/2 we got on our trip around Michigan's UP.

I read though this thread and never saw you post what the normal MPG where only that you were down to 5 to 5.5.
So did it ever get better mileage and then suddenly it got worse?

Headwinds really cream your mpg so I'd say between that, using the generator, and where you went over high passes, that's probably not too bad all things considered.

ArdraF


I kinda agree with Andra - that's a lot of things going against good MPG's.  So overall not too bad.

Title: Re: Diagnosing Poor MPG
Post by: Heli_av8tor on October 06, 2017, 10:45:45 PM
I bought the coach in March so I donít have mpg history.

Tom
Title: Re: Diagnosing Poor MPG
Post by: John Stephens on October 06, 2017, 11:34:49 PM
Tom - With headwinds and large mountains to climb, I don't think your mileage was out of range of what it should be considering your load, age of vehicle and the miles already on it. We took our three week trip from SW Florida to Asheville, NC to Pigeon Forge, TN and then to Columbus, OH before heading back home. We crossed the Smokies and had some pretty good grades, although nothing like you experienced in Colorado and Utah. I took accurate mileage from both the onboard computer's last 50 mile average and doing it the old fashioned way since auto computer mileage is known for being off. In our case, assuming .5 gal/hour going to the genset running our coach a/c, the computer was pretty accurate, giving me a reading of 7.4 mpg for the overall trip, while my manual calculations showed 6.98 mpg including the gas going to the generator. I kept a pretty steady 62 mph when I could.

My coach is heavier than yours with its W-24 chassis and additional length and my toad weighs in at 4,500 lbs, so I'm not sure how that compares with yours, but I should be getting worse mileage than you since I'm running the same engine. I now have 38,000 miles on the coach and it's a year newer than yours, so I would expect those facts to give me slightly better mileage. I anticipate my mileage getting worse with age and use and plan on extending the life of the engine as best I can by regular oil changes and routine maintenance. If you're like me, you didn't receive any service records on your coach when you bought it, so you may not be aware of the kind of service it received. If the previous owner didn't take care of it and provide timely maintenance, you may have a weaker engine that will never give you optimal mileage.

But if you can get 6+ while towing in mountains and headwinds, you're doing okay.
Title: Re: Diagnosing Poor MPG
Post by: Heli_av8tor on October 07, 2017, 09:09:02 AM
John - Yes, you should be heavier than me. A thousand on the toad alone. I believe you should have the 6 speed Allison whereas I have the 5 speed. Numerous times I thought having that extra gear with closer ratios would have helped. I too drive 61 / 62 mph.

I ended our trip with 52K miles on the coach. I have good records of maintenance from the original owner but they stop 3-4 years ago. I think it set for several years before I got it. Perhaps illness or death of the original owner?

My manual says the AVG MPG computer gives a 50 mile rolling average. I swear it has to be averaging over a much shorter distance, perhaps 5.0. Normal interstate grades, both up and down, across Nebraska and Iowa just have too much effect on the readout. (Yes, I'm sure it's not in INST mode.) I noticed a strange thing with the mileage computer pulling mountain grades. It would start out at 6 something and decrease to 0.0, then to "---". Then it would go to the 80's or 90's and work it's way back down.

I did try to stick with "Top Tier" gas and started this trip with a double concentration of Chevron Techron fuel additive.

Several suggested that I could have fuel injectors leaking when they are supposed to be closed. I'd like to clean the injectors using something like the OTC Tool ( https://www.amazon.com/gp/product/B00GZ0H9VC/ref=ox_sc_act_title_1?smid=ATVPDKIKX0DER&psc=1 ) But it's pretty pricey for a my limited use.

I won't have another trip until January and it will be across flatter ground. I hope to see a little better mpg, but you're probably right that I'm about where I'm going to be.

Tom
Title: Re: Diagnosing Poor MPG
Post by: Arch Hoagland on October 07, 2017, 11:45:12 AM
Tom...I'm at about 25,000 lb with toad and have basically your same RV. The MPG readout on the dash is, let's say, very optimistic.

I calculate my MPG by hand and have recorded every gallon of gas I've ever put in it.  7.1 is the average and I always use the cheapest gas.

I live and travel in the west where every trip involves a mountain of some sort, there's no getting out of the central valley of California without going over a mountain. 

So track your mileage for the next several thousand  miles and you'll get a good feel for it.  I don't think there are any modifications that will improve it.

Let us know what you find out on the next trip.
Title: Re: Diagnosing Poor MPG
Post by: John Stephens on October 07, 2017, 12:49:27 PM
John - Yes, you should be heavier than me. A thousand on the toad alone. I believe you should have the 6 speed Allison whereas I have the 5 speed. Numerous times I thought having that extra gear with closer ratios would have helped. I too drive 61 / 62 mph.

I ended our trip with 52K miles on the coach. I have good records of maintenance from the original owner but they stop 3-4 years ago. I think it set for several years before I got it. Perhaps illness or death of the original owner?

My manual says the AVG MPG computer gives a 50 mile rolling average. I swear it has to be averaging over a much shorter distance, perhaps 5.0. Normal interstate grades, both up and down, across Nebraska and Iowa just have too much effect on the readout. (Yes, I'm sure it's not in INST mode.) I noticed a strange thing with the mileage computer pulling mountain grades. It would start out at 6 something and decrease to 0.0, then to "---". Then it would go to the 80's or 90's and work it's way back down.

I did try to stick with "Top Tier" gas and started this trip with a double concentration of Chevron Techron fuel additive.

Several suggested that I could have fuel injectors leaking when they are supposed to be closed. I'd like to clean the injectors using something like the OTC Tool ( https://www.amazon.com/gp/product/B00GZ0H9VC/ref=ox_sc_act_title_1?smid=ATVPDKIKX0DER&psc=1 ) But it's pretty pricey for a my limited use.

I won't have another trip until January and it will be across flatter ground. I hope to see a little better mpg, but you're probably right that I'm about where I'm going to be.

Tom

We have the same transmission, most likely. I have the Allison 1000, which is a four gear with overdrive, or 5 gears.

You're lucky you got some records. When I bought my coach, it had 23,005 miles on it and I thought it had been fairly well taken care of because it was a one owner that was former military. But I got no records, had no idea when the last oil change had been done, and didn't know if anything else, such as tranny fluid change, tune up, air filter, etc. had ever been performed due to its low mileage. So I had to spend the money to get it all done whether it was needed or not, just to be on the safe side.

I know exactly what you mean about the rolling average seeming to change drastically quicker than you would think if it was taking a 50 mile average. When I coasted down a mountain this last trip, the computer might have read 6.0 from the climb, but after going downhill only one or two miles, the average was now up to 10.5. Of course, if you have ever switched it to "instant" mode, you have found that when coasting, you'll register in the 20's or 30's. That reminds me of a diesel that consumes no fuel when idling. I never experienced what you mentioned about the reading going to zero or blank and wonder if you may have an issue with your computer, or at least the mileage readout.

I have always used the cheapest gas I can find in all my cars and the coach and have (knock on wood) never had any problems as long as I keep the fuel injectors clean by giving the the required service every 30,000 miles. Now that you mention it, I can't remember if that was done on my coach when they did the tuneup. I'm pretty sure they did.

You can read the many posts from a variety of people who want to make everyone think their coach is better than anyone else's by stating they are getting outrageous mileage. But the simple fact is, nearly every gas coach made is going to get between 6 and 8 mpg. Variances will occur with age of coach because many of the newer ones are built lighter and with more efficient power plants, size and weight will make some difference but not much, whether or not you tow, and the highway, traffic and weather conditions of your trip. One leg of our last journey, I got almost 9mpg because I had a strong tailwind, and one leg, I got 5.5 because I was battling headwinds. That, it itself, in my opinion, gives the greatest variance when driving because when you're traveling in the mountains, the downhills are going to neutralize the uphill climbs to some extent. I actually found myself getting better mileage in the mountains than on flat ground. Reason: I turned the cruise control off and anticipated the climbs by going faster than my norm before hitting the grade. If I was coasting downhill and saw a climb coming up, I would speed up to 68-70 mph so I could make the grade all the way up or most of the way before having to downshift. When you leave the cruise control on, it cannot anticipate those grades coming up and will maintain the same speed all the way down the hill you're on, and then downshift almost as soon as you start the climb. It's a trick I learned as an OTR truck driver. My mileage on flat ground, such as the interstates in Florida, was about 7.0-7.5 on the computer readout, while my mileage in the mountains without the cruise on was over 8.0 most of the time.

Good luck with your mileage. The best thing you can do is simply keep it maintained properly and it will give you its best performance.
Title: Re: Diagnosing Poor MPG
Post by: ArdraF on October 07, 2017, 05:35:46 PM
Quote
I noticed a strange thing with the mileage computer pulling mountain grades. It would start out at 6 something and decrease to 0.0, then to "---". Then it would go to the 80's or 90's and work it's way back down.

That's probably an instantaneous reading as opposed to actual gas mileage.  It tells you that you're doing well at 6 and not so well at 85, which is normal on lower and higher hills.  When we had gas engines we always added a vacuum gauge which was a similar way of telling us how much fuel we were expending.  Sometimes if it's really a high number you can lessen your foot pressure on the gas pedal or perhaps drop back on the cruise control.  We usually drop our cruise control back to 55 or maybe less in rolling terrain because the transmission wants to "hunt" (e.g. shift gears frequently) too much.  When going over mountain passes like you did on I-70 up to the tunnel from Denver we disengage the cruise control and drive it ourselves.  Even with our big 500 hp Cummins engine we slow a lot and of course use more fuel at the higher elevations.

ArdraF
Title: Re: Diagnosing Poor MPG
Post by: Stephen S. on October 08, 2017, 02:37:51 PM
I like the new readouts that vehicles have for performance, mileage, and such. Most vehicles on the road these days have this info available from the main computer. They just don't have a handy display. To get the readouts in my 1999 Chevy 3500 Winnebago I bought the UltraGauge dongle. The screen I set as primary on my smartphone has both instant and average fuel mileage gauges, along with RPMs and actual coolant temp (preferred over the idiot light).

In the hills I watch as the instant MPG goes from 0.5 to 40 (pegged), then watch the average settle down to around 9.2 or so during a long trip.

The secondary screen has dials for engine load, RPM, coolant, and instant MPG. I use this to train myself to feel when the RV is doing its best in various traffic and terrain.

[edit] There is the added bonus of any codes from the system (flashing engine light) are displayed in easy to understand sentences.
Title: Re: Diagnosing Poor MPG
Post by: ArdraF on October 08, 2017, 05:29:20 PM
We've used a Silverleaf for quite a few years and like being able to see how the engine and transmission are functioning.  We usually keep six items on the display with engine coolant temperature being one of them so we can see how hot it's getting when ascending our western mountains.  We also like that it shows what gear we're in because that was dropped by Cummins for a few years.

ArdraF