Diagnosing Poor MPG

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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
 
"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.
 
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.
 
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.
 
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
 
Heli_av8tor said:
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
 
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

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
 
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.
 
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
 
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
 
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 :)
 
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 Hoagland said:
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
 
Bill N said:
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.
 
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.
 
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
 
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
 
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
 

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