Diagnosing Poor MPG

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Heli_av8tor

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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
 
Heli_av8tor said:
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.
 
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
 
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. 
 
Heli_av8tor said:
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.
 
Heli_av8tor said:
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.
 
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
 
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
 
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.
 
Heli_av8tor said:
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
 
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.
 
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?
 
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
 
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.
 
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
 
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?
 
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.
 
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
 
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..
 

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