Consumption problem

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DrPepper

DrPepper

Member
Joined
Feb 20, 2011
Posts
5
Location
Morges area, Vaud County, Switzerland
Hello, I own a Fleetwood Flair RV Chevrolet 1997, 7.5 liter engine. My problem : the vehicle consumes 6-7 MPG while other identical models show 10 MPG. In Switzerland, the mechanics do not know enough about this type of vehicle and engine to find the cause of this problem. Any information can be useful. I do not know what to do, change the Lambda sensor or the electronical "boxe"? Results of pollution's test are very good and indicate no dysfunction. Thanks you in advance for your help.

For information, here in Switzerland, the fuel costs $ 8.20 to $ 9 one gallon ....
Best regards from Switzerland.

Jisse
 
I think those claims of 10mpg are a bit optimistic. From my experience 7mpg sounds about right for your motorhome. I had a 1998 truck with that same engine that weighed 8000 pounds and I averaged 10-11 mpg. I'm thinking you're in the low teens for weight and not exactly aerodynamic. I try to make my trips closer to home and stay longer to use less fuel per vacation.
 
DrPepper,
I seriously doubt you will improve your mileage much, if any. I've had a small motor home (22 foot) with the same engine and got about 8 mpg. My current unit also gets 8 mpg on a good day, and probably 7.5+ overall.
Ernie
 
Our 25 ft 1997 Winnebago Warrior with the 454 engine gets a consistent 11-11 1/2 mpg when driving 60-65 on cruise control. Guess we should feel lucky!
 
How fast are you driving? I only go 55 and I am lucky to get 9 miles per gallon with a 454 in a 27 foot class A. If you are driving over 55 that would be one reason why your mileage is so poor.
 
Hi Tom, thanks for your answer. I drive mainly around 55 mph. Unfortunately, Switzerland isn't a flat land... but I drive an economical way, that doesn't help. As I wrote, the previous owner said he never passed the 10-11 mpg, and the person who imported the RV in 1997, who is a mechanical, says the same thing.

I'll work to find the problem. Thanks again. Best regards. Take care.

Jisse
 
Thanks Gary for the help. I thought about a problem with LAMBDA "probe"(?)....  The tests are good when engine is runnimg slow. And if the "probe" gives wrong informations when driving at 50-55 mph ?......  Hard even impossible to say...

Take care. Regards from Switzerland.

Jisse
 
Unfortunately as others have said, I don't think you'll ever see 10mpg on a regular basis in that rig.  I have a similar vintage Class A (1994 although mine is the Ford 460 chassis) and average around 7-7.5mpg on flatland driving around the Midwest U.S.  My average cruising speed is around 55-60mph.  Only one time has my math shown a trip average of 10.1mpg on about a 150 mile stretch, and to this day I wonder if I did the math wrong.  If not, then the conditions were JUST perfect.  No wind (or possibly slight tailwind), cool summer evening driving, no towing.

$8-9/gallon is ROUGH, and I think I may complain a little less about the $3-4 we are seeing and anticipating for this summer here in the States.  :eek:
 
We had a 1994 30-ft. Lazy Daze Class C with the Chevy P30 454 engine.  For the first 50,000 miles we got about 9.7 miles per gallon which included trips all over the U.S. and Canada.  A few trips it was only about 8.7 mpg.  After we started towing the Geo Tracker it dropped to 9.3 on at least one trip and the average at 57,684 miles was 9.6 mpg.  We thought this was very good mileage!  We're conservative drivers with easy throttle.  We had a vacuum gauge so we would know when we were using fuel efficiently.  How hard you use the gas pedal when starting and how hard you brake can have a huge effect on your mileage.  Our trips with many mountains and/or windy conditions consumed more fuel.  You may be able to tweak it some, but I really don't think you'll do a whole lot better in mountainous Switzerland!  Be sure to look at how you're driving.  If you're making what we call "jack rabbit" starts (0-60 in xxx) that uses a lot of fuel.  Also, using lower gears on mountains helps.  You may go a little slower, but your fuel consumption will be better.

I sure hope our fuel prices don't increase that much!  Ouch!

ArdraF
 
Check tire pressures, check brakes for drag and alignment, if all these check good then you are probably getting as good of fuel economy as possible under your driving conditions. I had the 454 in my old Pinnacle and got 9.2mpg average but most of my driving was in Florida where it is relatively level.  Good luck.
 
When you find that 2.5 mpg and can truly document it, patent that formula. You will get so rich$$$$.
J
 
That is what my Pinnacle got right out of the box...,... The only time I ever seen the economy drop was when I had an exhaust manifold leak that burnt a pug wire which caused a misfire. I believe the newer fuel injected models are capable of better MPG. 
 
With the cost of fuel over there I'd guess you're doing all the right kind of driving to conserve fuel. One thing I'd do is check the vacuum with a vacuum gauge. Check all the vacuum lines. You might have a vacuum leak that the computer is compensating for but at the sacrifice of fuel economy. It's an easy check and one more thing you can eliminate as a possible problem. Good luck and if you ever find the problem (if there really is one) please let us know what you find.

BTW, how long is your motorhome and how do you have it loaded (lots of extra stuff to add to the weight)?
 
The "Lamda sesnor' is the oxygen sensor and it basically tells the ECM how efficient the combustion was by measuring the exhaust gas content. It doesn't work any different at high speed or low. I suppose it  could begin to fail at high exhaust temperatures, but the usual failure mode is either shorted or open circuit. Rarely does it give just slightly wrong information. And if it gets stuck on one reading, the ECM will notice that it cannot get the calibration to change and turn on the Check Engine trouble light and store an O2 sensor code.  It could be some other sensor, though, maybe air mass or some such. Maybe you need to invest in a service manual for that engine and chassis, since your mechanics are not too knowledgeable in US built vehicles.

By the way, can your mechanics check for and read the diagnostic codes for a US engine?
 
Actually Gary, GM reccomends some high speed driving or loaded driving to burn the carbon off of the sensor so it is unlikely it will fail at high temps. Normally it is deposits that cause failure of the O2 sensor. The key to good enginge performance is proper tuning, including timing set right, No vacumn leaks (MAP sensor) vacumn advance on engines not computer controlled (With proper centrifugal advance curve) Engines with mileage on them seem to see the Centrifugal advance springs get weak and also binding in the distributer. 1999 should be totally computer controlled so it needs it's initial timing set correctly. Being an OBD II system most parameters are easily read through the OBD II cpnnector. Another robber of fuel economy is tire inflation and alignment with brakes dragging and emergency brake set to tight. Also on automobiles, tire pressures were often recommended by the manufacturer to improve ride rather than fuel economy, on MH the tire pressure should be set to loaded weight.
 
The man that sold it to you said it would get 10 MPG?? The dealer that sold me mine said it would get about 10 to 12 MPG. Mine gets 6 MPG all day long. Sounds like my dealer and your seller went to the same school for salesmen
 
Hmm, probably not the problem, but are you talking Imperial gallons? I think that would be about 5 quarts? Which would then be about 10 mpg for that engine. I have that engine in my 1983 HR and I get about 7.75 mpg.
 
I think that would be about 5 quarts?
Click to expand...

Only 4 imperial quarts  ;D

1 imperial gallon = 1.2 US gallons. So your 7.75mpg (us) = 9.3mpg (imperial).
 
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