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Loss of fuel economy

14K views 83 replies 18 participants last post by  silvertank 
#1 ·
Lost about 4 to 5 mpg since last oil change . Started off about 2mpg average and has been increasing slowly since, brought it up to the dealer and they told me nothing they can really do about it as its super hard to diagnose but to test the oil when i change it again and let them know if anything abnormal shows up. gonna change he oil this week and up date to see whats going on my average mpg stays pegged around 30 now instead of around 39 to 42. Im averaging 37 to 39 where i was averaging 41 to 42 calculated by each fill up. Anyone got any idea it should have went up not down
 
#41 ·
ummm 31-35 is not that far off at those speeds when in such hilly highways... when i went to lords town through Tennessee my mpg dropped since its very hilly. overall the whole trip i did 67.3 mpg but through Tennessee and Kentucky it was tough doing so even at 55mph. top off your tank, run factory tire pressure remove all non essential clutter in the car then try driving 100 miles on the highway at 55 mph cruise control and then top off again see what its at. my diesel compensated for altitude but i could feel a little under normal power compared to my home of Florida
 
#42 ·
Depending on where i go usually i pull out 52 in the hills have since day one, but i have been averaging 42 on the same trip back and forth to work and on the trip to ms same route same speed every time then all of a sudden it dropped now im lucky to pull out 37. I keep track of every tank of fuel and lately it seems to be on the decline for no apparent reason.


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#43 ·
ahh ok so its not so much a gradual decrease its a huge step off. hmm thats a strange one. do you think the computer may not be compensation properly for the high altitude? i could see it running a bit richer then it should thus burning more fuel and more frequent regens
 
#45 ·
Anything under 3,500 ft (1,067 m) is considered low altitude. This has to do with a combination of overall air pressure and partial pressure of water vapor. The interesting thing is that turbo powered cars tend to do better as the altitude increases. This is because the turbo can maintain a consistent air pressure in the cylinders but the aerodynamic drag is reduced.

It's not altitude - something else is going on.
 
#47 ·
I think the one question that was never asked is about the oil. You indicated that the oil was changed, then the MPG dropped. Improper oil can cause excessive regens which can cause poor fuel economy. Do you know 100% for sure what oil was used?
 
#51 ·
OK, no issues there. What about fuel? Could biodiesel have been introduced into your fuel supply?
 
#53 · (Edited)
B20 has more potential energy which should translate to better economy.

If anyone has a link that shows more soot being produced from Bio diesel I would like to read it.

Bio cleans everything incredibly well so if you dont use it from new you will get extreme cleaning action that could produce a little extra soot in the short term due to all the dirty deposits getting cleaned out of the fuel system.
My 2 cents.

I would rather run bio to be honest.
 
#54 ·
#57 ·
ive not researched it much but i never run bio. here in Florida we have straight diesel. during lords town trip i had to use bio from pumps on highway. not a huge drop that i noticed but i did feel my cruze a bit weaker on throttle.
 
#58 ·
Anecdotally and from my "butt dyno", I'd have to agree that non-bio diesel is better from an acceleration standpoint than bio. Still getting frequent regens, but it's only been one tank on non-bio diesel.

Only a real dyno could tell if non-bio is better...
 
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#61 ·
You just reminded me, tire pressure could be a meaningful factor.
 
#64 ·
Tire pressure makes a difference, but not as much as you might think.
 
#69 ·
I have read on other automotive forums that the computer(s) can be reset by disconnecting the battery for X amount of time/ and possibly even touching the disconnected cables together. I am not sure if that holds true here so do so at your own peril. Maybe someone more familiar with that sort of thing can chime in.

Here's a referenced article that seems legit, but again do so at your own peril!

How to Disconnect a Car Battery to Reset the Computer | eHow
 
#70 ·
Resetting the computer by pulling the plug works on the gas models. I don't see a reason why it wouldn't on the diesel models as well.
 
#72 ·
Wow, that's scary because a blown turbo seal can cause a runaway diesel in some cases. This is the first I've heard of any turbo issues on our cars. Can you go into more detail about what exactly happened to your turbo?
 
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#74 ·
Thanks for the update. Keep us posted. That sounds like something different than a seal inside the turbo.
 
#76 ·
Great point!
 
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