Cant figure out my bad gas mileage!!?? - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


1992-1999 Series I L27 (1992-1994 SE,SLE, SSE) & Series II L36 (1995-1999 SE, SSE, SLE) and common problems for the Series I and II L67 (all supercharged models 92-99) Including Olds 88's, Olds LSS's and Buick Lesabres Please use General Chat for non-mechanical issues, and Performance and Brainstorming for improvements.

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Old 03-15-2008, 06:07 PM   #1
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Default Cant figure out my bad gas mileage!!??

My last fill-up I averaged 12.84mpg with mostly city driving. Damn near everything is new. The engine has about 6K on it so it should be broke in by now. The trans has about 3K on it. I do have the lower gears, but it wasnt that big of a drop. I started with 3.06 and went to a 3.33. The engine was completely rebuilt and runs real strong. It has a new cat, O2, K&N, Syn. oil. It has a custom chip which corrected the trans shift points so it goes into OD and TCC lockup correctly. I turn about 1900 rpm @ 60mph. It doesnt seem to matter how I drive, my mileage stays about the same. I was getting about 16 mpg on the highway. Any ideas???
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Old 03-15-2008, 06:14 PM   #2
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I am at a loss there..

Did you use an AC Delco or Denso O2?

When you had your chip burned did you make any Fueling changes?

I don't have the cold weather like you guys do.. But my inner city still nets me 19-21mpg

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Old 03-15-2008, 06:25 PM   #3
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I used a Denso O2. But it was just as bad before I changed that and before the chip. I didnt change anything with the fuel except to allow more spark advance. Would the gears make that big of a difference???
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Old 03-15-2008, 06:38 PM   #4
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Gears will make a difference... But not like you are seeing..

This time of year I would fully expect to see 14-16 Mpg from your car around town.. On the open road I would expect 27 Mpg with your gears, and even that is being conservative ...

Not sure what direction to tell you to look in..

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Old 03-15-2008, 06:43 PM   #5
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I don't know a lot about motors, I admit that.

So here'* my shot in the dark. Is it possible that the P&P could have messed up the A/F ratio or something like that.

And this may seem stupid but is all connections hooked up, not loose?

Trying to help out here.
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Old 03-15-2008, 06:50 PM   #6
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what do your plugs look like. I would expect them to be black, running pig rich. They should give us a clue anyway.
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Old 03-15-2008, 06:51 PM   #7
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My first guess woudl be your O2 sensor. Do you have a way to have it tested with a wide band O2? That would give you an idea as to what your ratio is.
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Old 03-15-2008, 08:24 PM   #8
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I checked the front plugs around the time I put the tranny in. They were an almost perfect tan color. I plan on putting some Iridim plugs in soon so I will see what the others look like. I have a snap-on scanner at work. The last time I scanned it my O2 was reading correct. All my data looked good on the scanner. There are no vacuum leaks, all emission systems are on and working, and the motor runs real strong. Im stumped.
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Old 03-15-2008, 08:50 PM   #9
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pcv?
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Old 03-15-2008, 09:10 PM   #10
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Quote:
Originally Posted by phatklan
pcv?
Its new along with the UIM.
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