Another Bad Gas Mileage thread... - 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 10-02-2007, 02:09 PM   #1
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Default Another Bad Gas Mileage thread...

I know the gearheads has to be sick tired of them by now.

Initially, I thought it was due to the way I drove. But after many tankfuls, I concluded it is not. THe most recent tank, I got 17 mpg and that'* with about 70% highway (at 5 mph over max) and the rest in city. That was with grandma-driving style. For some reason I have not been able to get over 23 mpg combined.

I had a new O2 (Delco from INTENSE) not too long ago and the plugs/wire are only 25,000 miles old.

I'm ready to blow it up.
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Old 10-02-2007, 02:11 PM   #2
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Remember your evap issue thread. I'm thinking it has something to do with the bad mileage.

http://www.bonnevilleclub.com/forum/...ic.php?t=86523
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Old 10-02-2007, 02:14 PM   #3
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I think so but I wasn't sure if that would majorly affect the bad mileage.

That'* why I made a new thread.
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Old 10-02-2007, 02:16 PM   #4
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I'm not 100% sure it is..it'* something malfunctioning on the car and it is part of the fuel system.
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Old 10-02-2007, 04:39 PM   #5
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Would vaccum leaks play a part in bad mpg?
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Old 10-02-2007, 06:05 PM   #6
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Default mpg

What brand and heat range of plug are you running. It has a lot to do with the mileage.
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Old 10-02-2007, 07:04 PM   #7
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Default Re: mpg

Quote:
Originally Posted by SSEBONNE4EVA
What brand and heat range of plug are you running. It has a lot to do with the mileage.
I honestly have no idea, they were just stock replacements, I think they're Champions but that was way before these happened.
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Old 10-03-2007, 11:21 AM   #8
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Assuming you have already done the obvious tune up items, filters, pcv, etc.
Pull them and see how they are burning. If they are dark colored the plug is probably too cold of a heat range.
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Old 10-03-2007, 09:42 PM   #9
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I pulled a plug, turns out it'* a AC Delco unit and the spark plug looks white turning to tan...so it'* a-okay
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Old 10-04-2007, 10:55 PM   #10
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My mileage is close to yours. I've driven other 3800 powered cars that get great gas mileage, but my Bonnie has always been under 20 in town, low 20'* on the highway. The car runs fine, Transmission was rebuilt about 20k miles ago, O2 sensors have been replaced,plugs,wires,etc. I've had the car(97 NA 3800) for about 75k miles and it'* always been about the same. I'm stumped. I'll check my fuel evap system, but I'd be suprised if it made more than 1 or 2 mpg difference. Maybe weak coils, or partially clogged cat? If you fix yours, I'd love to hear what you find.
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