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 10-29-2002, 10:03 AM   #1
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Default Bad gas mileage

I have a 95 SC SSE1. I have Delco Plat plugs, new O.E. wires, semi synth oil. I keep vehicle in performance mode and I have played with octane ranging from 87-94 but cant seem to get more that 13mpg out of tank. My tank is 18gal i usually run out at abt 230miles. Could this be normal. I usually drive about 80mph. I have no special exhaust system, I have gone to a 180 t-stat. K&N filter and recently took the front portion off of my airbox so its just a big hole. Never done O2 sensor, although my CE light comes on after about 10min of driving and stays on till i restart. The last time it was checked it came up as code 640 QDM. Please help!
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Old 10-29-2002, 01:00 PM   #2
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Hey, welcome to the club. I got the same year and trim line that you do.

As far as gas mileage, my guess would be O2 sensors. I am pretty sure mine need to be replaced and my gas mileage is roughly the same as yours. I know if I keep it to 60 and highway speeds I can get up to 400+ miles from a tank, but I tend to give it alot of gas requently which I know will hurt my mileage, but it'* SC'd, so who cares!

So what it the cars mileage now? If my memory serves me right, O2'* usually need to be changed ever 60k - 70k miles.

CE light? I'm probably just haveing a menatl lapse but which light is the CE light?
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Old 10-30-2002, 03:21 AM   #3
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It'* the check engine light!!!
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Old 10-30-2002, 03:41 AM   #4
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Don't ever run below 91 octane in a SC bonneville....bad news. I don't have my book handy right now, so I can't tell you what that code is, but a couple months ago, my mileage too about an 8mpg nosedive. I changed the O2 sensor and it came right back up. Sucker had me running richer than snot from a 4 year old.

I'd check the code first (I can look this up in the morning), and then maybe think about O2'*.
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Old 10-30-2002, 03:37 PM   #5
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Default Re: Bad gas mileage

Quote:
Originally Posted by G2SSEi95
I have a 95 SC SSE1. I have Delco Plat plugs, new O.E. wires, semi synth oil. I keep vehicle in performance mode and I have played with octane ranging from 87-94 but cant seem to get more that 13mpg out of tank. My tank is 18gal i usually run out at abt 230miles. Could this be normal. I usually drive about 80mph. I have no special exhaust system, I have gone to a 180 t-stat. K&N filter and recently took the front portion off of my airbox so its just a big hole. Never done O2 sensor, although my CE light comes on after about 10min of driving and stays on till i restart. The last time it was checked it came up as code 640 QDM. Please help!
That code doesn't show up in my book, and nothing even comes close. Are you sure about the reading?
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Old 10-30-2002, 11:41 PM   #6
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I would go for the whole 9 yards . O2 , pcv and the EGR valve , How long since your last plug change ? Might wanna get the injectors clean too! See ya at work!!!
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Old 10-30-2002, 11:47 PM   #7
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Default bad gas mileage

Whats a PCM? Is that the same as a ECM Electronic Control Module? I purchased the car in DEC 2001 at 58,500miles. I have 74,500 now. I rexcently took care of the PCV and at AC Delco platinum plugs have about 7000,00 miles on them. I tried to get a O2 sensor at Pep Boys, AutoZone, and Strauss, but they all carry BOSCH. I know u guys told me to stay away from Bosch O2, right?
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Old 10-31-2002, 12:05 AM   #8
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Default Re: bad gas mileage

Quote:
Originally Posted by G2SSEi95
Whats a PCM? Is that the same as a ECM Electronic Control Module? I purchased the car in DEC 2001 at 58,500miles. I have 74,500 now. I rexcently took care of the PCV and at AC Delco platinum plugs have about 7000,00 miles on them. I tried to get a O2 sensor at Pep Boys, AutoZone, and Strauss, but they all carry BOSCH. I know u guys told me to stay away from Bosch O2, right?
PCM stands for powertrain control module. Controls the trans, too. Your plugs could be part of your problem. The SC engines prefer Copper plugs for some reason. I wouldn't worry about your EGR. It would be throwing a code if it was bad. Stay the HELL away from bosch plugs AND O2. If you have to, get the O2 from the dealer, or www.gmpartsdirect.com you'll need the 8 digit part number at the site. Cheaper from the web.

HOWEVER....if you have a PCM problem, I wouldn't mess with the O2 yet. Kinda like putting out the match before the forest fire.
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Old 10-31-2002, 12:09 AM   #9
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About how much would a new one cost? Can I do it myself? Where can I get copper plugs?
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Old 10-31-2002, 10:53 PM   #10
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Hey Will, wouldn't Iriduim plugs work a little bit better ? They also make a chip for your car ! Ask me about it at work .
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