Bad Gas Mileage driving me nuts! - 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 02-22-2004, 12:46 AM   #1
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Default Bad Gas Mileage driving me nuts!

Hey guys I am having a real problem. I am only averaging about 17-18 mpg on average. This is with city and highway combined but mostly highway. During the summer I would get around 25 mpg highway almost 26. Now no matter what I do I can't even get 20 mpg on average. I used to get about 21-22 mpg on average. I do have a smaller supercharger pulley and the exhaust and those are my only performance mods on the car right now. I took out the taylor wires, the ngk plugs, the superchip, the cone air intake and the 180 deg t stat. All that stuff is all out of the car and replaced with delco wires, delco plugs, stock airbox, original chip, and 198 t stat. I am pretty much back to a stock setup.

Do you think that the supercharger pulley is causing me to get 2-3 mpg worse than I used to? I have gone through just about everything on the car including all vacuum lines.

I replaced the 02 sensor about a year ago and the fuel filter the same time. The catalytic converter is aftermarket Hi flow and was installed at 125,000 miles or so and the car now has 155,000 on it. I don't know if the cat could have anything to do with it or not but I am running out of options.

Thanks guys for any suggestions in advance.
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Old 02-22-2004, 02:23 AM   #2
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Well sorry to be so obvious but ITS WINTER in MA especially

I am positive you lose some mpg from:

-The Heater
-Warming the car up
-Traction i.e. with engine over-revving
-WIntertime, traffic is worse

Just some quick ones for ya.

But on a side note, no the pulley should not affect it that much. Also the stock parts might be old (unless you bought new ones to put in).
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Old 02-22-2004, 02:51 AM   #3
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i get 15 steady in town, and 25 on the hwy, summer conditions. If your getting poor mileage its very hard to say whats causing it. Could be a vast number of things!
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Old 02-22-2004, 03:00 AM   #4
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The SC pulley will have no noticeable effect. Have you ever had an unexplained coolant loss (since the O2?)? O2'* don't like antifreeze. The sensor may be crap, anyway. Everyone makes a lemon from time to time. If it'* not an AC Delco, put one in.

How much TC slip are you getting? Do you have access to a scantool? A slipping TC can KILL mileage really quickly.
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Old 02-22-2004, 02:59 PM   #5
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yeah i can get access to a scan tool but I dont know if its going to tell me if I have a slipping TC. I will try though. Also I have no leaks on the car. And finally all the stock parts are new on the car. New delco wires, new plugs, new air filter, and the 02 sensor was bought new from GMPD along with the fuel filter. Thanks though guys
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Old 02-22-2004, 03:36 PM   #6
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Get a scantool on that thing, and tell us how much slip you get in TC lockup. Also how much slip you get under moderate accelleration with the TC unlocked. How many miles on this trans, and how long since the last service?
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Old 02-29-2004, 01:43 PM   #7
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ok i hooked it up to a scantool. The duty cycle never went to 100% it was always at 0% whether the traction control was on or off. the rpm slip at idle was s254 rpm, which sounds like a lot. Under acceleration it was the same. However, under braking the rpm slip went to negative numbers -254 at the most. So what does all this mean because i am very confused. thanks
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Old 02-29-2004, 01:53 PM   #8
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Above 48 mph in TCC lockup, how much slip do you have?
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Old 02-29-2004, 11:42 PM   #9
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OK! it seems i have to do some more investigating. I know the computer didnt call for lock up because there was a NO next to that. I didn't notice the TPS values. I will get back to you guys when i have more info. The only code I am getting now is code 257(overboost) Its a weird code and it doesnt show up in alldata or most other places and if it is listed its not listed as overboost but something different. I think i am going to take the smaller pulley out for the SC and see what happens.
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Old 03-01-2004, 06:27 AM   #10
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17-18 is what i get in the city so its normal
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