fuel milage - 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 08-21-2007, 02:30 PM   #1
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About 2 months ago, i installed a 180 degrees t-stat. After that, my gas milage droped drastically(i filled up 2 miles after installing it) i was getting aroun 25-28 mpg to work and back, and it dropped down to about 20.
I drove it around for a little over a month like this with no improvement. I drive 100 miles a day, 5 days a week and every mpg counts, so because of this, i wanted my milage back, so i dropped the 195 degrees t-stat back in. Its been a few weeks now and my milage is still stuck at around 20 mpg. I was wondering if anyone know what the cause could be. I've had a full tune up within the last 10,000 miles, cleaned the tb, maf sensor, and air filter recently, checked for vaccum leaks, and yeaterday i hooked up a scan tool

LTFT and STFT readings were both right around 0%, 02 sensors were cycling good, and spark knock was at 0, so at this point i'm am confused at what would cause the bad gas milage. I drive an average of 55mph with cc set and have not had a lead foot. I have written down all scan tool data so if anything else is needed i will post it
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Old 08-21-2007, 02:42 PM   #2
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When you say the O2'* are cycling good.. I interpret that as quickly. How high and low are the readings? Are you seeing 80-150 as a low number?
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Old 08-21-2007, 03:30 PM   #3
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yes and around 800 or 900 as a high
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Old 08-21-2007, 03:42 PM   #4
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What part of NH are you from?

Close enough that meeting up with the NE crowd to look over the car might be feasible?
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Old 08-21-2007, 03:54 PM   #5
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concord area, about an hour north of the mass border along 93
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Old 08-21-2007, 03:57 PM   #6
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Hmmm well..we are probably gathering at the usual place (aka my house) Saturday in Enfield CT on the MA border. You've checked for vacuum leaks and it runs great? Just crappy mileage?
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Old 08-21-2007, 06:19 PM   #7
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yep, no vaccuum leaks, bad milage, no drivability issues, i wouldn't be able to make it down there for a while, my boss gave me a can of sea foam with a vaccuum injector so i'm gonna give that a try tonight, , and maybe use the moto vac tommorrow. If i didn't put on so many miles a week i wouldn't bother,
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Old 08-21-2007, 10:12 PM   #8
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Check the reading for the Coolant Temp sensor. Are you sure your shifiting into OD?
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Old 08-21-2007, 11:33 PM   #9
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coolant temp stays beyween 195 and 205 and im sure OD is working properly
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Old 08-22-2007, 09:50 PM   #10
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i used the seafoam yesterday, and filled up halfway to work. Now i've only been 75 miles since i filled up and i'm already noticing a difference. Usually at 75 miles the gauge is halfway between full and 3/4th, and now at 75 miles its barley moved off of full, only time will tell, i'll know for sure if it did anything after the next few tanks, i'll keep you updated
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