Bad 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 11-20-2004, 11:32 AM   #1
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Default Bad Mileage

Well the LSS was getting good mileage but the past 3 tanks get worse every time. First of the third was about 22mpg, 2nd was 18mpg, and the last one was barely 15mpg. I put sea foam in but the car doesn't seem to like that. I think it got a tune up before I bought it and remember I had the fuel sender and fuel pump replaced. Any ideas?
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Old 11-20-2004, 12:04 PM   #2
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is you tranny working ok? i'm not that good with mechanics but i was reading about the tcc not looking up so you get worse gas milage. just a thought.
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Old 11-20-2004, 12:28 PM   #3
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have you gotten gas at the same place?? you could just have gotten a bad mix of gas. I know that a few days ago. I got gas and then i was driving and my O2 gauge I have showed that hte car was running lean. It did this for a few days and then the car became normal. THe car also ran like crap
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Old 11-20-2004, 12:33 PM   #4
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When was the last tune up? Could be one of many things. Clogged air filter, fouled plugs, dirty TB, lazy o2.


-justin
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Old 11-20-2004, 01:55 PM   #5
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don't sweat it too much. All of our MPG'* are dropping steadily.. know why? It'* the time of the year. They put out a different blend of gas for winter and gas mileage always goes down significantly. Mine has gone from 28 MPG highway to around 24 MPG. :(
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Old 11-21-2004, 11:44 AM   #6
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Default gas mpg

Ditto on the gas mpg. My cars do the exact same thing every winter when they
change the gas in the NE. Also longer warm up times before the car goes into
closed loop at full temp.
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Old 11-21-2004, 01:00 PM   #7
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What type of seafoam and where was it put? They have top end cleaner, oil treatment, trans tune, gas additives, etc. Be more specific!
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Old 11-21-2004, 01:57 PM   #8
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It was a gas additive for cleaning fuel injectors and such. It said on the bottle it could be used for anything from lawnmowers to diesil trucks.
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Old 11-21-2004, 10:33 PM   #9
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That kind of a drop is more than winter coming on. I have never noticed more than about 2 MPG difference between summer and winter (Cadillac Northstar). You are burning unmetered fuel. Check for a leaky FPR and/or injector(*).
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Old 11-23-2004, 01:40 AM   #10
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Run the seafoam through your induction system. Use the vaccum line that goes into the break booster. Start the car, drive for about 20 mins so it is completely warmed up (this is important, don't use seafoam with a cold engine), pull the vacuum line from the booster and cover it quickly with your thumb so the car doesn't stall. (2 people will make this job easier if it does stall). Move your thumb so the vacuum line is halfway open, and VERY SLOWLY pour 1/3 can into it. This is important, if you do it fast, or if you stick the end of the hose into the can you run the risk of hydrolocking your engine. It took me about 5 mins to pour 1/3 can when I did it to my Tacoma and Bonneville. Shut off the engine. (or remove your thumb so the car stalls). I had an issue on my tacoma once when I moved my thing the engine was bucking but did not stall. I turned off the ignition and it took it about 20 seconds to stop dieseling. But there was no problem afterwards. 5 minutes later restart the car (will be more difficult). Let it run untill the smoke clears and take it for a drive. You will see a difference.
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