Poor 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 03-16-2004, 06:46 AM   #1
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I'm completely stumped. My 97 LSS SC, w/80,000 miles has been getting 18 mpg, mostly highway driving. Took it to my mechanic to have the ecm scanned and he found no codes and all parameters in tolerance. I changed the tranny Fluid, plugs, PCV, fuel filter and put in a K&N but there was no improvement. The plugs that I took out btw were fine, and the engine starts, idles and accelerates fine. So I knuckled under and took it to the dealer to see if they could fined anything and the tech says everything scanned within tolerance except #4 was missing and the plug wires needed to be changed. He also said that the fuel injectors were aftermarket because the tops are white and GM injectors are black; probably higher flow injectors that the first owner put in for power gains. So I replaced the wires with Delcos and still 18 mpg. My wife was driving this car until three months ago so I don't know when the mpg deteriorated, but I recall a few trips when we first got it where I was getting 30 mpg. Any suggestions?
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Old 03-16-2004, 08:19 AM   #2
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Maybe O2 sensor. Just a guess. It may be running rich
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Old 03-16-2004, 09:15 AM   #3
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O2 checked out OK
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Old 03-16-2004, 12:20 PM   #4
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Doesn't matter, even if a scantool says it'* 'ok'. A weak O2 wont' set an SES light, but can cause a slightly rich condition within normal operating parameters when it reaches a certain age. For some reason (might be the higher operating temnps of the 3800), our O2'* only seem to last an average of 50-60k miles.

Aftermarket injectors won't affect your AFR (which must be rich for you to get crappy mileage). They still deliver the proper amount of fuel based on the IAT, MAF, and O2 measurements.

You either have a misfire or a weak O2. That'* my take on it.
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Old 03-22-2004, 12:15 PM   #5
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Here'* an update. I changed the plug wires with Delcos and drove the car for a week. MPG did not improve so I replaced the o2 sensor with a Delco. Prior to replacing the o2, I disconnected the neg battery terminal to reset the computer and reconnected it about 30 minutes later. I don't know if it was disconnected long enough because the radio presets were not erased, however the trip odometer was. The o2 that I took out was chalky white. MPG still the same. Does it take a while for the pcm to relearn before I see an improvement? Did I have it disconnected long enough? Is there a procedured as far as how I'm suppose to drive after resetting the PCM? Also, the engine revs at 2000 @ 65 mph, but I recall seeing posts where members said they rev 1800 @ 70
Please help.
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Old 03-22-2004, 12:47 PM   #6
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Pull your battery for 30 minutes to completly clear it. It'll take around 100 miles or so to re-learn the AFR correctly.

White o2 =
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Old 03-22-2004, 01:34 PM   #7
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Thanks. Once the AFR re-learns will the MPG improve? What color should the o2 have been and what does the white indicate?
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Old 03-22-2004, 01:45 PM   #8
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Your milage should increase after the AFR returns to operating setups by the computer.

Mine was pretty white, and i'm getting better milage (usually) now. Mods help with milage as well, till you put yer foot in it..
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