Bad HWY 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 01-30-2005, 09:35 PM   #1
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Default Bad HWY gas mileage

I am getting the same gas mileage on the hwy as in the city, about 17 mpg. I have changed the spark plugs, to 1 step colder (recently). It has a newly oiled air filter. Tires all have good pressure. What could be causing such a drasticly low hwy MPG? I go to back and forth to NIU (dekalb) a lot, and it costs me much more than it should.
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Old 01-30-2005, 09:37 PM   #2
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Bad O2, a little air filter oil on MAF, bad FPR...?
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Old 01-30-2005, 09:39 PM   #3
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What RPM'* are you running at 55-60 mph?
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Old 01-30-2005, 09:40 PM   #4
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Check your fuel filter.
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Old 01-30-2005, 09:41 PM   #5
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A fuel filter won't cause poor mileage. Starving for fuel cuts power, but not efficiency.
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Old 01-30-2005, 09:44 PM   #6
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I forgot, the fuel filter is recently changed too (1 month ago). I think there may be a problem with the injectors, on random occasions, the car refuses to start, until i give it some gas. Performance seems to be fine (acceleration, that kind of thing). But the starting problems happen like 1 out of 10 times.
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Old 01-30-2005, 09:46 PM   #7
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Willwren, I was thinking that it could be my torque converter too. I will check the rpms tommorow morning, and post them. I believe they are fairly low, like around 2000-2500. Also I generally do about 70 mph for about 70 miles each way.
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Old 01-30-2005, 09:54 PM   #8
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Ok. Check to be sure. Need to get a scantool on that thing and find out how many rpm'* of slip you have in TCC lockup, also. If it'* too high (above 10 or 15 rpms), it can kill mileage.

Other than that, I'd think O2 sensor as the likely culprit, assuming you KNOW you don't have a bad injector or an intermittent ignition miss.
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Old 01-31-2005, 12:23 AM   #9
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Hmm, I will see if i can get some of my local Grand prix club members to scan it for me. I am thinking that my gaps might be slightly off (.55-.57) but i do not think that will make a huge difference. I can change my spark plug wires. But I think I need to scan it first.
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Old 01-31-2005, 12:24 AM   #10
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With a 3.4" pulley, you should be at least one heatrange colder, and gapped at .055".
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