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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 05-03-2005, 04:43 PM
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Default MPG Help!

Hey what parts have mainly to do with poor mileage? I am gettign about 9 mpg it seems and that kinda sucks. I have already replaced spark plugs and wires what else could be causing this mileage. The car has about 170K on it
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Old 05-03-2005, 04:54 PM
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the O2 has a major part on milage. DO NOT use a Bosch replacement, use the AC delco
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Old 05-03-2005, 05:02 PM
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Old 05-03-2005, 10:06 PM
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Leaky FPR.
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Old 05-04-2005, 12:31 AM
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A leaky FPR will have maybe a MINOR effect on mileage, more of a serious performance issue at WOT. There'* no way that can be the problem, at least by itself.
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Old 05-04-2005, 12:42 AM
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I would definantly go with an 02 sensor as stated above. And Definantly don't go bosch......Granted I'm not having any better luck with AC DELCO'* either.

Could try running some fuel system treatment through it too.
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Old 05-04-2005, 01:04 AM
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Fuel system treatment won't affect MILEAGE. Mostly power.
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Old 05-04-2005, 09:11 AM
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Originally Posted by willwren
Fuel system treatment won't affect MILEAGE. Mostly power.
PCV Valve?
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Old 05-04-2005, 09:49 AM
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PCV shouldn't effect mileage. ????

Check the IAT, doesnt that default to like 40 degrees and run us rich if unplugged? Go with the easy free visuals first. Dirty air filter. These would both use extra fuel. The O2 is a real possibility, and usually the biggest MPG killer.
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Old 05-04-2005, 10:09 AM
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My Bosch 02 and plugs are holding up well so far. Now that the warm weather is here I'm getting 19 mpg and 27 highway. I am concerned that the Delco Ign. is going to cook the plugs.
I have found that the new cleaner formula of gas was dragging my numbers down so I am adding a fuel saving additive that works well.
It is organic and coats the molecule of the fuel to make a super clean burn guranteed to pass any emission test.
At 9 mpg you have a major fault in the fuel and air ratio.
Add fuel injector cleaner and remove the 02. If the tip is black its shot. Clean the MAF with a Q tip and alcohol.
Procede to do all the normal tune up changes (filters, pcv, etc.) and see where this takes you in mpg.
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