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 04-24-2007, 07:27 PM   #1
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I drive 90 miles a day to and from with an average speed of 55 mph with cruise set and i seem to be only getting 23 mpg. does this seem low or is it to be expected, i only have 70,000 miles on the car, it runs great and my only mods are a k&n panal filter and a dynomax mufflar
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Old 04-24-2007, 07:57 PM   #2
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Thats pretty good for that car.
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Old 04-24-2007, 08:11 PM   #3
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I actually think that'* quite low. I have 171,000 on my 99 and drive 85 miles round trip, going 80-85 and get 28-30 MPG.

How old are the spark plugs, spark plug wires and oxygen sensor?

With only 70,000 miles they're probably original, but still 8 years old. It'd be wise to freshen those up with AC Delco wires, NGK spark plugs and an AC Delco oxygen sensor. You'll see the MPGs go up.
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Old 04-24-2007, 08:35 PM   #4
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My wifes Olds (in sig) gets above 30mpg on the highway and its a boat. Which gearing do you have?
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Old 04-24-2007, 08:47 PM   #5
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SLE would have the higher ratio. 3.0X?? Same as my 99. I think it'* 3.06 or 3.08. I get the numbers mixed up...
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Old 04-24-2007, 09:10 PM   #6
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Yeah, you're way low.

I agree with vital49 -- start with the plugs. Are they all nice and tan? Or are one or more sooty from missing or too rich a mixture.

Then get into your air intake. Seriously, rodents can fill it with food and nesting in a matter of days, if left outside -- and you're in that kind of country. Even in the garage, if you keep pet food in there.

Really, you just have to get under the hood and go through things critically, if you haven't done it in a while. Look at everything -- check for loose, chewed, or broken wiring.

Then go through your regular maintenance and tune-up procedures. Also do a general search on the BC forums. There'* lots of posts and good responses regarding poor gas mileage.

Good luck !
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Old 04-24-2007, 11:25 PM   #7
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the tune up stuff is original, but the service interval for plugs isn't till 100,000 miles, but true, it is 8 years old, i'll get some tuneup stuff tommorrow, as for the plugs, NGK platnium or iridum, or does it make a difference, the air box is good, i just put a k&n filter in and it was spotless and all the electrical looks good to as i just did an LIM gaskit 5000 miles ago

it idles smooth, has plenty of power, but fuel milage just bites, and i fill up twice a week and $42-$45 a fill up...i'll try the tune up and keep you updated...thanks for all the input
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Old 04-25-2007, 01:10 AM   #8
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In our regular old Bonnevilles, I just put in the recommended AC Delco plugs or Champion equivalents. Everyone seems to agree not to put in the standard Bosch, and so I never do either. Their fancier ones may be ok, but I don't know. The only problem I ever have with plugs (except for below) is breaking a back one now and then, and breaking the boots loose from the porcelain.

By the way, I just put in the same UIM/LIMs you did in our '98. I did it because I was losing Dexacool apparently into the combustion chambers. The plugs were so tortured from the Dexacool, they actually looked worse than the text-book examples of bad plugs. Oddly, the car had been running well and mpg had not suffered yet noticeably.

Did you check your plugs at that time?

Also, I'm kind of an idiot about hooking things back up again, like hoses, and even putting the pcv valve back in. Think about anything you might have missed -- especially air leaks. Sometimes the check engine light doesn't always respond to reassembly goofs. I sure had plenty !

Oh, and make sure to get rid of the Dexacool. Put in the new replacements or the old green stuff.
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