16-18mpg? - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


1987-1991 Parley with regards to your 1987 to 1991 Bonneville, Olds 88 or Buick Le Sabre Please use General Chat for non-mechanical issues, and Performance and Brainstorming for improvements.

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Old 03-07-2006, 02:46 PM   #1
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Default 16-18mpg?

No matter what I do I can't get good milage with my -88 SSE.. It'* always around 16-18 mpg city/highway. This is what I have replaced so far to find improvement: Air filter, Oil and filter, spark plugs, plug wires, delco coils, themostat and a new delco oxygen sensor.
Engine runns good and the converter lockup is working. What am I missing here?
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Old 03-07-2006, 07:44 PM   #2
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Looks like you've hit the most likely stuff already. Some other ideas: Plugged catalytic converter? Tire pressure up? No dragging brakes? Leaking fuel injectors? Gentle acceleration. No WOT. And, wintertime. All of us in cold climates see reduced fuel economy in cold weather. Especially if we warm up the cars before driving and only do short trips. Just kills mileage.
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Old 03-07-2006, 07:57 PM   #3
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With cole weather the older Bonnies will not do as well as the 94+... It takes a bit to get them warm enough to operate effiecently...

That milage for cold weather is not bad... Thats all city miles right? With a 2.97 Ratio trans 17-18 would be the norm for around town driving...

I might suggest a good bottle of fuel system formula to help clean things up a bit.. But other than that it seems as if you are doing ok...

I am in Florida where its warm most of the time and I usually pull off 19-21 mpg with a 2.84 gear ratio, and 31+ on the road...

so I'd say your bonnie is not too far off the beaten path..

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Old 03-07-2006, 08:50 PM   #4
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how does the mileage of these old cars compare to a series 2 */c? The numbers seem very similar, despite the extra 80hp...
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Old 03-07-2006, 09:00 PM   #5
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Quote:
Originally Posted by phoenix_flame220
how does the mileage of these old cars compare to a series 2 */c? The numbers seem very similar, despite the extra 80hp...

supercharges only take maybe 1mpg but you have to pay for premium so it is more like 2 mpg....
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Old 03-07-2006, 09:05 PM   #6
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I mean just as a power/fuel consumption rate. Ide definately sacrifice 1-2 mpg for the extra kick.
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Old 03-08-2006, 03:36 AM   #7
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Quote:
Originally Posted by bill buttermore
Looks like you've hit the most likely stuff already. Some other ideas: Plugged catalytic converter? Tire pressure up? No dragging brakes? Leaking fuel injectors? Gentle acceleration. No WOT. And, wintertime. All of us in cold climates see reduced fuel economy in cold weather. Especially if we warm up the cars before driving and only do short trips. Just kills mileage.
Yes this is with inflated tires and gentle acceleration. I don't know about the cat, might have a problem since the car feels weak on the highway.

Quote:
That milage for cold weather is not bad... Thats all city miles right? With a 2.97 Ratio trans 17-18 would be the norm for around town driving...
This is for both city and highway. My best highway driving only is 23,5mpg. Seems kind of weak. I know some people get close to 30mpg..
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Old 03-08-2006, 08:24 AM   #8
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Quote:
Originally Posted by grandam75
Quote:
Originally Posted by bill buttermore
Looks like you've hit the most likely stuff already. Some other ideas: Plugged catalytic converter? Tire pressure up? No dragging brakes? Leaking fuel injectors? Gentle acceleration. No WOT. And, wintertime. All of us in cold climates see reduced fuel economy in cold weather. Especially if we warm up the cars before driving and only do short trips. Just kills mileage.
Yes this is with inflated tires and gentle acceleration. I don't know about the cat, might have a problem since the car feels weak on the highway.

Quote:
That milage for cold weather is not bad... Thats all city miles right? With a 2.97 Ratio trans 17-18 would be the norm for around town driving...
This is for both city and highway. My best highway driving only is 23,5mpg. Seems kind of weak. I know some people get close to 30mpg..
If thats you best milage than I have to agree you have an issue some place... With the 2.97 gearing if everything is functioning correctly we have seen as much as 31 from a 2.97 88-91 Bonneville... I would consider 27-30mpg normal for a good long highway cruise for these cars...

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Old 03-08-2006, 11:09 AM   #9
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Default MPG

What spark plugs did you install? You could switch to a hotter plug and improve MPG in colder climates.
Also the front brake caliper slides tighten up on these cars causing the front brakes to drag decreasing MPG.
A complete cleaning of the throttle body and IAC, Maf sensors along with fuel injector cleaner in the tank will help.
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Old 03-08-2006, 10:25 PM   #10
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5W-30 vs 10W-30 motor oil in the winter might be good for an additional one-half mpg. You said that you replaced the thermostat. I'll have to assume it was a 195 F degree unit. Yes? I mean if you're in Sweden, you would want all the BTU'* a heater could put out! Also, you're probably taxing the electrical system by running the heater fan on High, The headlights are ON most of the time. And you probably have the rear defroster baking the rear glass too. Still, you should be getting closer to 20mpg if you're driving mostly highway.
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