2000 Bonneville SLE getting terrible gas mileage - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 05-15-2011, 11:17 PM   #1
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Default 2000 Bonneville SLE getting terrible gas mileage

im not sure how long ive had the problem but i recently noticed im getting 160 miles to a full tank. its all city miles but my screen says 18.5 mpg avg. its doesnt add up. im thinking i need plugs and o2 sensors, any other advice before i put money into it? thanks in advance
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Old 05-15-2011, 11:19 PM   #2
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Start with the basics.Air filter spark plugs spark plug wires coils o2 sensors..By start with i mean inspect and replace if needed Do you have a check engine light on?
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Old 05-15-2011, 11:25 PM   #3
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If your computer is telling you that your getting 18.5mpg, but the math don't add up, I would start looking for a leak. Check the top side of your injectors, check where the fuel line connects to the fuel rail. Check where the filter is connected, check over the tank real close. Perhaps a locking gas cap might be a good idea. You wouldn't be the first person to have someone siphon gas out of a tank.
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Old 05-15-2011, 11:42 PM   #4
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Thanks for the fast reply, ill check into those things
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Old 05-15-2011, 11:43 PM   #5
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and no CEL sseidriver97
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Old 05-15-2011, 11:44 PM   #6
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Alright definatley check over the basics including looking for a gas leak and if all checks out we can go from there
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Old 05-15-2011, 11:58 PM   #7
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We'll also assume that you're driving like a grandma, and not doing burnouts, drag racing other Bonnevilles, and the like.

I wouldn't count out people stealing gas, but if it'* continuously bad mileage, you can probably assume that'* not it (unless one of your co-workers is always popping breath mints at the same time every day).

As far as tune-up parts:
  • Spark plugs are rated at between 30,000 miles (coppers) and 100,000 miles (other metals).
  • Spark plug wires are generally rated for 30,000 miles: check the box.
  • O2 sensors will last forever, but lose effectiveness after 60,000 miles. (I got an extra 40 miles per tank by replacing my upstream one.)
  • PCV valves are cheap enough to replace, but as long as they work (rattle), that'* fine.

Also, check your tire pressure and alignment. Refill and realign as needed.
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Old 05-16-2011, 12:27 AM   #8
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As far as I know most GM fuel tanks after 1995 have a ball in the connector where your filler neck connects to the tank that would stop any gas theft. The reason for the ball is in the case of a roll over the fuel would not flow out.
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Old 05-22-2011, 12:09 PM   #9
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I took the car into tuffys for a free inspection and they didnt find anything wrong. I got some MAF sensor cleaner(worked on the crown vic) and throttle body cleaner. While cleaning them i got oil coming out of the throttle body, lots of oil. I checked the dipstick and im 3 quarts low. I just had an oil change. and i was low before the last one too. Any ideas on the problem now?
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Old 05-22-2011, 06:41 PM   #10
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could possibly be blow by..if thats the case its time for a rebuild sorry to say. if you're really paranoid try an compression and leak down test. That should reveal if you've do have piston ring problems..

But but usually if getting oil into intake it means that your pcv isnt functionind properly,So i wou;d start with checking it to make sure its in working order and if not replace it before you go teraing into your motor
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