Really bad gas mileage - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 02-11-2003, 01:08 PM   #1
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Default Really bad gas mileage

Have any of you encountered a sudden drop in gas mileage before? My car used to get around 20-22 mpg in the city, but now it'* getting around 15. The change wasn't even a gradual one. One tank I got about 20, then down to 15. Any ideas about what could be wrong?
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Old 02-11-2003, 02:32 PM   #2
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well, it might be your little LEAD FOOT! ok, thats jus my problem, but could be yours... could be spark plugs, or better yet, FUEL INJECTORS!
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Old 02-11-2003, 03:07 PM   #3
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I'll back Jr up again 100%. A sudden drop in efficiency without a noticeable power loss is almost certainly an O2 sensor. Up in Techinfo you'll see a list of part numbers and prices.

But just save yourself a headache later, like Jr suggested, and do a full tuneup.
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Old 02-11-2003, 09:31 PM   #4
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Heh. My leadfoot can't account for all of that.

Now that I think about it, I haven't had this one tuned up yet. I got it a few months ago and just forgot. I think I'll do that before too long. Thanks.
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Old 02-11-2003, 09:33 PM   #5
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My O2 sensor is on its way from gmpartsdirect. Saved at least $40 for parts. Sorry to ask again, but can someone run through the O2 replacement process, I can't find a manual for my car anywhere! Thanks a lot.
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Old 02-11-2003, 09:42 PM   #6
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15 mpg has been normal for me since I got the car new.
Around town driving....lead foot....short runs.

It gets a lot better on highway runs...25 mpg.
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Old 02-11-2003, 10:20 PM   #7
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To change the O2 sensor:

Look behind the rear spark plugs. You'll see a spark plug looking thingy sticking up out of your rear manifold that looks suspiciously like the one you just got from gmpartsdirect.

Unplug the harness. Use a crescent wrench or Metric (is it 19mm) combination wrench to remove the old one. Righty tighty lefty loosy.

Put the new one in, but don't touch the element end. Tighten it SNUGLY and Firmly, but don't move the car with your tightening.....

Hook up the harness. If you haven't burned yourself by now, it'* because you were smart and let the car cool down first. You're done.
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Old 02-12-2003, 01:12 AM   #8
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Does my car have only 1 O2 sensor, and is it usally the one behind the manifold.TY.
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Old 02-12-2003, 01:42 AM   #9
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Series 2 is OBD 2, so you should have 2. The one behind the cat only sniffs to make sure the cat is doing it'* job. For effieciency problems, just change the one in the rear manifold.
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Old 02-12-2003, 12:13 PM   #10
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Shhhhhhhhhhhhheeeeeeeeez my milage has drooped too...........hmmmmmmmmmmm wonder if its cuz i now have 355bhp.. :P one day when i fill up i`ll try to drive the speed limit ( i said i`ll try ) and see what i can pull for mpg.i just did a trip to Vancouver from Kelowna and only got 600klms from my tank.well i was avg. around 160kph + ( 100mph) with bursts` up to 220kph ( 130mph+) gas milage suck`d on that trip.....................but i will do a normal driving routine to get a good reading...but it sure is hard
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