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Cavalier poor mpg

Tiny
BAT KID
  • MEMBER
  • 2000 CHEVROLET CAVALIER
  • 4 CYL
  • FWD
  • AUTOMATIC
  • 55,500 MILES
Hi, I have a 2000 chevy cavalier and it all of a sudden started getting horrible gas mileage. I was getting 26mpg in the city. Now I have getting 17mpg highway and lesser than that in the city. I've checked for fuel and vacuum leaks, the spark plugs and wires, air filter, tire pressure and have found nothing wrong. The car runs great and doesnt smell like gas or idle rough. The car runs perfectly normal like it always did but before winter started it suddenly dropped. I have a flowmaster delta 40 muffler on it but I took my calculation of 26mpg after it was installed. I understand the winter takes away some fuel mileage but by how much I lost I think something else could be wrong with it. Can you help?
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Monday, February 14th, 2011 AT 9:02 PM

29 Replies

Tiny
JDL
  • MECHANIC
  • 16,098 POSTS
I'd use a gage and check fuel pressure. Also, check that the pressure doesn't drop like a rock when pump stops. If it drops quickly, check for leaking injector or pressure regulator.
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Monday, February 14th, 2011 AT 9:37 PM
Tiny
BAT KID
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  • 40 POSTS
Ok thanks. I go to college for auto and I will to a test tomorrow bu I was wondering how to check the fuel pressure because cavalier don't have test ports for a gauge. Do I have to T into the fuel line? If so how
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Monday, February 14th, 2011 AT 11:29 PM
Tiny
JDL
  • MECHANIC
  • 16,098 POSTS
There is kit that modifies the fuel pressure gage with quick connect fittings, so that it will work with your vehicle. I didn't realize till just now, that you don't have a schrader valve. Yes, you have to tee into the supply line.
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Tuesday, February 15th, 2011 AT 3:31 PM
Tiny
BAT KID
  • MEMBER
  • 40 POSTS
Ok I tested the fuel pressure and its fine. I also looked for gas smells or leaks by the fuel injectors and there's nothing. I check my spark plugs again and I noticed they are turning white. What could be wrong?
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Wednesday, February 16th, 2011 AT 6:58 PM
Tiny
JDL
  • MECHANIC
  • 16,098 POSTS
Any applicable trouble codes? If you check codes yourself, at the least, you would need an obd2 code reader. Some of the national brand autostores used to check codes for free. Next, I'd want to see if system is running rich or lean and why?
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Wednesday, February 16th, 2011 AT 7:07 PM
Tiny
BAT KID
  • MEMBER
  • 40 POSTS
I used a scan tool in my auto class and nothing comes up for anything but for the BCM code u1000 pops up. Im just getting a little confused because the car is getting bad mpg, runs perfectly normal but the signs of the spark plugs are showing that it's running lean
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Wednesday, February 16th, 2011 AT 10:15 PM
Tiny
JDL
  • MECHANIC
  • 16,098 POSTS
What are the fuel trims doing, adding or subtracting? Do a compression test. I can't think of anything else, without looking at it.
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Thursday, February 17th, 2011 AT 4:52 PM
Tiny
BAT KID
  • MEMBER
  • 40 POSTS
Ok I did a compression test and one on my cylinders had only 70 lbs of compression
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Thursday, February 17th, 2011 AT 8:36 PM
Tiny
JDL
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The lowest cylinder reading should not be less than 70 percent of the highest. 100 psi minimum. Could rings or bad valves causing low compression.
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Thursday, February 17th, 2011 AT 9:00 PM
Tiny
BAT KID
  • MEMBER
  • 40 POSTS
Ok do you think this could be the problem with my gas mileage? Also I noticed the head is covered in oil. And if the spark plugs are white, does that mean the car is running lean? If so would there be a problem with the o2 sensor?
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Thursday, February 17th, 2011 AT 9:05 PM
Tiny
JDL
  • MECHANIC
  • 16,098 POSTS
If all cylinders aren't producing like they should, yes it will effect gas mileage. In closed loop the 02 sensor tells the computer what's going on with the exhaust. If the computer thinks the system is running rich, it will lean the system, that's why I asked you about the fuel trims.

Does the engine use oil?
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Thursday, February 17th, 2011 AT 9:52 PM
Tiny
BAT KID
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  • 40 POSTS
Yes
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Thursday, February 17th, 2011 AT 10:47 PM
Tiny
JDL
  • MECHANIC
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Maybe the engine needs an overhaul?
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Friday, February 18th, 2011 AT 4:26 PM
Tiny
BAT KID
  • MEMBER
  • 40 POSTS
Oh god I hope not. I redid the compression test but I accidentally stripped out the hole for the 4th cylinder but I got reading for the first 3 cylinders dry and wet. Cylinder #1 was 105,125. #2 was 115,155. #3 was 125, 145. Is that bad?
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Saturday, February 19th, 2011 AT 2:25 PM
Tiny
JDL
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Doesn't sound as bad as it could be. Do you use more than a quart of oil every 1K miles?
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Saturday, February 19th, 2011 AT 5:11 PM
Tiny
BAT KID
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  • 40 POSTS
I put four quarts in every oil change which is every 3000 miles. If I ever run low its usually less than a quart
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Sunday, February 20th, 2011 AT 12:10 AM
Tiny
JDL
  • MECHANIC
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That doesn't sound bad. Your engine may give you many more miles of service? Whether or not that is efficient service, I don't know?
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Sunday, February 20th, 2011 AT 4:03 PM
Tiny
BOBINEHT
  • MEMBER
  • 1 POST
  • 1998 CHEVROLET CAVALIER
  • 4 CYL
  • FWD
  • AUTOMATIC
  • 110,000 MILES
I did a tune up last week on my Cavalier to hopefully fix the problem but it didn't make a difference. I replaced wires, plugs, filters and cleaned injectors best I could. The gas millage went from say 25mpg to 10! It also is hesitating real bad if I try to push the pedal down during driving. If I let off the gas pedal and slowly press it back down it will pickup and run great again. Can anyone tell me why I would lose so much gas now along with engine hesitating the way it is. It doesn't stall or at least hasn't yet.
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Friday, January 25th, 2019 AT 6:59 PM (Merged)
Tiny
JAYBRIDGES
  • MEMBER
  • 1 POST
  • 2000 CHEVROLET CAVALIER
  • 141,361 MILES
Im going to do this in KMs due to me being in canada, but Ive got a 2000 cavalier, I get about 425kms to a tank or 264.082 to a full tank, ive had it put on to a computer that reads codes, firing of the plugs and wires and everything, no codes came up or anything, even read both of the o2 sensors and came up with the right amount of flow. What could it be?
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Friday, January 25th, 2019 AT 6:59 PM (Merged)
Tiny
RASMATAZ
  • MECHANIC
  • 75,992 POSTS
Could be needing a major tune-up, tires, wheel alignment, suspension, brake and transmission problems-running too rich of a A/F mxture-etc
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Friday, January 25th, 2019 AT 6:59 PM (Merged)

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