Horrible V6 mpg (12-15mpg avg) and rough idle w/ ac on
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  • 1 Post By ddgm
  • 1 Post By EChas3
Cadillac STS Forum - 2005 through 2012 Discussion, Horrible V6 mpg (12-15mpg avg) and rough idle w/ ac on in Past Cadillac Vehicle Discussion; Hi everyone, thank you for reading and helping out. I did search but didnt find many answers. I have a ...
  1. #1
    Tfreestyle is offline Cadillac Owners Member
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    Horrible V6 mpg (12-15mpg avg) and rough idle w/ ac on

    Hi everyone, thank you for reading and helping out. I did search but didnt find many answers. I have a 2006 Cadillac Sts v6 that is constantly getting just horrible gas mileage. Manually calculating the mpg im getting between 13-15 avg per tank. about 50/50 city highway. Best I have ever gotten was 19-20mpg on the freeway to vegas at 70-75mph, one tank there and one tank back both with the same results. Also to add I have a rough idle that is amplified when I turn on the a/c that has been going on for a long time too. I replaced the spark plugs about 2k miles ago as a fingers crossed fix for the idle, helped a bit but not that much. The car now has 98k miles. No codes.

    Im not an overly aggressive driver but not a grandma so a few mpg off id understand but this seems very excessive. Any ideas of what could be going on? I am bringing the car in for the rear diff bushing to a friends shop(not a cadillac shop) so if there is anything we can check over while there i will have access to lift for a few hours.

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  3. #2
    Wojciech is offline Cadillac Owners Member
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    Re: Horrible V6 mpg (12-15mpg avg) and rough idle w/ ac on

    If you saying it's happing when A/C is ON maybe there is problem.
    Problem with compressor? Maybe belt is not stretch enough or tensioner fail?

  4. #3
    KRSTS is offline Cadillac Owners Enthusiast
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    Re: Horrible V6 mpg (12-15mpg avg) and rough idle w/ ac on

    I average 19.8 on mine combined. You have a malfunction. Take it to a dealer for a proper diagnosis.

  5. #4
    ddgm is offline Cadillac Owners Fanatic
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    Re: Horrible V6 mpg (12-15mpg avg) and rough idle w/ ac on

    I would try a good cleaning of the throttle body. The V6s are equipped with the 3.42 rear differential which does have an effect on the mileage. However when I drove mine
    1500 miles back and forth to Florida, I was getting somewhere in the 25-27 MPG range. I was considering switching to a higher gear ratio but as I now fly to Florida, I have abandoned that approach. By the way, mine doesn't use any oil.

    Doug
    kcaughron likes this.

  6. #5
    REALTORBENJAMIN's Avatar
    REALTORBENJAMIN is offline Cadillac Owners Fanatic
    Automobile(s): 2005 Cadillac STS V8 RWD
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    Vacuum leak? Like the N* motors possibly, loose intake plenum bolts?

    Mine gets poor mileage and it's been getting worse since I got it a year ago due to the vacuum leak which I need to address. Getting the P0174 code on the reg.

  7. #6
    Tfreestyle is offline Cadillac Owners Member
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    Re: Horrible V6 mpg (12-15mpg avg) and rough idle w/ ac on

    Bringing the car in tomorrow. Going to pull the plugs and see what they look like after looking for a vacuum leak.

    It's not only when I have the ac on its just a little more amplified when on.

  8. #7
    EChas3's Avatar
    EChas3 is offline Cadillac Owners Master
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    Re: Horrible V6 mpg (12-15mpg avg) and rough idle w/ ac on

    Did you use OEM plugs?
    KRSTS likes this.

  9. #8
    Tfreestyle is offline Cadillac Owners Member
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    Re: Horrible V6 mpg (12-15mpg avg) and rough idle w/ ac on

    Oem plugs were used. Ended up being the fuel filter was causing the rough idle. Once we got a new one in there no more vibration. Time to see if that is the reason for the bad fuel economy too. Putting in another air filter as a precaution.

  10. #9
    Eldo1953's Avatar
    Eldo1953 is offline Cadillac Owners Enthusiast
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    Where is the fuel filter on these and how do you change it?

  11. #10
    dannyquest is offline Cadillac Owners Fanatic
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    Re: Horrible V6 mpg (12-15mpg avg) and rough idle w/ ac on

    Under the car on the passenger side rail. A special tool to remove. Very easy. Danny

  12. #11
    EChas3's Avatar
    EChas3 is offline Cadillac Owners Master
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    Re: Horrible V6 mpg (12-15mpg avg) and rough idle w/ ac on

    Inboard of Right Rear - at least on V8's. I believe they are all the same.

    It used to be part of recommended service but was dropped ~ '07..

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