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Thread: high fuel trim long % in both banks

  1. #1
    Join Date
    Jan 2013
    Location
    Charlotte, NC
    Posts
    6
    My Cars
    2005 Bmw Z4 3.0i

    high fuel trim long % in both banks

    Hey, been reading the forums for years. So far I have been able to search and find solutions to problems I have come across thanks to the threads on this site. But not this time, so I had to make an account to ask.

    Ok, so I have a 2003 325i. It hesitates and will idle rough, acting like it has a vacuum leak. I ran a scan tool on it in generic mode and the fuel trim long % is 8.3 in bank 1, and 9.4 in bank 2. I don't know what the threshold is on these readings but It looks like there is a problem. I examined intake boots and vacuum lines and couldn't find any cracks. I ran a smoke test through the entire intake and ccv lines. I couldn't find anything. I also sprayed brake cleaner all over and still nothing. The ccv was replaced on this car acouple of months prior.

    I also have a 2005 Z4 3.0i that is butter smooth, and runs like it has no problems what so ever.
    I ran the scan tool on it to compare the readings and was surprised to see that my fuel trim was 11.7 in bank 1 and 9.8 in bank 2. I ran the same tests as before and found no leaks. Both cars have suction when removing the oil cap.

    Has anyone run into similar readings and symptoms? I don't understand how the z4 has no symptoms yet higher fuel trim %. Any help would be greatly appreciated.

  2. #2
    Join Date
    Feb 2011
    Location
    wilmington nc
    Posts
    204
    My Cars
    89 turbo e30. 86 535is
    Quote Originally Posted by neildavid View Post
    Hey, been reading the forums for years. So far I have been able to search and find solutions to problems I have come across thanks to the threads on this site. But not this time, so I had to make an account to ask.

    Ok, so I have a 2003 325i. It hesitates and will idle rough, acting like it has a vacuum leak. I ran a scan tool on it in generic mode and the fuel trim long % is 8.3 in bank 1, and 9.4 in bank 2. I don't know what the threshold is on these readings but It looks like there is a problem. I examined intake boots and vacuum lines and couldn't find any cracks. I ran a smoke test through the entire intake and ccv lines. I couldn't find anything. I also sprayed brake cleaner all over and still nothing. The ccv was replaced on this car acouple of months prior.

    I also have a 2005 Z4 3.0i that is butter smooth, and runs like it has no problems what so ever.
    I ran the scan tool on it to compare the readings and was surprised to see that my fuel trim was 11.7 in bank 1 and 9.8 in bank 2. I ran the same tests as before and found no leaks. Both cars have suction when removing the oil cap.

    Has anyone run into similar readings and symptoms? I don't understand how the z4 has no symptoms yet higher fuel trim %. Any help would be greatly appreciated.
    My understanding is you want your long term fuel trims as close to zero as possible. So that is quite baffling. I have seen a lot of pcv style hoses with quick disconnects on the bottom of the intake deteriorate and crack giving the same type of drive ability issues and not showing an leaks during a smoke test.

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