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Thread: Diagnosing cause of 1996 Ford Mustang GT - P1151

  1. #1
    Carb and Points!
    Join Date
    Jun 2015

    Diagnosing cause of 1996 Ford Mustang GT - P1151

    Hello everyone. New to the forum.

    I have a 1996 Mustang GT that had stored 2 DTCs. P0151 and P1151. I replaced the Bank 2 forward O2 Sensor and the P0151 code is no longer being set. However, the PCM still sets the P1151. Additionally, the Autotap PCM interface I use shows that the last time the P1151 was set I was going approx 50mph and the STFT was a -77%.

    My interpretation of that is an over rich condition and the PCM is in a state of asking for much less fuel from the Bank 2 injectors. AM I'm correct? If so at least one injector is likely leaky. How do I figure out which injector?

    All comments welcome. I'm not all that experienced with EFI either, so my interpretation may be incorrect. I can accept that..:-)

  2. #2
    Carb and Points!
    Join Date
    Jun 2015
    I run Autotap real time data option this morning and it expresses short term fuel trim (STFT) as a percentage rather than a change in voltage as is discussed in some EFI articles and/or blogs.

    What I saw was Bank 2 (drivers side on a Ford) the STFT was swinging further in each direction zero % to a greater degree than Bank 1 was.

    What is your opinion of what this is indicative of?


  3. #3
    STFTs in Fords aren't the same as GM. In GM computers, STFTs are the short term fuel corrections in %, while on Fords they are that and are also the commanded lambda, called lambse.
    Think of lambse as a commanded lambda which includes the fuel correction from the hegos (O2). The KAMRF's are closer to what STFTs are in GM jargon.

    I think P1151 in Ford is for a lean condition which can't be corrected adding fuel.
    Without going into much detail, you can see if the afr is rich or lean watching the O2 volts. If they go above 600 for long periods of time, it means you have a rich condition. If the volts peg below 300 then it's a lean condition.
    Swap O2 sensors from side to side and retest. If you get the same results, swap injectors from bank to bank. If the problem follows the injectors there is a problem with one of them (or more). If the problem remains in the same bank, check your plugs, coils, cables and test for vacuum leaks.

  4. #4
    Carb and Points!
    Join Date
    Jun 2015
    Thanks for the reply Montecarlodrag. Swapping sides is a good idea to see if the problem travels with the hardware. Good advice and technique.

  5. #5
    Carb and Points!
    Join Date
    Jun 2015
    Turns out however, that on this Mustang, I cannot swap O2 sensors side to side. The 2 sides have connectors that are keyed differently and won't allow swapping. I may still try swapping the injectors side to side if nothing else cures the issue.

    I noticed the last time the P1151 was set, it occurred after a barely detectable stumble in the engine after shifting into 3rd gear under light throttle.
    Last edited by tdf46; 06-25-2015 at 06:17 AM.

  6. #6
    A wideband also helps a lot

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