I just finished doing a bunch of work on my Mustang (replaced all gaskets down to head gaskets on engine, had the heads milled and a valve job done, replaced all the fuel injectors, new fuel injector, new plugs, wires, coil pack, new radiator, and new vacuum hoses).

After all this I am getting a P0172 check engine. Using a bluetooth OBDII and the torque app I checked out the fuel trims while running and noticed they were in the high negatives. On the off chance the MAF was dirty I pulled it and sprayed down the 2 resistors with some electrical contact cleaner and put it back.

Unfortunately I am still seeing LTFT values sitting at around -18 to -20 on both banks when at idle(750ish RPM at idle) if I rev it up the LTFT will steadily get closer to 0 the longer I accelerate but as soon as it steadies out or I go back to idle it goes back to -20.

I replaced The MAF last and still have -18 to -25 LTFT. I reset the ECU after putting the new one in and watched my LTFT and STFT numbers. LTFT started out at 0 as expected but STFT sat at -18 to -20 and as it got acclimated STFT dropped while LTFT got worse to now I am back to small negative STFT and high negative LTFT

Not really sure what could be the culprit at this point as the O2 sensor data looks like the ideal curve for both.

Anyone have an idea of what to check/test?

I have added some Data logs of a short trip where I let it idle in park a while, drove around at 20-45MPH for a couple mins, and then a couple WOT to 65 MPH. They are here.

  • What's the freeze frame data look like? Can you confirm MAF voltage output at idle?
    – Ben
    Oct 6, 2016 at 22:10
  • can you post a datalog that includes fuel trims, load, rpm b1/2 os2 and maf gm/s? do a idle to cruise to wot log if you can.
    – Ben
    Oct 6, 2016 at 22:18
  • I added the logs you asked for, let me know what you think.
    – OPcorki
    Oct 6, 2016 at 23:46

1 Answer 1


Cleaning the MAF won't help here because the fuel trims are negative. A dirty MAF would typically under-read mass air flow and subsequently force a positive fuel trim correction.

A few possibilities:

  • oversized fuel injectors

    If the fuel injectors are pushing through more fuel than what the fuel management is expecting, this would force a negative fuel trim to compensate for over-fueling. Given you just replaced the fuel injectors, I would suggest confirming that the injectors are indeed rated as per what the engine management is expecting.

  • excessive fuel rail pressure

    The mechanics behind why this would cause negative fuel trims is similar to the oversized fuel injectors scenario.

    Confirm if the fuel pressure regulator functions correctly.

  • over-reading MAF sensor

    It can happen. I've heard of this happening with an M5 MAF after a DIY-er doused it with brake cleaner (or was it electronics cleaner?) to "clean" the MAF. If the computer "thinks" there is more air than expected, it will inject more fuel than required, until negative fuel trims kick in and try to compensate for the situation.

    I know you've replaced this item already without success, but thought to mention it here for sake of completeness.

  • I took it to a local shop when it was running badly after changing the head gaskets and they changed the injectors. I will have to ask them tomorrow for the part number but here's a picture of them imgur.com/a/cnaDP They have orange tops and a silver bottom and are remanned.
    – OPcorki
    Oct 6, 2016 at 23:54
  • 1
    The orange top injectors are EV1 19# injectors and are commonly used in the 98 V6 Mustang. While you have the right injectors, that doesn't mean they are working correctly, though. Oct 7, 2016 at 0:11
  • Could you expound a bit on working correctly paulster2? I would have thought if some of the injectors were bad I would not see such identical fuel trim numbers on both banks?
    – OPcorki
    Oct 7, 2016 at 2:02
  • @OPcorki the body of the injector may look identical doesn't mean that it flows identically. I would double check the part numbers to make sure they are the same
    – Zaid
    Oct 7, 2016 at 16:42

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