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Thread: missfires, flat performance, etc.... MAF replaced

  1. #1
    Join Date
    Jul 2016
    Location
    adelaide
    Posts
    30

    missfires, flat performance, etc.... MAF replaced

    hi guys,

    i recently purchased a thor 4.6, which had suffered a head gasket failure (loose bolts, compression leakage under the exhaust manifold). the car came to me with the right head off.

    i rebuilt both heads and replaced the cam/lifters.

    during cam break in, it was clear it wasn't running happily - missfires, flat spots which come and go, etc.

    i put it on a code reader which returned -

    * catalyst damaging missfire multiple cylinders
    * catalyst damaging missfire cyl 7
    * missfire with low/empty fuel
    * lambda sensor upstream bank 1
    * lambda sensor upstream bank 2
    * emission relevant missfire multiple cylinders
    * emission relevant missfire cyl 5

    i cleared the codes and reset the adaptations. for about 20km the thing drove amazingly, then deteriorated again.

    as an experiment i fitted a MAF from a similar vintage mercedes, which gave a good idle but flat performance. figured the latter was down to calibration and that a new MAF would sort it.

    ordered a bosch 4.0 MAF from ebay, thinking 4.0 and 4.6 was the same. the inserts had the same part number so i swapped over the housings.

    with the new MAF the car runs as it did from the beginning. sometimes it will clear its lungs and start to pull, but a far cry compared to how it was after i reset the adaptations. seems unlikely both MAFs are faulty, but then again, it ran quite consistently with the mercedes sensor.

    temp gauge reads at the end of the blue, but diagnostic had it idling at approx 90C. does a code reader base that on the gauge side of the sensor, or EFI? which is to say, could it be a faulty temp sensor?

    i feel it runs slightly better on LPG, which to my mind rules out ignition.

    any thoughts appreciated. i was certain the new maf would sort it. that magic 20km after resetting adaptations must return!!

  2. #2
    Join Date
    Jan 1970
    Location
    Avoca Beach
    Posts
    11,186
    AFAIK the MAF doesn't do a lot in closed loop.

    The O2 sensors set the mixture and the TPS gives the ECU a load signal.

    maybe a clue is fuel low/empty fuel tank.

    The FPR is in the fuel pump assembly of a Thor. Have you checked the fuel pump, and or the pickup gauze. AFAIK there is no other fuel filter.
    Regards Philip A

  3. #3
    Join Date
    Jan 2016
    Location
    Netherlands
    Posts
    232
    Have you checked exhaust gasses (or can you?) I had a similar problem what turned out to be (largely since it is still not 100% solved) false air beeing sucked in after the MAF sensor causing all sorts of troubles. If you have O2 still in the exhaust gasses that might indicate a problem. Also check the values of the lambda sensors, if one of them is broken one bank might be running rich and the other might be running lean. Not sure what kind of tester you have but with my faultmate (on a gems admittedly) I can see the fuel pressure as measured by the car.

    Also, when I exchanged the MAF sensor I had no choice but to reset the adaptive stuff since it would'nt run properly with the new unit

    Good luck!
    Cheers,
    -P

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