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Engine not holding fast idle - Failed NCT

  • 02-05-2016 9:47pm
    #1
    whizbang
    Registered Users, Registered Users 2 Posts: 1,974 ✭✭✭


    Logn story, but this has been a factor in the last few tests, sometimes just a comment, only once before was it not tested/failed becouse of it.

    Once the RPM's get over about 1500, engine speed will continue to increase to over 3000rpm's at which point even the tiniest reduction of throttle position causes the rpm's to drop to idle.

    Car is a Hyundai accent, 1.3 Fuel injected, 2001. >100K miles.
    Engine is well worn, using more than a drop of oil, but still runs well.
    Throttle body, TPS, MAF have all been changed, I havent changed either of the O2 sensors, becouse I dont believe they are bad.

    No fault codes stored persistantly, there was an occasional error relating to MAF/TPS/RPMs

    So i logged some data and plotted on the below graph.
    As you can see the increase in Throttle causes the RPM's and MAF to rise, but as the pedal is reduced the rpms continue to rise(around 106 - 146 mark)
    until they go above NCT test range.

    I had initially thought i had fixed it with a bad alternator connection causing electrical noise, but still no good.

    AccentOBD.jpg?dl=0


Comments

  • #2
    blackbox
    Registered Users, Registered Users 2 Posts: 4,703 ✭✭✭blackbox


    Did you replace MAF with a brand new one? Could there be an air leak at or after the MAF?


  • #3
    whizbang
    Registered Users, Registered Users 2 Posts: 1,974 ✭✭✭whizbang


    Its possible its the MAF, but;

    From the OBD data , the MAF tracks the RPMs almost perfectly, and looking at the fuel trims, seem to indicate the expected values are present. I might be wrong on this..

    Does anyone know how to calculate MAF in g/sec for a 1.3L at 800RPM (tickover) ??

    Air leak seems to be the main cause generally. I am convinced there is no air leak. I have also removed and blocked the PCV port, and the other crankcase vent. This seemed to have a small positive effect, but not much really.

    Google throws up a few complaints, mostly blamed on the manufacturer, with no cure. Turn on all the electrics defo helps, but not allowed according to NCT.
    Still looking...


  • #4
    whizbang
    Registered Users, Registered Users 2 Posts: 1,974 ✭✭✭whizbang


    Just to update , it was the MAF.

    The replacement one we tried earlier was also bad! Actual numbers in the good unit are much higher.
    The USA based service info gives the MAF readings in different units, and i found it impossible to convert.

    The ign timing also seemed to be too far advanced, this was the clue for a more knowledgeable bloke.


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