V12 Rough running

    I have a slightly rough idle with a regular stumble. Ive replaced the HT leads, plugs, done a intake gasket overhaul, replaced the vacuum hoses, and dismantled and cleaned up the DK's.


    Now after I do a 'magic reset' (batteries have been disconnected for a period), the car runs better for a period of time before then resuming to its slightly rough idle.


    I note that it takes a few seconds to fire on starting from cold, and even longer when hot.


    Im thinking either PCV valves, coils, or lambdas, but would anyone be able to hazard a guess as to where to start first?


    Is there a way to check the fuel pressure regulators?


    Thanks


    Matt

    After a 'magic reset' the engine will run on default values during idle, it does this as the lambda sensors have not reached temperature and any idle mixture adaptations have to be learned. As the car runs fine initially after a reset then there must be a sensor that is skewing the adaptations due to incorrect sensing of that parameter.


    This could be the lambda's themselves or could be a MAF failing to read the air mass correctly.

    Hmm,


    I was thinking perhaps the same thing.


    Thing is that the lambdas were replaced not too long ago in the service history.



    I wonder if a removal and clean would help.


    Thanks for the help.

    I thought crankshaft sensors were pretty binary, they either work or stop working completely? I've had problems on other cars when they would stop working when they got hot then started working when the cooled down but I've not heard of them starting to fail. I've not heard of lots of things though, so if they do on our cars can you let me know as I'll probably replace mine.


    It sounds like a mixture problem to me, and if it's slight this should be fixed by the lambdas. Is the service history BMW? I had problems with pattern lambdas on my car, I replaced them again with Bosch and there was an improvement (at least in my head). You should be able to tell by looking at the connectors, pattern ones need the old connectors bodging on (at least mine did) which is pretty easy to spot. Otherwise, maybe you have another air leak downstream of the MAFs and the lambdas can't correct quickly enough, so i would go with VCVs and the other rubbery bits. If they are 20 odd years old it wouldn't hurt to replace them anyway and they are (comparatively) cheap.

    disconnect the lambda sensors and force the electronics this way to run on preset values. If anything changes you're on the right track - if not look for another route.
    Personally I think the lambda sensor's role is way overvalued. If they work or don't work doesn't make a lot of a difference. ONLY if the connections are swapped between the two banks you really really notice it because the engine runs like shit. This is at least my personal experience.

    Zitat von Matt;101935

    Thanks guys.

    Would that work for the MAF's aswell?




    I found that you can disconnect a MAF while the engine is running and it will continue to run, albeit a bit rough, but it's very hard to start the engine without the MAF connected (850cSi)

    Disconnect the coils one at a time so the car runs as a straight six. It should be able to do this smoothly but if one bank does not, you know where to start.


    Remember the coil feeds the bank nearest to it but the MAF and throttle body swaps sides.


    It could be sensing an overrich mixture and then going into limp mode.


    Only use OEM Lambda sensors.


    An exhaust gas analysis from an MOT station might help and would guide towards either a rich mixture (high CO) or a misfire (High HC).


    Are there any stored codes?


    8Tech.

    I need to borrow an OBD thing to check.. really annoying that our cars cannot do the codes on the flashing engine light!!


    Thanks for the help, I will check this wekeend...


    New problem for this morning, leaky heater hose!!:banghead:

    Zitat von NickF;102153

    Apologies if I'm dredging up an old conversation., but my M70 flashes it's check engine light, aka the Stomp Test. All it needed was the bulb adding.


    +1
    ECE cars just miss the bulb. Everything else for the CEL / stomp test is on board.

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