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S54 idle problem... still

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  • S54 idle problem... still

    I'm determined to get this problem nailed this summer. It's not something which really affects the driving of the car as such but it's starting to annoy me now.

    When the engine speed is dropping from say >2500rpm back to idle (like for example when approaching a junction and dropping the clutch) the car almost stalls but then the ECU catches it and recovers to normal idle speed. Only takes about half a second or so and never actually stalls but I know this is not normal behaviour.

    It can also be demonstrated just by sitting the car in neutral, revving to 3000rpm or so and coming off the gas quickly. You get an undershoot of rpm but then it catches and idles normaly.

    So far I have replacd the idle control valve and the throttle potentiometer (the one on the engine rather than the one on the accelerator pedal). All to no avail.

    I mentioned it to the main dealer once when I was passing, but the chief tech there (Victoria BMW, Harrogate) said it didn't even have an idle control valve. All confidence in them even opening the bonnet evaporated in a puff of smoke at that point.

    Does anybody have any suggestions of what to try next? I was trying to find some tech data on the throttle pot on the accelerator but couldn't get anything from either cardata, RealOEM or the TIS disc???
    Last edited by spunkym; 08-05-2007, 06:31 PM.
    Black 02 reg S54 M Roadster

  • #2
    No ideas on this one anybody?
    Black 02 reg S54 M Roadster

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    • #3
      Sorry mate, no idea.

      Hope you get it sorted soon.

      David
      EX: S50 Black with Kyalami Orange/Black Leather. Becker Bluetooth/Ipod HU.
      Now: S50 Black/Black with Ap Racing brakes and Race logic

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      • #4
        Have you put the diagnostics machine on it?

        If not then that will be the 1st thing to do. If the diagnostics does not pick anything up then I'll try the ff before throwing money at it

        1. Injector cleaner
        2. clean the MAF
        3. see if you can swap the ECU , someone might have a spare somewhere.


        Greenbat

        Ex MC Owner

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        • #5
          it could be the lamda sensors but the best way to sort the problem is to hook it up to a diagnostics machine

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          • #6
            Have the diagnostics at home on my laptop - no fault codes stored and everything looks normal.

            By ff do you mean fuel filter? I've already tried injector cleaner but I will give cleaning the MAF a go this weekend, see if that helps.

            Cheers.
            Black 02 reg S54 M Roadster

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            • #7
              I have had this same problem on my S50 aloong with a rough idle. I had changed Lambda Sensors, had idle control valve cleaned, tried a different MAF and tried injector cleaner. 2 specialists later the problem has been finally diagnosed as a leak in the Brake Servo. There is a pipe going form this into the inlet manifold area.

              I have ordered a new one so should be back to normal soon.

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              • #8
                Interesting. Could be something like that - although mine does idle fine once it has settled. If the engine is pulling air in which hasn't been passed the airflow meter then the fuelling could be reduced or cut. This is probably a lack of fuel problem rather than a lack of air (i.e. idle control valve could be wide open but if airflow not registering passed AFM then no fuel will be injected).
                Black 02 reg S54 M Roadster

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                • #9
                  This was on my S54 ... replaced the MAF and all was well again with it, it gets worse ... mine got to the point where the ECU would not catch it ...
                  Stalling at junctions and the like ...

                  Way BMW found it was to remove the cable to the MAF and run the engine without (Engine warning light comes on and needs to be reset !) if the problem is less or goes away then the problem is with the MAF ...

                  Mine was changed as under BMW cover ... suppose you could clean it ..

                  Good to see some names here helping still ...
                  Ben that is two Coupes you have broken now ... one my old car and now your new beast! ...

                  I am back in UK and now turned to the dark side ... Porsche 996TT 420hp :)
                  Need help setting up a CMMS/EAMS system?
                  S54: P*** Yellow ;-)

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                  • #10
                    Brilliant - looks like the AFM clean is going to be first on the list this weekend then. If that doesn't work, a hunt around for vacuum leaks on the inlet.

                    Many thanks for the replies.
                    Black 02 reg S54 M Roadster

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                    • #11
                      AFM clean did not help - same goes for unplugging it altogether. No obvious vacuum leaks - brake servo is good and no change in idle speed squirting around with WD40.

                      I'm really stuck on this one :-(
                      Black 02 reg S54 M Roadster

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                      • #12
                        You could try calling the dealer who did mine this time last year ...

                        Sunningdale BMW, try to talk to them see if they can shed any light on your probs the guy there who worked on mine knew his stuff ..

                        mine was a 51 plate in PH Yellow .. cant think of plate on it ..
                        Need help setting up a CMMS/EAMS system?
                        S54: P*** Yellow ;-)

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                        • #13
                          Try a replacement ECU. A few companies will be prepared to give you a replacement ECU on trial.

                          Greenbat

                          PS. Can you list everything you've done so far when you have a moment

                          Ex MC Owner

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                          • #14
                            UPDATE......

                            Been having a play around this weekend to try to eliminate a few things.

                            1) First of all I removed the electrical connection from the AFM. You don't actually get an ECU warning light by doing this unless you stop the car and restart it again with it disconnected, but what this does is force the ECU to substitute 'average' values for the AFM as it has no real data to go on. I wouldn't recomend driving the car with the AFM disconnected but as I only need to rev the car in neutral then it was useful to see what difference it made. The answer - none at all! This would lead me to believe that the AFM is in fact working. I did subsequently remove the sensor itself to inspect it and it did have a thin layer of oily dirt on the 'hot film' element which I cleaned off with carb cleaner, but unsurprisingly it made no difference back in the car.

                            2) My attention then turned to a vacuum leak as sugested by Mike. First of all I removed the hose connecting the brake servo to the inlet manifold and sucked on the end of the hose to see if the brake servo was leaking air at all..... no problem there. There are a few other pipes and valves on the S54 feeding into the inlet manifold on the engine side of the throttle flaps so I disconnected the main feed here and bunged the pipe with a rubber bung. This meant that the only way air could get passed the throttle flaps was via the idle control valve which should eliminate the majority of potential vacuum leaks. Revving the car in neutral saw exactly the same behaviour - an undershoot of revs before returning to a steady idle. The problem was probably elsewhere.

                            I left the car at that point and went in for the day - feeling pretty dejected!

                            On Sunday I decided to put the laptop on the OBDII port again and have a look at the data. I had done this a few weeks before and everything looked normal. There was a fault code stored for the AFM (despite no indicator being lit on the dash) which I kind of ignored and I went on to look at the lambda probe voltages. The thing that occured to me at this point was that the car didn't really exhibit the fluctuating idle when it was cold - it was then that I realised this seemed to coincide with the changover between the lambda probes being run open loop (i.e. ignored when the engine was cold) and closed loop (after about 5 mins of the engine running at idle). It would seem only when the lambda signal was being utilised that I had the problem.

                            So this did point to a lambda sensor fault despite the car just going through the MOT with perfect emissions. I took the car out for a drive with the laptop on the passenger seat to get everything fully up to temperature and then pulled into a layby to look at the lambda voltages. What I did notice was that one of the lambda probes (the car has two sets, a couple of probes before the cat and a couple after it, along with an EGT sensor) was swinging in voltage between 0 and about 0.7V whilst the other was steady at about 0.7-0.8V. Does anybody know if this is normal?

                            I still had the self inflicted error code for the AFM stored so i decided to clear the ECU and set off home. This is when I got the surprise - as soon as I cleared the ECU fault code (which actually resets all the learned stored data) the idle problem completely disappeared!!! Reving upto 3K rpm and back to idle was rock solid, whereas before it would very nearly stall.

                            All the way back home the car felt smoother but I did notice a couple of times that the ECU put the lamda measurement into open loop due to a 'system fault'??? This did not store a fault code or seem to make any difference to the car at all but seems a little odd?

                            So, I'm left with a problem that seems to have completetly disappeared after nearly 18 months of annoying me (despite the battery being off a couple of times which I thought reset the ECU anyway) and I'm still not really sure of the culprit - or if indeed it is gone for good. It looks like the most likely culprit is one or more of the oxygen (lambda) sensors but as the car has four of them, replacing them isn't going to be cheap and perhaps is not even neccessary... unless the problem returns.



                            UPDATE- Drove to work this morning and the problem is back. Joy.
                            Black 02 reg S54 M Roadster

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                            • #15
                              OK - my current latest theory is that one or both of the pre-cat lambda sensors is dodgy.

                              I'm guessing that maybe if the sensor has gone slow or soggy, when the ECU flips from open loop measurement (ignore lambda sensor, like when revving up the engine) to closed loop measurement (observe lambda voltage, like at idle), the lambda reading is not what it should be and this is upsetting the idle temporarily. But it's just a theory!

                              Usually thouugh I would expect an emissions light to come on if any of the lambdas are dodgy? Thoughts?
                              Black 02 reg S54 M Roadster

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