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Rough Idle & Off-Idle Hesitation

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    #16
    Out of interest....what's the state of the C191 plug connections and wiring ?

    The round plug under the intake manifold. That plug feeds spark to the injectors, corroded wiring in C191 isn't unusual and can result in poor running if rot is causing high injector circuit resistance.

    Might be the cause of your slightly lean condition.

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      #17
      Originally posted by Z06&E30 View Post
      Any progress made on this??? Chasing something similar.
      Originally posted by Seawolf View Post
      Out of interest....what's the state of the C191 plug connections and wiring ?

      The round plug under the intake manifold. That plug feeds spark to the injectors, corroded wiring in C191 isn't unusual and can result in poor running if rot is causing high injector circuit resistance.

      Might be the cause of your slightly lean condition.
      Sorry, I totally missed these replies somehow. I haven't had a ton of time to work on this and my original smoke machine order from eBay didn't go through as planned (long story). I have another smoke machine on order from a different seller and should receive this one soon. Once I get it, I'll check for vac leaks, although I suspect I won't find anything. Famous Last words, right? :)

      I cleaned the C191 today. Not a lot of corrosion, but there was a little bit. First start from cold seemed smoother than before, but that may be placebo effect. We'll see how it does tomorrow after sitting overnight.

      Thanks for your suggestions!
      - Darrell

      2001 BMW M5
      1990 BMW 325is - Build Thread

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        #18
        Hello everybody.
        Darrel, have you solved the issue?

        I have exactly the same problem with a 1987 320i Cabrio with L-Jetronic injection (no ICV, no catalytic converter, no O2 sensor, mechanical ignition with centrifugal and vacuum operated timing on the distributor…), stable idle at 850 rpm but my engine still runs rough at idle and shakes following a non regular pattern, even after a complete overhaul of the engine (crankshaft bearings, piston rings, valve guides, valve seals, valve seats, head gasket, engine mounts, etc…) and replacement of many parts :

        new spark plugs
        new spark plug wires
        new distributor cap and rotor
        new coil
        new fuel pump
        new fuel filter
        new fuel injectors (refurbished, tested and flow matched)
        new air filter
        new timing belt and tensioner

        The roughness seems to be due to a not regular combustion that is very noticeable at idle and gets smoother at higher revs, disappearing after 1600-2000 rpm.
        Actually it is not a complete misfire, like disconnecting a single injector or a single spark plug. In that case the roughness and shaking is very strong and regular. It is more a slight combustion defect, with a random intensity and timing.
        It is weaker when engine is cold or warming, seems to be a bit worse when engine is completely warm and stopping at idle after a long trip.

        For sure it is not a fuel delivery issue related to fuel pressure regulator, pump or whatelse because the fuel pressure is OK at 2.5 bar and if I remove the fuel pump fuse while the engine is running, to stop any pressure variations due to pump and fuel pressure regulator working, it keeps shaking at exactly the same “rhythm” for some seconds, before it dies as expected.

        It is not an air/fuel ratio issue: leaning or enriching the mix through the idle CO screw or forcing the position of the AFM potentiometer doesn’t solve the problem, clearly the engine runs worse exaggerating the correction in both directions.

        It doesn’t seem to be due to a vacuum leak, the vacuum is high and stable in the intake ( -670 mbar at idle ) , a slight enrichment of the mix doesn’t smoothen the idle.

        Definitively it is not an electric / electronic related issue, after changing most parts I measured everything with an oscilloscope: primary and secondary ignition waveforms, fuel injector voltage waveform and current ramps, air flow meter control voltage, inductive trigger pickup on distributor. All waveforms show regular patterns and values as expected.

        Ignition timing is OK and stable, I checked it both with a timing light gun and then with an oscilloscope with a TDC reference and a capacitive probe on ignition wire of cylinder 1.

        I tested with a pressure transducer the pressure in both the intake manifold and in the exhaust as well.
        The intake pressure waveform associated with openings/closings of intake valves is very regular and even.
        The exhaust pressure waveform on the contrary is relly uneven, the pressure waveform varies continuously from cycle to cycle and from cylinder to cylinder, the same cylinders show a quite regular pattern on a cycle and a irregular pattern the next cycle, confirming what seems to me an irregular combustion problem.

        The exhaust gas analyzer doesn’t show any problem: CO 2,9%, lambda 0,92, CO2 13%, HC 250 ppm .

        Crancking compression is OK, 11 bar , the same on all cylinders.

        Valve clearance was adjusted two times: the first time before engine overhaul, then some time after engine overhaul because the engine was really rough.
        Strangely, both times the mechanic told me that he found very tight valves, without any clearance. Last adjustment improved things a bit, but didn’t solve the issue.

        Finally I ordered a high pressure transducer to measure running in-cylinder compression and detect any valve / cylinder leakings or cam timing issues with the scope.

        Help me, I spent a lot of time and money and I’m still at the starting point!!! This car is bewitched….

        Stefano

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