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    Idle problems m20b27

    I've done plenty of searching but I need some help.

    I have an 85 325e and I am having idle problems.
    Whenever I am in park or neutral my idle goes to 2k but in drive and reverse it is right around 700 rpms.

    I have cleaned my ICV (even though it wasn't too dirty) I also disconnected the ICV while running and it actually got worse so it does work, maybe it's on its way out? I dunno

    I also took apart the intake to check for any cracks in the boot but it was fine, I cleaned it and put it back on.

    I looked at the MAF and used carb cleaner on that and it seemed fine.

    I seafoamed my car about a week ago hoping it would correct the idle but it didn't. I did regain some lost power and it is a lot more responsive but still has that funky idle.

    I looked at all my vacuum lines and they all look fine, no worn lines or holes ive noticed. The only line I didn't take off is the one from the valve cover to the throttle body.

    I am stumped here, if anyone could help me I'd much appreciate it. Thanks in advance.

    #2
    Idle or hard starting problems are most commonly caused by intake leaks and/or
    a sticky or defective Idle Control Valve (ICV). The only reliable method of
    locating intake leaks is to have a smoke test run on the intake and crank case
    and to test the brake booster with a gage and vacuum pump. The complete list
    of possible causes of an intake leak is:

    Intake boot
    Throttle body gasket
    ICV hoses & connections
    Brake booster, hoses, and connections
    Crank case breather hose
    Evaporative control hoses, valve, and expansion tank
    Fuel pressure regulator & hose
    Injector seals
    Valve cover gaskets & bungs
    Oil filler cap
    Dip stick o-rings
    Oil return tube o-rings
    Pann gasket or oil level sensor gasket

    While leaks in some of those can be found by inspection or by spraying carb
    cleaner on suspect areas, not finding leaks that way doesn't eliminate the
    possibility. Only a smoke test will really work.

    Once the possibility of intake leaks is eliminated, the ICV needs to be
    removed and cleaned with carb cleaner until the vane inside moves freely. When
    the ignition is switched on you should be able to feel vibration from the
    ICV. If no vibration the ICV is bad, there's a problem with its wiring or
    connector, there's a problem with the TPS, or the DME (or Idle Control Module
    (ICM) on an ETA car) is faulty.

    For the DME (or ICM) to control idle, the idle switch in the TPS must work
    correctly. The switch should close when the throttle stop is 0.030-0.060" off
    the idle stop screw. Hearing a click doesn't mean the TPS is working. Check
    with a meter or test light.

    The fuel system should be tested via the suite of tests in the Bentley manual
    as invalid rail pressure can be a contributor to idle and starting problems. A
    simple injector check is to pull the injectors, jumper the fuel relay to run
    the pump, and see if the injectors are leaking. You can also point the
    injectors into a towel, remove the coil wire, and crank the engine to see if
    all of the injectors appear to be spraying in a similar fashion. The best
    approach to possible injector problems is to have the injectors cleaned and
    flow tested. Since raw fuel can or will be released in these tests, have a
    fire extinguisher handy.

    While a bad check valve in the high pressure pump can result in longer than
    normal cranking, if the fuel system is working as it is supposed to the rail
    will reach normal pressure in a few turns of the engine. A weak pump, clogged
    filter or leaking FPR in conjunction with a failed check valve can result in
    longer cranking and/or idle poroblems.

    The O2 sensor can be a contributor to idle problems. The O2 sensor is a
    scheduled maintenance item with a useful life of no more than 100k. If the
    sensor has that mileage or more (or is of unknown age), replace it.

    The AFM can be a contributor. If the vane doesn't move freely or the
    resistance track is worn the DME may be receiving invalid data from the
    AFM. And if someone has fiddled with the bypass air adjustment the DME may be
    unable to stabilize idle. The bypass air adjustment should only be adjusted
    per the procedure in the Bentley and with an exhaust gas analyzer. And even
    then everything else associated with engine management has to first be
    operating properly. If the AFM becomes a suspect, replacement with a good used
    unit is the best approach.

    Improperly adjusted or malfunctioning valves will affect idle and starting. As
    can compression issues from ring or cylinder wear. A valve adjustment is
    called for every 15k. A useful diagnostic is to run compression (dry and wet)
    and leak down tests on the engine. Aged ignition wires, plugs, distributor
    cap, or rotor can cause problems. Insulation does break down with time and
    heat. And since the youngest E30 is going on 20 years old, if the ignition
    system is original or the plugs are old replacement is indicated.

    Although not usually a problem, a bad DME temp sensor is a possibility. That
    generally won't cause an unstable idle, but can cause hard cold or hard hot
    starts and/or a rough idle. As can problems with the timing reference
    sensors. Although not commonly encountered, a bad harmonic balancer on an
    M20B25 or M30 engine will cause problems.

    When all other possibilities have been eliminated and idle or starting
    problems persist, replacement of the DME, or if applicable the ICM, is
    indicated.

    In many cases the cause of problems will be a combination of factors. So it is
    important to test and repair all of the possible causes.
    The car makes it possible, but the driver makes it happen.
    Jim Levie, Huntsville, AL

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