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Thread: 735iA rough idle and running very rich

  1. #1
    Join Date
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    1991 BMW 735iA

    735iA rough idle and running very rich

    Another rough idle thread... I know

    I've tried searching the forums the past months to find a solution to my problem, but it appears everyone seems to have the same symptoms but different solutions.

    I have a '91 735iA for 3 years now, and it always had a rough idle. Not a change in the rpm at idle, just a bit shaky and rough (like a oldschool V8). This would sometimes be a bit better and sometimes a bit worse. Once it's on the go, it is very smooth compared to idle, no hesitation whatsoever (well, until recently). WOT is also no problem and it still pulls strong once past 4k rpm.

    I've already changed spark plugs, distributor, lashed the valves, water temp sensor for DME, fuel filter and I can't remember if I also did the plug wires
    It would be weird if I didn't change them with the plugs and distributor....

    Anyway, it all didn't really do much for the idle, maybe it was a little bit better but it was definitely still very rough to modern standards. (and even to M30 standards)

    The last couple of weeks however a new problem showed its ugly head, it now intermittently has a overfueling issue I think. At idle it will start to misfire (sounds like it's not running on 6 cylinders), really shake and drop the idle to the point of almost stalling. Black smoke also appears from the exhaust and it smells like paint thinner....
    You can hear a strong air sucking noise when it does this with the hood open and it will do this with a cold and hot engine.
    It now does this also while driving sometimes, it will bog completely down and then suddenly pick up again. It does seem to get worse when the weather is colder and/or more moist. The fuel consumption is also way, way high.

    I'm not a mechanic, but I do have time and some basic maintenance experience. What tests can I do to exclude certain sensors or problems?

  2. #2
    Join Date
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    11/88 E32 750iL+98 E36M3
    Does the engine have a normal throttle valve or EML? Differences here https://www.bimmerforums.com/forum/s...osition-Sensor
    If no EML, did you already test the throttle position sensor?
    How to test TPS https://www.bimmerforums.com/forum/s...&highlight=TPS

    air sucking noise: check all vacuum hoses in engine bay, especially the hoses under the intake
    https://www.bimmerforums.com/forum/s...ght=M30+intake
    http://www.nmia.com/~dgnrg/hose_map.jpg
    Last edited by shogun; 12-05-2018 at 04:41 AM.
    Shogun tricks and tips for the E32 series are HERE!

  3. #3
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    1991 BMW 735iA
    It has a normal throttle valve with cable, no EML.

    I haven't tested the TPS with a voltmeter yet, will check this out.

    I suspect it does have a vacuum leak somewhere, but when I tested this with carb spray around all the vacuum hoses, this didn't gave me a clear sign of a leak somewhere. Will recheck the vacuumlines under the intake.

    Could this also be a fuel pump related issue? I hear it buzzing at the rear of the car since the beginning I had it.

  4. #4
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    well, the fuel pump must have a minimum pressure, and volume. Nominal data: The M30 fuel pump operating pressure is 3 bar, conveying capacity at 12 V 1.9 liter/minute, conveying capacity against counterpressure 875 ccm/30 seconds,
    Usually a new or good operating pump has more than 3 bar, to control the pressure for the injectors, there is the fuel pressure regulator, and that is controlled by the engine vacuum. That is why the vacuum hoses are so important. If they leak, fuel pressure cannot be controlled the proper way. Have you tested the fuel pressure already?
    Did you make a simple check of the fuel pressure regulator? Remove the vacuum hose from the fuel pressure regulator, if there is fuel inside, the FPR is damaged.

    That is a good analysis. How to verify the cause of an intermittent long crank and a start/stall condition. No need to have the electronics as shown first (because we backyard mechanics do not have them anyway), just remove the vacuum hose and sniff if there is fuel smell or even fuel coming out. Watch for fuel. Takes over a minute that fuel is coming out of the port in this vid, diaphragm cracked. https://www.youtube.com/watch?v=AKtR_yF7bi8
    Shogun tricks and tips for the E32 series are HERE!

  5. #5
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    Well, that just might have been a very easy fix...

    On the youtube video you definitely hear the sucking noise out of the vacuumline. When I tested this with my car... it did nothing. Held my finger to it... nothing.
    Just to be sure, I waited a couple of minutes for the FPR to see if gas came out, but it didn't. Does smell a little bit like gas if I put my nose on it, but this might be normal?

    The vacuumline however, was ruptured beneath the intake. So after I cut the bad piece off and with a bit of fiddling it was reattached. Will need to replace the lines anyway, but it will do for testing purpose now.

    And voila, we've got suction! Still not nearly as strong as on the video, I can't hear it. But I can feel it. Idle is smoother now, will need to make a testdrive to verify the problem is solved.

    *edit: went for a short drive, at startup it started to sputter again and spew black smoke after about 10 secs for roughly a minute or so. When I pulled away it sputtered some more and then got back to normal. It needs less throttle now, responds a bit crisper and idle is definitely smoother. Didn't have the issue of sputtering during the testdrive, it does smell a bit rich at the exhaust though. The problem was intermittent to begin with, so it might still come back.

    Something different catched my attention however, someone has been tinkering with the charcoal cannister, there's a vacuumline going to... well, nowhere. When I plug the line with my finger, the engine wants to stall. I know the charcoal cannister is for emissions purposes, but could it affect the idle as well? (must be, since it's connected to vacuum?)
    I don't have a gas smell in the interior.
    Last edited by BlackFox; 12-05-2018 at 08:26 AM.

  6. #6
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    fuel tank breather valve testing I posted yesterday, here is the link, so no need to type again https://www.bimmerforums.com/forum/s...ve-how-to-test

    Also check those hoses, otherwise thru that hose the engine also can suck uncontrolled air. http://www.realoem.com/bmw/enUS/show...diagId=13_1374
    Shogun tricks and tips for the E32 series are HERE!

  7. #7
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    1991 BMW 735iA
    So far so good, seems like the vacuumleak was indeed the problem.

    Now to tackle the other problems

    Thanks shogun!

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