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Thread: Rough idle

  1. #1
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    bmw 850i 1991, 635csi

    Rough idle

    I have a 1991 bmw 850i, its been off the road some years but now i am bringing it back to life. I have changed plugs, caps, rotors , fuel filters and vacuum hoses. but it has a slight misfire at idle and it will sometimes over fuel on one bank of cyclinders.
    I have been told it could be the fuel regulator, or oxygen sensor , but I am not sure I have oxygen sensors on this car---It has no cats
    Any ideas would be very welcome

    Steve

  2. #2
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    Quote Originally Posted by steve.jordan View Post
    I have a 1991 bmw 850i, its been off the road some years but now i am bringing it back to life. I have changed plugs, caps, rotors , fuel filters and vacuum hoses. but it has a slight misfire at idle and it will sometimes over fuel on one bank of cyclinders.
    I have been told it could be the fuel regulator, or oxygen sensor , but I am not sure I have oxygen sensors on this car---It has no cats
    Any ideas would be very welcome

    Steve
    Well with no 02 sensors it will run rich because there is no adaptation so preset map its running on is very rich. Start there.

    Sent from my SM-G892U using Tapatalk

  3. #3
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    The cats aren’t monitored on your car because you only have pre cat O2s on an OBD1 car so someone probably cut them out. They would have left the O2s intact unless they were retarded because there’s no reason to mess with them. I doubt that’s your issue

  4. #4
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    Quote Originally Posted by killian665 View Post
    The cats aren’t monitored on your car because you only have pre cat O2s on an OBD1 car so someone probably cut them out. They would have left the O2s intact unless they were retarded because there’s no reason to mess with them. I doubt that’s your issue
    You would be surprised, someone retarded did precisely that to my new car in my garage. Backfires like a cannon

  5. #5
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    Some previous owner removed the cats on my 850, but had enough brains to install O2 bungs on the replacement pipe. If some your car's PO removed the cats, be sure they reinstalled the O2 sensors in the replacement filler pipes.
    Mike Drives:
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    E32 - 740iA - (1) 1994
    E32 - 750iA - (1) 1990 (Not an "L") (EC German National)
    E32 - 735iL - (2) 1990
    E32 - 750iL - (1) 1990

    E38 - 740iLA - (2) (1) 1997 & (1) 1998
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  6. #6
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    It would seem my car does not have cats or oxygen sensors from new(1991 european car ) , but a code read dispays "Oxygen sensor-heating relay".
    The car seems to run fine, slight misfire every now and then at idle, but after a while it coughs and then one bank seems to pump excess fuel fumes from the exhaust ??
    This is driving me up the wall---any ideas welcome
    Steve

  7. #7
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    Why do you think the engine is over-fuelling? Does the engine stop running when that happens? Explain the fault symptoms a bit more.....
    Timm..2007 E64 650i Individual Sport..1999 E31 840ci Individual Sport..ex owner of 2000 E38 740..1999 E38 740i V8 M62..1998 E38 735i V8..1993 E32 730i V8..1988 E28 518i


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  8. #8
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    Hi
    No the engine does stop, but after a while at idle, it might hesitate and then you get an increase of fumes from one side (always the same side) and this really smells of fuel its white fumes in colour ---at the MOT it did the same and the emissions were way off--but this happens now and again at idle . Apart form this it has a very slight misfire on the same side every now and then . When I replaced the injectors these were really in a bad way some were just a single hole in the middle instead of three , all 12 were replaced with Bosch gen 3. It runs better now but is still not quite right. All the vacuum pipes to the fuel regulators have been replaced, but it migh be that I have a sticky fuel regulator or some other electrical fault
    Any thoughts
    Steve


  9. #9
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    Hi Sorry to quick to reply, this should read does not stop, it just faulters
    Steve

  10. #10
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    Those symptoms sound more like a simple HT problem - the misfires are the cause of the petrol fumes at the pipes rather than over-fuelling causing the misfires and the fumes. Always best to start with the simple things first such as the distributors, HT leads and plugs....
    Timm..2007 E64 650i Individual Sport..1999 E31 840ci Individual Sport..ex owner of 2000 E38 740..1999 E38 740i V8 M62..1998 E38 735i V8..1993 E32 730i V8..1988 E28 518i


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  11. #11
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    Hi , some more advice needed , to re-cap my 1991 BMW 850I is suffering from poor idle, but perhaps more critical is the real hesitation on acceleration, to date( and the problem does seem to come from one bank ) I have changed all the ign leads, plugs , caps, injectors and fuel filters, and these have all been BMW parts, all this has made no difference,
    I have pushed down on the inlet manifolds to try and detect a leak but no engine note changes , so I don't know if it has a leak but I know they suffer from this .
    I am really running out of ideas now, any more suggestions before I completely give up and sell the car
    Cheers
    Steve

  12. #12
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    Quote Originally Posted by steve.jordan View Post
    Hi , some more advice needed , to re-cap my 1991 BMW 850I is suffering from poor idle, but perhaps more critical is the real hesitation on acceleration, to date( and the problem does seem to come from one bank ) I have changed all the ign leads, plugs , caps, injectors and fuel filters, and these have all been BMW parts, all this has made no difference,
    I have pushed down on the inlet manifolds to try and detect a leak but no engine note changes , so I don't know if it has a leak but I know they suffer from this .
    I am really running out of ideas now, any more suggestions before I completely give up and sell the car
    Cheers
    Steve
    Do you have an eml light on?
    What spark plugs have you used?
    Have you connected diagnostics to the car? Could be one of the sensors like a cps.

    Sent from my SM-G892U using Tapatalk

  13. #13
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    I have no EML light on , the spark plugs were bosch F8LCR part number 12129061870 from BMW ,
    I have some diagnostiics read but not faults, and it does not go into limp home mode----it just have rough idle , smells of pertrol from one exhaust bank and won't accelerate
    Is the cps(is this crankshaft position sensor ) often a problem, we did disconnect and reconnect it to fit the new ign leads (to get the ign lead sensor fitted ) would this effect one bank of cyclinders.
    The problem seems to be that on pressing down hard on the accelerator, one bank hestitates to pick up so you get a lag and then it revs up and runs lumpy . Which is why I thought I have the dreaded inlet manifold leak. I have no problem doing the work, but would like to get some evidence that it is this before buying the gaskets etc
    If I need to do the job waht else should I do whilst I have it apart ---
    any ideas help thoughts or suggestions would be very welcome


  14. #14
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    Start at fuel pressure, then check the distributor caps and rotor arms.....850s eat these. You can swap MAFs and ignition components over to see if the symptom moves to the other side, be aware the right MAF is the left bank.

    If you don't have O2 sensors then you should have idle adjustment screws on your MAFs.

    What colour are the inside tips of the tail pipes?

  15. #15
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    I have changed the disributor caps and rotar arms, I have also changed the fuel pressure regulator on the the offending bank of cyclinders
    Your question on inside tips of the tail pipes is interesting , I have run the car for 20 mins and the offending side has more exhaust gas and the inside is wet but I would not say oily or water wet , it is a thin liquid and does feel and smell like fuel and leaves a sooty black mark on you hands not an oily grease mark .
    Do ignition coils partly fail---ie they either work or they don't , or can they give poor output and cause intermitant misfire or weak spark.

    I will change over the MAFS and coils one at a time to see the impact
    Thanks for the suggestions

  16. #16
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    If one is black then it would indicate its running rich, overly rich can cause misfires also. Coils can give weak output but usually fail completely.

    How about the temp sensors on the back of the engine? checked those? wiring could be bad as the looms go brittle.

  17. #17
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    Thanks . I will check

  18. #18
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  19. #19
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    Update , so I have now tracked this down to the MAF, which appears to be fine for 10-15 mins then fails and floods one bank. My next question is the MAF I have is Bosch 0280 212 025----very expensive from the dealers, can these be refurbished by experts or can you go direct to Bosch, or are there any other good makes that can be trusted
    Do we need to replace them as pairs-----I am really happy to have solved the probelm with help from this forum and will spend the money with the dealer if I have to, but thought I would check with everyomne else first

  20. #20
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    I read somewhere on this forum that Bosch changed specs on a part number, is it correct? if so, which NGK plugs & Bosch plugs are recommended for M73?

    - - - Updated - - -

    Did you try to clean it with MAF cleaner? new one is about $1K, try to get it tested used.

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