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Thread: 1994 750iL Limp Mode - your input please

  1. #1
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    1994 750iL Limp Mode - your input please

    I came across an issue this weekend and my car went into limp mode, here's what happened:

    - driving perfectly fine then parked the car for a couple of hours, when I re-started the car it was instantly in limp mode
    - engine running rough and at <1/2 power
    - I can accelerate till redline by manually shifting the trans in the first 1 & 2 gears, so I'm NOT limited to either 3000 or 4500 rpm - which can happen in limp mode sometimes
    - otherwise transmission shifts normally in Drive - so luckily it's NOT stuck in 3rd gear
    - check engine light flashes on/off intermittently while driving, but does not stay illuminated
    - strong smell of gas in the exhaust
    - occasional small backfire in the exhuast
    - RH cyl bank 1 cold (relatively) to the touch, drivers side bank 2 hot to touch
    - EML comes on for 2 secs when started
    - cannot get any codes via stomp test

    - started the car this morning after the car has been sitting for almost 24 hrs, and a big cloud of blue-ish grey smoke came out of the exhaust
    - exhaust smelt very rich of gas
    - engine still in limp mode
    - shut down immediately and Google searched symptoms for the next few hours

    At this point I'm ready to go through the motions of swapping parts across cyl banks, reading resistances of sensors, visually checking fuel/ignition components, etc....

    Before I spend the many hours to check everything = My question is that based on what I've written above, is there an specific component or system that is highly suspect, which I should focus my energy on first?

    TIA for any input/ideas!!!

  2. #2
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    I’m by no means an expert on the V12s, but with the symptoms you described, my first guess would be the O2 sensor. I think the M70 has two, one for each bank - so maybe that would explain one cylinder bank being hot and one cold.

    take that with a grain of salt because I’ve never had a failed O2 sensor myself, but I’ve researched the symptoms on an M30 to try and combat my rough idle. I remember hearing it could cause an excessively-rich mixture.

  3. #3
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    Definitely a possibility, but isn't the DME in open loop when started cold, and the o2 sensors bypassed until the engine reaches a certain temperature?

    The more I think about it I think it’s a FPR that’s defunct and stuck wide open, or most likely, a fuel injector that is stuck open.

    Why I say those is because of the big cloud of smoke at the cold startup + strong smell of gas in the exhaust. I deduce that that would only happen immediately at startup if a cylinder or cylinder bank is saturated with fuel. And when the engine was started this morning, the excess fuel was burnt off.
    Last edited by whiteghost1; 10-19-2020 at 04:00 PM. Reason: More info

  4. #4
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    Check for spark at the plugs. sounds like your getting plenty of gas.

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    Quote Originally Posted by tsmcg161 View Post
    Check for spark at the plugs. sounds like your getting plenty of gas.
    For sure, ignition system is on the list. Please bear with me here as I'm thinking this through.....

    1. If the coil or cap/rotor failed wouldn't the fuel injected in bank 1 (suspect bank) not even burn, thus I would not get the cloud of smoke at startup?
    2. If the DME in bank 1 detected a no spark condition for that bank - wouldn't the DME shut down fuel for that bank and not saturate the cyls with fuel?
    3. Or are you thinking that perhaps it's providing spark, albeit extremely weak, thus enough to make smoke but not enough to fire the cyl(s)?
    4. If the spark/ignition of a specific cyl failed, from the secondary ignition part of the system - wouldn't the engine misfire on that cyl only, and not shut down the entire bank 1?

    I'm trying to rationalize what the cause would be, so I appreciate you listening to my rambling on.
    Last edited by whiteghost1; 10-19-2020 at 04:28 PM.

  6. #6
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    Im thinking you lost a crank posistion sensor. See if you have spark on any of the plugs on the dead bank.

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    Quote Originally Posted by tsmcg161 View Post
    Im thinking you lost a crank posistion sensor. See if you have spark on any of the plugs on the dead bank.
    I thought a dead crank position sensor would cut fuel instead of spark, or am I mistaken?
    You're providing good insight, I appreciate any input so keep em coming!

  8. #8
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    There are several things that can cause this problem. The CPS is usually public enemy #1. Do you have access to a "noid" light? Its used to check injector pulse.

  9. #9
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    must be ignition related, not O2 sensor. check CPS sensor, donut for that side, ignition coil, distributor, rotor. The donut sensor is not for the ignition section of the DME, that is the crankshaft position sensor, but it is for the fuel injection section. If the inductive sensor on the plug wire fails then it goes into gang firing, since the m70 is a semi sequential design meaning that it triggers the injectors in each side in 2 banks of 3 every other revolution, when it goes into the gang fire, then it triggers all six at the same time every revolution but decreases the volume of fuel by 50% .
    Do you have a inline spark plug tester? I have a cheap HF one for less than $10. Tester connects between the spark plug and spark plug wire to troubleshoot dirty spark plug connections, defective points and bad cables or connections. Connects in seconds.
    Last edited by shogun; 10-19-2020 at 08:43 PM.
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  10. #10
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    How do you connect the spark plug tester when the spark plug has a threaded connector type? It doesn't have the screw-on tip, at least that's how it is on the M30.

  11. #11
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    maybe you insert a small bush to make the hole smaller, or solder one of these terminals to the tester https://www.kingsbornewires.com/prod...04.652.001.htm
    or use an old spark plug from M30 and connect it somehow to the tester.
    Last edited by shogun; 10-20-2020 at 01:35 AM.
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  12. #12
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    Quote Originally Posted by shogun View Post
    must be ignition related, not O2 sensor. check CPS sensor, donut for that side, ignition coil, distributor, rotor. The donut sensor is not for the ignition section of the DME, that is the crankshaft position sensor, but it is for the fuel injection section. If the inductive sensor on the plug wire fails then it goes into gang firing, since the m70 is a semi sequential design meaning that it triggers the injectors in each side in 2 banks of 3 every other revolution, when it goes into the gang fire, then it triggers all six at the same time every revolution but decreases the volume of fuel by 50% .
    Do you have a inline spark plug tester? I have a cheap HF one for less than $10. Tester connects between the spark plug and spark plug wire to troubleshoot dirty spark plug connections, defective points and bad cables or connections. Connects in seconds.
    Great input Master, here’s what I have found when focusing on diagnosing bank 1 (cyl 1-6):

    - tested for spark at the secondary side of the coil = nice strong blue spark
    - all 6x spark plugs in bank 1 were saturated with raw fuel
    - unplugged bank 2 CPS, then started the car: to test if bank 1 CPS is working = engine ran smooth for a couple seconds, then started to run rough and blow rich exhaust smoke; activated the CEL. Bank 1 CPS (OE BMW) has only a few hundred Kms on it, going under the assumption that it’s good, for the fact that the engine ran while only on bank 1 CPS’s signal
    - tried to check the ign coil and CID (cylinder identification) resistance = I discovered that my multimeter’s fuse is blown, could not take resistance measurements
    - visually checked the cap and rotor = in good physical condition

    That’s where my diagnosis stands. I’ll purchase a spark plug tester shortly to confirm that the secondary ignition on bank 1 is firing properly.

    At this point, the bank 1 CID is highly suspect as the root cause for why I’m loosing bank 1’s power and running rich. Before I purchase a replacement, is there any comments, steps to try or questions to my logic?

    I’m all ears if you have any suggestions.
    Last edited by whiteghost1; 10-20-2020 at 09:23 AM.

  13. #13
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    Quote Originally Posted by tsmcg161 View Post
    There are several things that can cause this problem. The CPS is usually public enemy #1. Do you have access to a "noid" light? Its used to check injector pulse.
    The noid light is next on my diagnostic tool list, as soon as I can get my hands on a set. Great suggestion.

    - - - Updated - - -

    Quote Originally Posted by Cactuar View Post
    How do you connect the spark plug tester when the spark plug has a threaded connector type? It doesn't have the screw-on tip, at least that's how it is on the M30.
    Not sure if this is the case with all years of M70, but my 1994 M70 wires/plugs has screw on tips. So the standard spark plug tester will work in my case.

  14. #14
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    standard spark plug tester works on M70, I have one from HF with the long extension.
    Some test data when you got your multimeter working again:
    M30/M70 ignition system data accdg to Bentley
    Coil primary, coil code # 2051118335 terminals 1 (-) and 15 (+) resistance 0.50 ohm, Coil primary, coil code # 20510171101 terminals 1 (-) and 15 (+) resistance 0.37 ohm
    Coil secondary, coil code # 2051118335 terminals 15 (+) and 4 (ctr. resistance 6.0 kohm, Coil secondary, coil code # 20510171101 terminals 15 (+) and 4 (ctr. resistance 9.0 kohm
    spark plug ends 5.0+/- 10% kohm, shielded plugs 1.0 +/- 20% kohm, spark plug wires 0 ohm (approx.), rotor 1.1 +/- 10% kohm

    M70 firing order: 1-7-5-11-3-9-6-12-2-8-4-10, crankshaft position/rpm sensor: 540+/- 10% ohm

    from workshop manual
    distributor rotor 1+/-20% kohm, angled/shielded connectors 1+/- 20% kohm, spark plug connectors 5+/- 20% kohm, cylinder identification sender coil resistance at 20 degree C (68F) <1 ohm, pulse sender/crankshaft position sensor coil resistance 540 +/- 10% ohm, temperature switch for e-box cooling E32 750: switch on at 44 +/- 3 degree C, switch off at 36 +/- 3 degree C.

    CPS and donut details https://www.bimmerforums.com/forum/s...ex-Sensors-M70
    Motronic info 1.1 and 1.3 (should be same for 1.2 and 1.7) says: When the engine is cranked or run, a speed signal from the CAS causes the ECU to earth pin 3 so that the fuel pump will run. Ignition and injection functions are also activated. All actuators (Injectors, ISCV, CFSV etc), are supplied with nbv from the main relay and the ECU completes the circuit by pulsing the relevant actuator wire to earth. https://www.bimmerforums.com/forum/s...ic-1-1-1-3-BMW
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  15. #15
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    There isn’t any reason why a CID donut sensor from an m20 or m30 wouldn’t work, is there?

  16. #16
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    Maybe the length of the cable is different? Connector same?
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  17. #17
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    Quote Originally Posted by whiteghost1 View Post

    At this point, the bank 1 CID is highly suspect as the root cause for why I’m loosing bank 1’s power and running rich. Before I purchase a replacement, is there any comments, steps to try or questions to my logic?
    Picked up a used M20b25 spark plug wire set, to test my theory, and installed #6 cyl spark plug wire from the set, which includes the CID donut sensor. Thankfully this solved my problem and I’m happily running stoich and smooth once again. Cheers to those who contributed to this thread!

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