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Thread: Electrical issue causing occasional misfire. Help!

  1. #1
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    Electrical issue causing occasional misfire. Help!

    I've been trying to find the cause of an intermittent misfire for some time. I have replaced the entire ignition system including cables, plugs, caps, rotors and coils. There are no vac leaks, fuel pumps are both in good order and the injectors have been recently cleaned and serviced.

    The misfire seemed to come on randomly and once it had started it would rarely go away until the car had been left to sit for some time (sometimes a day or two). I've been logging the specific conditions when it occurs to try and find a pattern but until this weekend had not discovered anything consistent. However, this weekend the car was running beautifully until I stopped at a wash station to hose it down. After I'd cleaned the car the misfire began. Also, strangely a couple of other electrical gremlins occurred. Namely, the a/c controls took a few seconds to light up and activate after I turned the fan on and the mirror dip when reversing didn't kick in for about 10 seconds.

    These functions all returned to normal after a few minutes and after another 30 mins of driving home the misfire had become more infrequent. It seems pretty obvious that water is getting in somewhere and, although the car is under cover it is not garaged so moisture in the air could get in causing it on other occasions. I hadn't noticed it occurring only in rain previously but it's possible I ran the screen wash cycle. Now I just need to figure out where the leak is. The fuse box appears dry but seems an obvious choice is the lid isn't watertight and the blower relay is next to the ignition relay, I think.

    I guess it could also be a short in the main loom which is a much more worrying prospect.

    Anyway, long and the short of it is that I would love some advice suggestions of the best way to sort this.

    PS Can't scan the car. I think the LKM is faulty but haven't had a chance to unplug when I've got scanning equipment available to test.

  2. #2
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    Check the fusible links on right-hand positive battery cables. My engine would not start and Scott directed me to the links and I found one burned out, but they can also crack and cause erratic running issues. I bought a package of links at Adanced Auto for about $5, replaced them, and engine started right up.

    Have you replaced the capacitors in the General Module?

  3. #3
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    fwiw, The car will run with the GM removed from the car, Those hidden fuses do cause a bad mis or cut the engine off when they are cracked, Happened to me several times till i found the culprit.

  4. #4
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    Quote Originally Posted by 8eights View Post
    fwiw, The car will run with the GM removed from the car, Those hidden fuses do cause a bad mis or cut the engine off when they are cracked, Happened to me several times till i found the culprit.
    The fuses under the glove box where the GM is located? I can't remember which functions those fuses are for but will take a look.

    Quote Originally Posted by JDP530 View Post
    Check the fusible links on right-hand positive battery cables. My engine would not start and Scott directed me to the links and I found one burned out, but they can also crack and cause erratic running issues. I bought a package of links at Adanced Auto for about $5, replaced them, and engine started right up.

    Have you replaced the capacitors in the General Module?
    Yep, capacitors replaced last year. Have you got a part no for those fusible links. Been so focused under the hood that I hadn't considered water ingress in the boot - probably more likely now I come to think of it!!

  5. #5
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    There are two fusible links, one on each of two red cables coming off right-hand positive battery terminal. They are shrink-wrapped to cables so you will have to cut away the insulation to expose the fuses, well, that's how they were on my '91. And, there is a fuse box mounted to wheel well. Here is the fuse I replaced:
    http://www.realoem.com/bmw/enUS/show...diagId=61_0917

  6. #6
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    Quote Originally Posted by JDP530 View Post
    There are two fusible links, one on each of two red cables coming off right-hand positive battery terminal. They are shrink-wrapped to cables so you will have to cut away the insulation to expose the fuses, well, that's how they were on my '91. And, there is a fuse box mounted to wheel well. Here is the fuse I replaced:
    http://www.realoem.com/bmw/enUS/show...diagId=61_0917
    Thanks, found the one in the little fusebox but didn't notice this other one. I'll take a look tonight and test the fuses.

  7. #7
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    Here are photos of one fusible link under shrink wrap on positive cable close to battery:

    IMG_20180108_154012268.jpg
    IMG_20180108_154436826.jpg

  8. #8
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    Quote Originally Posted by JDP530 View Post
    Here are photos of one fusible link under shrink wrap on positive cable close to battery:

    IMG_20180108_154012268.jpg
    IMG_20180108_154436826.jpg
    Thanks. That's very helpful. Haven't been able to test with a multimeter yet but neither of mine look in good nick. Car has done approx. 90K miles so they are probably due for replacement anyway.

  9. #9
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    I was in a hurry to fire up my car since I had never driven it and had been working on it for some months! So I drilled out rivets just enough to run in screws with locknuts to hold in new link. You could use a audio fuse holder to replace the oem part.

  10. #10
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    Quote Originally Posted by JDP530 View Post
    Here are photos of one fusible link under shrink wrap on positive cable close to battery:

    IMG_20180108_154012268.jpg
    IMG_20180108_154436826.jpg
    Had a good look at mine last night and it seems a previous owner has 'modified' at least one of the fusible links. The little box appears to contain the large 80A fuse. And, after unwrapping a few layers of electrical tape, I found that the other smaller 50A fuse has been removed altogether.







    Not convinced the missing fuse is the root of the issues as it ran fine for a 2-3 years before I even noticed this but it have impacted other electrical systems that are subsequently causing the fault?

    There's also this other shrink wrapped connection from the +ve terminal right next to the batter connection.


  11. #11
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    Haven't yet replaced fusible links. Decided to buy OEM ones that came with a 3 week delivery time ex-Germany (gotta love living in New Zealand!). I have noticed something very odd.... The misfire fault still comes and goes, seemingly at random. Yesterday, I drove the car in the morning and it ran beautifully. A couple of hours later I went for another drive to the beach and she run rough from the get go. The weird thing is that on that second drive the aircon stopped blowing cold. The blower still worked as normal but no cold air. I have replaced the air con recently and had it tested just a few weeks ago so it is not a coincidental fault. So my question to the electrical buffs - where is the likely fault that would cause a misfire and cut off the AC compressor? I'm assuming DME rather than multiple relays.

  12. #12
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    Quote Originally Posted by jeffbebe View Post
    Haven't yet replaced fusible links. Decided to buy OEM ones that came with a 3 week delivery time ex-Germany (gotta love living in New Zealand!). I have noticed something very odd.... The misfire fault still comes and goes, seemingly at random. Yesterday, I drove the car in the morning and it ran beautifully. A couple of hours later I went for another drive to the beach and she run rough from the get go. The weird thing is that on that second drive the aircon stopped blowing cold. The blower still worked as normal but no cold air. I have replaced the air con recently and had it tested just a few weeks ago so it is not a coincidental fault. So my question to the electrical buffs - where is the likely fault that would cause a misfire and cut off the AC compressor? I'm assuming DME rather than multiple relays.
    Get that 50A fuse replaced ASAP! That's the kind of shit that PO's do to cars that start fires! If it blows, you're on to something. The other one doesn't look great either. Car Audio stuff works fine. There's a difference between 50A/80A in different model years, 1991 should be 50A IIRC.

    One of those links powers the e-box (the box with the 2x DME, EML and several relays). You might have a bad EML or DME, but that's rare.. Multimeters are not terribly useful when troubleshooting intermittent things (like measuring fusible links), but you can measure the + post in the ebox while someone flexes the cables.

    I think there's a ground cable from the chassis to the engine.. double check that.

    In my car, the connector to the fuel pumps under the seat was corroded and acting intermittent.

    You replaced your ignition system.. make sure you didn't use 'fancy' plugs. the originals are long gone, the new part number ones aren't great, NGK are the best ones for the 850i with standard ignition. Some folks (myself included) have had all sorts of problems with the cylinder ID probes on aftermarket wiresets.. Just sayin... If your ignition system replacement made things worse, suspect both of those (plugs/wires).

    Other seemingly random causes of electrical disturbances.. a shorted cell in the battery could do funny things, but it would be obvious within a day or two. Might have a bad alternator or alternator regulator/brushes, and the system voltage may be dropping (though this would be accompanied by a dummy light).

    Cheers!

    '89 735i, '91 850i, '81 MB 380SLC (For Sale), Tesla Model 3, and VW Passat TDI -- Yes, I still repair General Modules, DM for info!

  13. #13
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    Quote Originally Posted by EEDegreeToDrive View Post
    Get that 50A fuse replaced ASAP! That's the kind of shit that PO's do to cars that start fires! If it blows, you're on to something. The other one doesn't look great either. Car Audio stuff works fine. There's a difference between 50A/80A in different model years, 1991 should be 50A IIRC.

    One of those links powers the e-box (the box with the 2x DME, EML and several relays). You might have a bad EML or DME, but that's rare.. Multimeters are not terribly useful when troubleshooting intermittent things (like measuring fusible links), but you can measure the + post in the ebox while someone flexes the cables.

    I think there's a ground cable from the chassis to the engine.. double check that.

    In my car, the connector to the fuel pumps under the seat was corroded and acting intermittent.

    You replaced your ignition system.. make sure you didn't use 'fancy' plugs. the originals are long gone, the new part number ones aren't great, NGK are the best ones for the 850i with standard ignition. Some folks (myself included) have had all sorts of problems with the cylinder ID probes on aftermarket wiresets.. Just sayin... If your ignition system replacement made things worse, suspect both of those (plugs/wires).

    Other seemingly random causes of electrical disturbances.. a shorted cell in the battery could do funny things, but it would be obvious within a day or two. Might have a bad alternator or alternator regulator/brushes, and the system voltage may be dropping (though this would be accompanied by a dummy light).

    Cheers!
    Thanks, that's very interesting. I have been wondering a lot about whether it was a fuel issue - such as a blocked injector (even though I serviced them a year or so ago) - as the issue seems worse under load. I didn't think of the connector under the seat. It may be coincidence but I swapped the interior out for one from another 8er around the same time the issue started so I guess it's possible I damaged a cable, etc when pulling the front seat. Where is it located exactly? Presumably driver's side rear (in a RHD car)?
    Last edited by jeffbebe; 03-10-2018 at 10:24 PM.

  14. #14
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    Just found your post here: https://www.bimmerforums.com/forum/s...5#post23892295

    Will take a look this weekend and replace the connectors if required. Thanks again.

  15. #15
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    Quote Originally Posted by jeffbebe View Post
    Just found your post here: https://www.bimmerforums.com/forum/s...5#post23892295

    Will take a look this weekend and replace the connectors if required. Thanks again.
    Stealer in NZ wants $85 for those connectors you mentioned in that post!! All ex-Germany. Ouch.

  16. #16
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    Checked the fuel pump connectors and no signs of corrosion or fuel leakage (to be fair the fuel smell has always been outside, unburnt fuel from exhaust). There is a DIY-looking earth connected to the larger of the two connectors. Is that normal?

    Even if not, I can't believe it is contributing to the issue - unless the connection has gone bad - as it has run fine for three years previously.




  17. #17
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    Replaced the two fusible links. Reconnected the batteries and started the car and the idle went all kinds of crazy. Surging for 20-30 seconds, presumably as adaptions, etc had reset with the batteries disconnected. Drove off and the misfire was still there but disappeared after a minute or so of driving. All seemed great but then the misfire started to creep back, particularly at low revs (the opposite of what was happening previously). Also noticed the surging idles seems to happen every start up. I know there are no vac leaks as I’ve smoke tested. Could this point to faulty O2 sensors? I replaced them not long ago but I guess one could be faulty?

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    I'm not sure how much data you can get from the V12's - but checking the adaptations would tell you whether you have a vacuum leak or a misfire:

    http://www.meeknet.co.uk/E38/Diagnostics.htm
    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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  19. #19
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    Quote Originally Posted by Timm View Post
    I'm not sure how much data you can get from the V12's - but checking the adaptations would tell you whether you have a vacuum leak or a misfire:

    http://www.meeknet.co.uk/E38/Diagnostics.htm
    Unfortunately I’ve never been able to scan the car. That’s a different thread somewhere. I believe it’s a faulty module preventing the diagnostics from working.

    If I’m reading that article correctly bad O2 sensors could prevent the DME from adapting to a bad AF mix so it would run rough but there would still be a root cause.

    I’m 99% sure there are no leaks, could a faulty O2 cause the misfire if it’s feeding poor info to the DME. I can smell unburnt fuel when it’s misfiring.

    I’d thought electrical issue because it feels like a cracked rotor (mine are new) and weird that the new ac compressor has crapped out at the same time. Fuses and relays all test okay.

    Going to spend a weekend pulling modules to isolate the diagnostics problem next. Frustrating.

  20. #20
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    Lambda sensors rarely fail - but they get blamed for everything! Much more likely you have a simple HT fault.....
    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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  21. #21
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    Quote Originally Posted by Timm View Post
    Lambda sensors rarely fail - but they get blamed for everything! Much more likely you have a simple HT fault.....
    My thinking exactly but having recently replaced the entire ignition system - plugs, caps, rotors, coils and leads - I'm at a bit of a loss, particularly as the issue seems to come and go at random. When I've had vac leaks before they've been fairly consistent, but this occurs hot and cold with no warning and can disappear whilst driving instantly, sometimes for quite some time. An electrical short somewhere seems more likely, as you say. Locating it, not so easy. Now that fusible links have been replaced, I'll take a look at the ground strap. I remember it looked okay when I had the top off to do banjo bolts but that was a couple of years ago. I guess possible I've sprung a leak from the rocker cover, although that was redone at the same time so should be fine (with less than 5000 miles driven in that time). I'd also expect oil build up on the plugs to behave more consistently.
    Last edited by jeffbebe; 03-26-2018 at 03:39 AM.

  22. #22
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    Make sure you haven't connected the lambda sensors up to the wrong DME, easy mistake to make if you have removed the connectors at the sensors at some point. As in the left sensor is connected the DME that controls the right bank.....you will get all sorts of problems with low to mid RPM range but the car will run fine at full throttle because the sensors are disabled at that position.
    Last edited by Omega man 1969; 03-26-2018 at 06:13 PM.

  23. #23
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    Quote Originally Posted by Omega man 1969 View Post
    Make sure you haven't connected the lambda sensors up to the wrong DME, easy mistake to make if you have removed the connectors at the sensors at some point. As in the left sensor is connected the DME that controls the right bank.....you will get all sorts of problems with low to mid RPM range but the car will run fine at full throttle because the sensors are disabled at that position.
    Certain they're the right way around as the car ran fine long before the fault began occurring with the sensors as they are. I also remember how it ran once when the sensors were hooked up incorrectly and it was a very different kind of rough!

    The strangest thing that has occurred since replacing the fusible links is that the surging idle has become many times as prominent on start up, to the point where it stills when the idles drop. But then, once I drive for a a minute or two, the misfire seems to go away and it runs beautifully. So confusing.

  24. #24
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    So as it warms up? Have you checked temp sensor values using diagnostics from a cold start? also check the oxygen sensor values from cold start.

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    That's because you deleted the adaptations (the early DME's lost all learned values when the battery was disconnected) - and it will take a time to re-learn. That's why diagnostics can be so useful if you can find what the adaptations are as it would point to where the default values are lacking.

    Quote Originally Posted by jeffbebe View Post
    Certain they're the right way around as the car ran fine long before the fault began occurring with the sensors as they are. I also remember how it ran once when the sensors were hooked up incorrectly and it was a very different kind of rough!

    The strangest thing that has occurred since replacing the fusible links is that the surging idle has become many times as prominent on start up, to the point where it stills when the idles drop. But then, once I drive for a a minute or two, the misfire seems to go away and it runs beautifully. So confusing.
    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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