Very annoying misfire issue

[Jon K]

Hey guys - running on stock ECU and an FMU for right now until I get my standalone finalized and can set aside the time to tune. Recently when driving the car, it will drive completely fine if I am 25% throttle or less. If I go to 35 - 50% throttle I get a "hickup" like a misfire and it happens pretty much every time I load the engine. I notice it happens specifically when at lower RPM say 1500 - 2700 rpm or so. If I stay off the pedal say like 20% throttle and let the revs go up when driving (staying off boost as a result) the engine will rev to 6500 rpm with no issue. It is only when I give it a specific amount of throttle with lower rpms. On the highway in 5th gear around 4000 rpm I can floor it and get the 4 or 5 psi (have low boost pulley on) and car pulls fine. I don't think the car is leaning out because its at such low RPM where there is really no boost to contest with.

Plugs are new. Valve cover leaked a bit not long ago maybe it ate a coil? Frustrating as ****.

[632 Regal]

whats the plug gap? pissible weak coils are causing the plugs to blow out under cylinder turbulance?

[granit_silber]

Jon,

Your condition sounds like mine (minus the cool stuff in your bay) when I lost an ignition coil.

-ashley

[Jon K]

Jeff - the plugs dont get gapped, the M50 requires no alternation from the OE plug gap. I think it could be a weak coil but what sucks is that I replaced all 6 coils about 70k miles ago.

[neil_004]

I have a 91 M50 that did exactly the same thing. I was cleaning the engine bay a week and a half ago in prep for replacing the valve cover gasket and it magically went away. The only thing I touched that I think could have caused it was the fuel rail and injectors and the electrical connectors. In the past 2 days it has started to come back a bit. Not really sure what is going on but I'm inclined to look in that area.

[632 Regal]

maybe you can test them and isolate the weak one?
Jeff - the plugs dont get gapped, the M50 requires no alternation from the OE plug gap. I think it could be a weak coil but what sucks is that I replaced all 6 coils about 70k miles ago.

[Jon K]

I have an extra 6 coils so I will swap them over tomorrow night and see what the deal is. I hope it goes away.

If not, I have an entire engine waiting to be built. argh i need money.

[winfred]

sounds like a coil cutting out, depending on when it happens sometimes you can get someone to power brake the car and get it to miss and pull coil wires till you find the miss, it helps to have all of the clips popped so you can pull em quick and not beat on the tranny too bad

[Jon K]

winfred its a 5 spd... it doesn't seem to happen much when the engine isn't loaded i dunno how i'd keep the engine loaded and brake on lol. Sounds like a bad thing waiting to happen. I think i t's #5... i dunno why , i just have this feeling (valve cover leaked there for a while).

[Booster]

Jon ,you know it can only be spark or fuel since its only during the loading demand. If you've already dealt with a new f-pump,filter etc,. and know all of thats good to go....I'd also be inclined to inspect the coil spark delivery on each.
I know on the Skyline RB26 inline six's with multi-coil packs that they are prone to that behaviour as a result of intermittant response and failures.
Vincent:)

[zygoteer]

... pissible weak coils ...

hmm ... that would do it for sure !

[winfred]

****, well it's easy on a automatic, they don't usually show up till you have a load on the motor, a dyno would be handy


winfred its a 5 spd... it doesn't seem to happen much when the engine isn't loaded i dunno how i'd keep the engine loaded and brake on lol. Sounds like a bad thing waiting to happen. I think i t's #5... i dunno why , i just have this feeling (valve cover leaked there for a while).

[RichO]

I had a similar issue with my 91 m50. It turned out to be the plug boots not the coil pack itself.
One was damaged and the plug was actually shorting intermittently to the cyclinder head. Cheap fix.

[Jon K]

Yeah - like I said i have 6 extra coils and 6 extra boots... hopefully its coils.

Winfred - I noticed tonight that one a cold start, the car drove fine... I even put the pedal in the boost section of pedal travel and it pulled great. Then, about 2 - 5 mins later after it was warming up it started to miss again.

[Bill R.]

the burp you describe is indicative of a lean misfire... Wide open goes much richer and if you actually do have a weak coil then a richer mixture requires less voltage to fire. A lean mixture especially combined with boost will require much more voltage/spark to light.. If changing the coils doesn't work then try using some aftermarket single electrode plugs and close the gap up some to see if you can deliver enough energy to get them to light the mixture. Along the lines of what Jeff was saying.



Yeah - like I said i have 6 extra coils and 6 extra boots... hopefully its coils.

Winfred - I noticed tonight that one a cold start, the car drove fine... I even put the pedal in the boost section of pedal travel and it pulled great. Then, about 2 - 5 mins later after it was warming up it started to miss again.

[Jon K]

the burp you describe is indicative of a lean misfire... Wide open goes much richer and if you actually do have a weak coil then a richer mixture requires less voltage to fire. A lean mixture especially combined with boost will require much more voltage/spark to light.. If changing the coils doesn't work then try using some aftermarket single electrode plugs and close the gap up some to see if you can deliver enough energy to get them to light the mixture. Along the lines of what Jeff was saying.


Thanks Bill - I am going to be swapping plugs 1 step colder with slightly less gap when I get my standalone ECU back in.

As for the misfire, I fixed it.

http://www.bimmer.info/forum/showthread.php?t=20883