M20 Misfire (Please help....)

[iamsam]

Updated 8-1-09, still misfiring, need help!

Here is the down low: My friend (username Finn on here) has a misfire with his M20. It has a consistent miss under load, and it seems to sometimes cut in and out randomly from running fine to missing, but always misses under a load.

So here is what we did to try to fix the problem:

-Replaced spark plugs
-Replaced spark plug wires
-Replaced distributor cap
-Replaced timing belt
-Adjusted valves
-re-did engine grounding wires

We swapped these following parts off of a perfectly running M20 to try to diagnose the problem:

-MAF sensor
-Coil
-Entire fuel rail, including injectors and FPR
-ECU

What else could it possibly be?? the car ran fine when he got it, and one day it randomly started missing, with no significant event to trigger it. This is pissing me off as much as it is pissing my friend off, and we really want to be able to fix it before he has to bring it to the BMW specialist.

This thread is a very similar problem i believe: http://www.r3vlimited.com/board/showthread.php?t=123558

[slo325i]

I dunno if this'll help but it's free... Try a good ecu??

[iamsam]

I dunno if this'll help but it's free... Try a good ecu??

I actually thought of that when we were swapping stuff from my brother's good M20 car.... but i figured it couldn't possibly be the ECU, because ECUs don't just go bad..... do they?

[416stroker]

vacume leak?

[slo325i]

I actually thought of that when we were swapping stuff from my brother's good M20 car.... but i figured it couldn't possibly be the ECU, because ECUs don't just go bad..... do they?

Cured my problem on a 240sx...

[iamsam]

vacume leak?

thought of that, looked pretty closely, we may have to look closer. Would a vacuum leak cause this?

Cured my problem on a 240sx...

Hmmm.... in that case, i will try it asap.

[E30_fiend]

+1 on the ecu being the problem.

If you are looking for an easy way to check for a vaccum leak there is one way that has never gone wrong for me and that is to grab a can of brake cleaner and spray all your main vaccum hoses and gaskets around your throttle body and you intake manifold. When those gaskets go bad they can start to draw in more air in little doses.


Just for the record, do you have a lean running condition? this would signify a vaccum leak. Have you been getting really good gas mileage?






Taylor

[iamsam]

+1 on the ecu being the problem.

If you are looking for an easy way to check for a vaccum leak there is one way that has never gone wrong for me and that is to grab a can of brake cleaner and spray all your main vaccum hoses and gaskets around your throttle body and you intake manifold. When those gaskets go bad they can start to draw in more air in little doses.


Just for the record, do you have a lean running condition? this would signify a vaccum leak. Have you been getting really good gas mileage?






Taylor

will try the ECU. Sorry but how will the brake cleaner indicate a leak? And i have no idea if its lean or not. But it has been getting 24 avg MPG.

[E30_fiend]

spraying brake cleaner will locate the leak because if there is a crack in a vaccum line the your idle will jump up due to extra combustable material being in the system. so spray it all around gaskets and hoses. You SHOULD NOT hear you idle jump. If you do, suspect that hose/gasket(s).






Taylor

[iamsam]

spraying brake cleaner will locate the leak because if there is a crack in a vaccum line the your idle will jump up due to extra combustable material being in the system. so spray it all around gaskets and hoses. You SHOULD NOT hear you idle jump. If you do, suspect that hose/gasket(s).

ah, i see! this i will try.

UPDATE: replaced O2 sensor, did nothing to fix the problem.

[Finn]

Thanks for all the suggestions, unfortunately I have tried most of them. I am going to buy some ether to check for intake leaks. I suspect the ECU. It really seems like a timing issue and I am beginning to suspect that the spark advance is out of wack. I can smell unburnt fuel in the exhaust and it sure isn't getting good gas mileage.

Another bummer is that I can't check this at work since they blocked the site for some reason. Again thanks for all the help

[structured]

bump because I have almost the exact same problem. Replacing plugs and wires has done nothing to help. I am going to change the coil pack today with a used one that I got from a forum member. (Does anybody know what the coil pack should read in ohms?). The one in my car shoes about 0.35 (primary) and 5700 (secondary). The one that I just got shows about 0.35 (primary) and 6200 (secondary).

My mechanic also told me to check my afm. Actually he told me to replace it, but I told him to screw himself. He sucks at guessing problems and each guess costs me $500.

[iamsam]

bump because I have almost the exact same problem. Replacing plugs and wires has done nothing to help. I am going to change the coil pack today with a used one that I got from a forum member. (Does anybody know what the coil pack should read in ohms?). The one in my car shoes about 0.35 (primary) and 5700 (secondary). The one that I just got shows about 0.35 (primary) and 6200 (secondary).

My mechanic also told me to check my afm. Actually he told me to replace it, but I told him to screw himself. He sucks at guessing problems and each guess costs me $500.

thanks for the bump! (ok that sounded weird)

But yeah mechanics can rack you up quite a bill without actually doing anything at all. The trick is to find a good private mechanic who specialized in BMWs and won't charge unless he actually does something.

Next try to fix this M20 is to replace the ECU. Swiped it from my brother's M20 car and we are going to see if that fixed the problem. If it doesn't, its time to give up and hand her over to the BMW specialist.

[iamsam]

OK we just now replaced the ECU with one from a working M20 car and it didn't do shit!!!

so that was definitely not the problem. What else could it possibly be??? help!!!

[nando]

have you checked grounds? because they can cause issues like this.

does the check engine light flash at all when it misses?

have you run a compression test?