Warm Misfire

[IAmGary]

Fixed: Dirty CPS. Easy fix, cost nothing.

My m20 has a misfire that develops when the engine is warm, and so far I haven't been able to diagnose it. A friend of mine who is an experienced mechanic drove it and said he was 90% sure it was an ignition problem on the secondary side, which is something between the cap and the plugs. I'm open to alternate theories and maybe someone here had the same issue and can guide me.

Symptoms:

• initially idles rough, runs strong once the temp. gauge starts to move
• I've had it smoked and plugged all the leaks but who knows, it's an e30
• runs great for 5, 10 even 20 minutes
• at some point, driving or idling, CEL (1221 o2 sensor) comes on
• at the same time sounds and acts like a cylinder is missing
• loss of power especially at low RPM, poor fuel economy
• let it sit for 20-30 minutes
• it will initially start up like the cylinder is missing with CEL still lit
• after 20 seconds the CEL clears and it drives fine for another 5, 10 or even 20 minutes.
• Update: non-conductive spark tester thing seems to indicate it's cylinder 5 that is misfiring. How do you fix 1 cylinder?

Here's what's been done, in the last year:

• all basic maintenace (fluids, gaskets, hoses)
• temperature sensors
• belts
• timing adjustment
• smoke test
• refurbished AFM
• new ICV
• new battery
• new o2 sensor

Here's what I've done or replaced trying to fix this:

• new fuel pump (old one was dying anyways)
• new Bosch plugs (same as I've used for the last year)
• inspected and ohm tested HT wires (and replaced 1 that had a cut in it)
• new cap and rotor (old one was good but I had spares so...)
• used but good coil

There's so many different causes of misfires, but this is the list of things that might apply to my m20's particular issue:

• 1 clogged fuel injector (will check this weekend, hopefully)
• DME
• crank sensor

I'm receptive to any other ideas. Also, if anyone has links to good walkthroughs for testing the DME and crank sensor I'd really appreciate it. I have the Bentley manual, which I'm reading, but sometimes a procedure is clearer when put in layman's terms.

Thanks!!

Oh yeah, here's my car. It was kind of infamous around here for a while.

[enigmaticdream]

Grab a screw driver while the engine is running and put it against each injector. They should make a clicking sound if they're running. How do the spark plugs look?

[IAmGary]

I'll try the screwdriver to injector test. Cylinders #4, 5 and 6 all burn oil but #5 is the worst offender. I change those plugs every 2,000 miles but the kind of crud I've seen in the past would not exclude the valves from being the problem.

I know this engine is dying, I'm trying hard to keep it running until June or July when I can swap it.

[jlevie]

There should not be any intake leaks if the smoke test was conducted properly. Which would mean plugging the exhaust, replacing the AFM with a plug containing s fitting for smoke for smoke introduction, feeding in smoke until a pressure of ~4psi was reached, and holding that pressure for at least 5 minutes.

The ignition system (distributor & wires) should be replaced with new parts, a new set of NKG ZGR5A plugs installed, rail fuel pressure checked, and the injectors.

The 1221 code usually means the data from the O2 sensor is invalid. That could be a bad sensor (best to use an OE part), there is a fault in the engine wiring harness, or the DME is bad.

[IAmGary]

Thanks for the good advice, jlevie. I'm hoping to do as many diagnostics as possible and avoid throwing any unnecessary parts at the problem.

On that note, at lunch today I cleaned the CPS. It was easier to deal with than expected and took literally 2 minutes:

1. Used a hex bit and socket driver to remove single bolt attaching CPS to the car
2. Sprayed a bit of PB Blaster on a shop cloth
3. Removed all the greasy crud from the sides and face of the CPS
4. Wiped down thoroughly with a clean cloth
5. Re-installed the CPS reverse of removal

It was easier than I thought because the CPS really can't be installed incorrectly and requires no adjustment. Then I drove the car for 20 minutes without the problem appearing, which is the longest it's gone without symptoms. I would have kept driving but lunch was over, so I am not sure if the dirty CPS was the cause. From what I've read a dirty CPS can cause misfires and other problems and I know this was last cleaned a year ago when the timing belt was done. Perhaps it was due for a spring clean.

Giant Photo of the CPS location on late model e30s.

enigmaticdream, my engine is so loud and vibrates so much that the screwdriver test wasn't really helpful. I did use a stethoscope and heard ticking, but nothing was misfiring at the time so I'll have to wait until I get a misfire to check again.

Everything I've tried has made the problem either take longer to manifest or generally improved drivability so I think I'm on the right track. Hopefully I won't have to get into the harness or DME because... it sounds technical and I have no idea how I'd approach that. Does anyone have some links that might help?

[Bimmer_man]

I just wanted to say if that blue E30 is yours vary vary nice if possible can you tell me the paint code and good luck man my E30 has a misfire as well but my whole ignition system has been replaced. If you can maybe check your distributor cap?

[IAmGary]

That paint job... simultaneously the car's redeeming feature and the thing that had damaged it the most because the paint is so really badly done. The color is not a BMW color: it's Mitsubishi D06 Octane Blue.

BMW guys kind of look down on non-BMW colors but people ask about this color all the time.

[Bimmer_man]

That paint job... simultaneously the car's redeeming feature and the thing that had damaged it the most because the paint is so really badly done. The color is not a BMW color: it's Mitsubishi D06 Octane Blue.

BMW guys kind of look down on non-BMW colors but people ask about this color all the time.

well it looks nice well done man

[IAmGary]

I don't want to speak too soon and jinx it but what are the chances that all my problems are caused by a dirty CPS that took just a couple minutes and a shop cloth to clean?

Cold starting tomorrow will be the real test but my m20 has been on it's best behaviour all day.

[enigmaticdream]

Glad to see you solved it. Not that I'm having an issue but now I feel like seeing how clean mine is lol

[IAmGary]

Do it!! Doooo it!! Seriously, almost all my creeping, slowly getting worse driveability problems are gone. I'd challenge anyone who has any issues to just check and make sure it's clean. Costs nothing, takes minutes.