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:
Here's what's been done, in the last year:
Here's what I've done or replaced trying to fix this:
There's so many different causes of misfires, but this is the list of things that might apply to my m20's particular issue:
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.
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.

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