2ndof2

I'm posting because I've thrown a CEL for random misfires twice in the past 500-1000 miles.

Just a little background:
Top end was done about 6000 miles ago and I had everything you could possibly imagine done at that time, plugs, wires, etc even had the distributor rebuilt b/c it wasn't turning smoothly.

Important note: The car does NOT have OE Porsche belts. Contis, in very good condition. Read this is often the cause. Also read this typically is the result of high RPM driving and some imbalance caused by the wrong belts. This sounds like a reasonable assumption except both times my car threw the CEL it wasn't after any "spirited" driving.

The car was in for a 90K tune-up just prior to the first incident, and it passed with flying colors. My indie P-car wrench has the Porsche diagnostics software and nothing showed up out of the ordinary.

Lastly, the codes thrown were for different cylinders although I think #5 was a perpetrator both times. I believe it was #3 and #5 the first time and now #5 & #6 this time many miles later.

Other than not having an OE Porsche belt, are there any other probable causes of this issue?

Thanks,
David

Van1

Bad dual mass flywheel. I did plugs, wires, distributors, rotors, etc, and would still get random misfire codes. When I changed my clutch and flywheel to a LWF and GT3 clutch disk the random misfire codes stopped. While not much out there on it, there is some info in the archives about this.

2ndof2

Flywheel eh? Had my clutch replaced at the time the top end was done. What makes a flywheel bad? We replaced or carefully inspected all of the logical "while you're in there" parts.

mcipseric

How is your idle? Is is a little rough? Check if you have any vacuum links. This could cause some misfires. Take the oil cap off and place a rubber glove over the opening. You should see some suction.
Good Luck.

1pcarnut

Might want to go put on the Porsche belts. There have been multiple threads on this from guys pulling their hair out, then putting the Porsche belts on and the problems is cured.

chris walrod

We've solved a few misfire problems with new DMF's or LWF's. At around 100k miles the DMF gets mushy. There is a spec in the workshop manual if memory serves that outlines a service limit to the DMF.

2ndof2

Bumpy at idle and failed the oil filler rubber glove vacuum test. May be onto something here. Where should I look to find the source of the vacuum leak?

2ndof2

Thanks for the flywheel info Chris.

2ndof2

Definately going to have the OE belts installed next time the car goes in to the shop. Searching for the vacuum leak may be that next time unless sorting the leak is an easy DIY.

mcipseric

there is a smoke test that your shop should be able to do. smoke will come out the leaks during the test. Most likely cracked plastic connectors or the such.

mcipseric

update? I too am having the misfire codes.
It happens at idle. the idle goes rough then the CEL comes on. I have replaced the belts and fixed a couple of vacuum leaks. The CEL came on once with the AC at idle.

Any ideas?

timothymoffat

My 97 does throws a misfire code a few times a year. Usually after slow city driving. I've always found that an "Italian tuneup" on a regular basis keeps the light off. FWIW, the light always goes away after 3-4 cold start cycles. Car has ~80000 miles, new OEM belts, new plugs, new caps/rotors etc......still must change plug wires.

Steve Weiner-Rennsport Systems

Just to affirm what the others said and a few thoughts,.......

Dealer-sourced belts are a MUST. No exceptions for these OBD-II cars.

A proper top end rebuild should have included new rubber intake manifold sleeves as well as new plastic stub manifolds; I hope they replaced them as both are prone to vacuum leaks. Further, each & every rubber fitting in the intake system should have been closely inspected and replaced as needed since they are prone to cracking from heat degradation.

A proper smoke test will show most vacuum leaks.

++1 on a bad DMF. I always replace these during a clutch job as they do not last forever and will trigger misfire codes when they get old.

tonypai

I was getting random misfires and the engine would sometimes die. After replacing the clutch, flywheel and engine mounts. I haven't had a CEL in 6 months.

mcipseric

i just solved my mystery misfire problem. - bad dual mass flywheel.
lots of chatter at idle and at shutdown..... wish it was a cheaper fix.....