Code reader says random misfires, various cylinders. 1, 3, 4, 6, whatever.

Changed plugs, caps, rotors. Coils on the way.

No effect on performance, and the misfires are undetectable...

I thought it happened every time i filled the car with gas, but now it happened independent of that.

what now...

 

I would say no. My guess is spark plug wires. I test them by running the car in a completely dark garage to see the faint arcing. Usually the wires are cracked on the lower valve covers or under the intake.

 

I remember reading about another rennlister using a non-Porsche alt belt, Contenental I think?
Had this issue. I wouldn't think it would slip enuff to cause a string of misfires, but it did in his case.

 

Using all Porsche belts, parts, etc.

No visible arching. but of course, its hard to see everything under there..

 

What are the actual codes? Could be the O2 sensors.

 

Have you checked for vacuum leak?

 

Has anyone on here ever had to change sensors? It's pretty common on MB as it's a wear item.

 

I've changed several usually around the ~60K mark. I just changed all 4 on my current car (100K) since one was throwing an error and was reading richer (falsely) on one side. Seeing that the ones in the car were aftermarket, this must be at least the third set of O2 sensors.

 

I would also say plug wires. It might worth pulling out the MAF & cleaning it. Are the misfires @ low or high RPM's?

You can try to unplug the O2's & see if they go away, but it will need to be cleared when the cel pops up after.

 

One of my 993s got a CEL with 8k mi on the top end. My Porsche indie immediately said I needed to do the top end again. That made absolutely no sense whatsoever. I took the codes, did a search here and found some threads suggesting it might be the O2 sensors. Sure enough, $40 later, the CEL was gone and never came back. I never went back to that indie.

 

Hi,

You didn't say how many miles are on the car but if the plug wires are original, they should be replaced ASAP as they are one of the main causes for "mysterious" misfires and running problems. Test the wires by fogging them with spray bottle of water while watching the running engine in a dark garage or at night; it can be quite a light show,....

A proper 60K service always includes new plugs, wires, caps, rotors, oxysensor(s) depending on whether its OBD-1 or OBD-2, alternator and fan belts as well as filters.

 

I probably should elaborate.

96 coupe with 42,000 miles that is used as a daily driver. And I'm sure to see the rev limit a few times a day.

Getting frequent (once a week) CEL

P0300 (random multiple misfire)
and the subsequent P0301, 04, 06, 05.

Absolutely no noticeable effect (no stumbles, no loss of power, etc)

It happens at idle, or at speed...totally intermittent.

Recent (30 days ago) service included factory belts, pulley update, caps, rotors, plugs, all fluids, filters. No effect on the frequency of the CEL after this service.

I ordered a coil pack from Sunset last week, just to rule it out. Are O2 sensors and wires next? or should I install a low mileage turbo motor and see if that helps.

 

Turbo motor transplant gets my vote.....why take any chances

 

Ah, that's good to know, on my C benz I recently replace 4 @ $120. each. And the sensor is so conveniently located.

GL with getting it sorted out....