nine9six

I threw another CEL with the same misfire codes; P0300, P0304, P0305, & P0306. All misfires are coming from the right bank of cylinders.

Last time, I changed the distributor cap and rotor, due to what looked to be carbon traces about 150 degrees around the cap, along the same path of the rotor copper contact point.
This time the cap and rotor look completely clean...

Since the misfire codes are all on the right bank of cylinders, I am thinking maybe distributor belt play/slop. Should there be any rotational play in the rotor? It seems to be spring loaded and has radial play in the opposite direction of operational rotation.

Hmmm, right side O2 sensor issue maybe? (thinking out loud)...But my OBDII reader recorded O2 sensors as OK

Any rational thoughts would be greatly appreciated

P.S.
I have kinda ruled out failing dual mass flywheel, since all misfire are on one side of the engine. I'm thinking DMF issues would be more random, with misfires on both sides of the engine. Am I thinking straight?

Thanks in advance, and Happy Easter!

Lorenfb

"Hmmm, right side O2 sensor issue maybe? (thinking out loud).."

Yes, focus there. Also, compare the long term trim (FRA) for each bank.

nine9six

Thanks Loren...I was hoping you'd chime in; but I have to admit you are talking way over my head with regards to comparing the long term trim (FRA) for each bank.

Is this mentioned in the engine section of the Workshop manual, and if not, could you please direct me to the correct section?

Thanks again for your technical knowledge and consideration to my posted issue.

Best regards,
Paul

Jay J

What year is your car?

nine9six

1996

Loren,
No need to respond, as I think I found a very similar comment you posted to mgianzero's misfire issue.

Here is the link to your comment.
https://rennlist.com/forums/993-foru...ire-codes.html

Marc,
Did you ever confirm your issue as a failing DMF? Our issues sound VERY similar, with the exception that my misfires are still relegated to one bank of cylinders (4,5, & 6)

Happy Easter!

Mike J

Did you check all the usual culprits? can you give us a bit more data, like:
- how many miles since the last plug change?
- how many miles on the car?
- how many miles on the plug wires?

Have any diagnostics been run on the car or code pulled (i.e. how did you get the CEL codes - with a PST-2 or a generic tool?)

Look at the lower plug wires on the right hand side, they take a nasty 180 degree turn on #5 for instance, I often find that particular plug wire gets cracked first, it's once possible.

Anyways, it would be good to have more data - can you feel it misfiring? How often does it happen? Is it reliably repeatable? (ie. you will know if you fixed it).

Cheers,

Mike

BobbyT

Some owners have reported misfire codes that were very difficult to diagnose and correct. In some cases, the culprit was found to be the alternator/fan drive belts. Other than genuine Porsche belts somehow trigger a misfire code, even though they apparently fit correctly...

I don't know if this could occur on one bank only, but it might be worth checking the belts to see if they are correct for the car, and have the Porsche brand stamping on them.

nine9six

Hi Mike,
Yes, sorry about the brain freeze

Plugs were changed 5/7/2010, @ 68.4k mi. Current clock is 75k mi.

Plug wires appear to be original OEM

Codes were pulled with an Actron 9850 code reader, not a PST-2.

Car threw same codes a few months back (10/13), but I thought I had the issue solved with new Bosch caps and rotors. There appeared to be arc traces across approx 150 degrees up at the contact areas inside the caps. Although I dont have a complete set of records, caps and rotors appeared original OEM, which is why I replaced them recently.

Car seems to run fine, although admittedly I have not driven more than a handful of different 993's, so my seat O' the pants comparison base is somewhat limited.

I will check the plug wires tomorrow, after Easter Sun festivities ...Seems most might guess at plug wires, but the misfires being relegated to only the right bank of cylinders has me thinking something else...

Could this be an intermittant (before cat) O2 sensor issue, on the right side?

nine9six

Thanks BobbyT...
Records invoice reflect Jerry Woods Enterprises replaced 1 AC belt and two fan belts (5/10) @ 68.4k mi. There is no p/n on the invoice, but will check the belts for the p/n tomorrow. Jerry Woods is very reputable with the early P-car crowd on Pelican, and I'd like to think he uses Porsche belts.

EDIT: Both fanbelts are Porsche parts. They were replaced in 2010, 7k miles ago.

nine9six

Bump to the top for any other ideas/thoughts?

BesideTheBox

Last Spring, I was getting P0306, misfire on cylinder 6.
What cured it was cleaning the grounding points.
I think the difference here is that you haven't said anything about your engine running poorly.
My engine ran really bad during its warmup phase and would buck, CEL flashing even. Then clear up after 10 or 15 minutes.
At any rate, cleaning grounds is inexpensive. Just takes time.

2ndof2

I get some misfire codes from time to time...different cylinders as I recall but its repetitive. I had all kinds of stuff replaced with a full top end refresh, valve guides, resealed entire motor. Had stuff replaced like plug wires, had alternator rebuilt, etc. But even though this was all done the car had this issue shortly afterward I spoke with my mechanic who is highly experienced with 993s and owns one himself and he thinks a bad flywheel may cause this problem. I had a clutch replaced when the motor was out for the top-end job but not the flywheel. Only tossing the idea out there. Have no idea how this may cause the problem but thats what I was told is a possibility.

cgfen

remove and reseat both ends of the plug wires?
replace the plug wires?

That's what i would do.

Mike J

What can happen is on the dual-mass flywheel (ie. the stock flywheel), the mechanism that couples the inside and outside portions of the flywheel gets worn, and can result in vibrations and very slight changes in the rotational speed of the flywheel - that is picked up by the sensor at the outside edge of the flywheel, and those slight variances are interpreted as a misfire. The position of the flywheel as detected by the sensor allows the computer to figure out the piston. If you can hear a rattle when stopping the engine, it may be the inside of the DMF rattling around a bit when the outside part of the DMF stops.

On a RS clutch, which consists of a single mass flywheel (ie. the flywheel is one part, not two), this would not happen. Another advantage of that setup.

Cheers,

Mike

mgianzero

Paul - we spoke about this, but I'll post my findings on here for all to see as well.

I agree with Mike (as usual). But, if you remember, I posted a thread on RL months ago about my random misfires. Here's what was done on the car - plugs, wires, caps & rotors, new Porsche belts, flywheel sensor, and a 1997 DME swap. I've brought my car to several Porsche shops and they were all impressed with how relatively smooth my car idles and runs compared to most. I was getting misfires in all cylinders and ones at both idle and 10,000 rpms (which we know is not possible).

Having several conversations with Steve Weiner has led us to believe the flywheel is shot. My car had a clutch replaced but same flywheel at 41,000 by previous owner. Again, that's probably as sign that PO was hard on the driving my car. A car that's 18 years old and poor driver habits could certainly both lead to a bad flywheel.

So my next big project (yes ... another one!) would be an engine pull - RS flywheel, clutch and various "while-you're in there" repairs - improvements. I've got a list a mile long of things I "should" do. Just hope my wife doesn't leave me over this list!