samurai_k

This is a great thread.

I have noticed we all went straight to solutions and I am wondering if someone can explain what each of these solutions do to remove the CEL.

Excuse me in advance for the ignorant question, for example why does a bad vacuum, bad DMF, bad idle valve, bad ignition, bad vacuum, etc cause a misfire? What would you observe as a symptom when one of these components is bad or failed?

The engineer in me would try to inventory the possible CEL symptoms, isolate the actual ones observed and then choose the right solution?

sacman

Will do. I did clean the ICV. Today, I plan on replacing the fuel filter and I did also order a new DME relay just to rule out the cheap culprits as I read almost all the threads about misfires. Talking to a gear head friend of mine, he mentioned that I should feel the misfires if they truly happen but that's not my case. I can't feel anything different while driving the car. On my way back, I had the GoPoint BT OBD reader plugged in and scanned in real time and I noticed that while the CEL is on the fault code changed back and forth between a P0302 and a bunch of P0300, P0304, P0305 and P0306. It seems like something electrically flickering or sensors on its way out. I'm recording all these down for future help to anyone.

QN

nine9six

Do yourself a favor, and take your car to a qualified 993 specialty shop with a Porsche diagnostic tester. (Bosch Hammer, PST2, or PIWIS) and have the issue properly diagnosed.

From your description, it sounds like a failing DFM... a dual-mass flywheel has a rubber center, that when failing, allows the outer portion of the flywheel, where the sensor contact resides, to be out of sync with the flywheel position sensor and with the actual crankshaft position; and this can cause misfires.

Or

Please pull your distributor caps if you have not already done so, to make sure you have no carbon traces to the contacts on the inside of the caps.

pp000830

Nope, you won't feel it as an emission revenant misfire that will cause a check engine light is not often enough for you to hear or feel it as a stumble. Most engines misfire all the time, it is more about when the misfire count is higher than the count programmed into your ECU a relatively low number,

sacman

All, I just want to drop an update on the misfires issue with my car for future reference because I did search a lot of posts for resolution. In my case, it has been an expensive path to the solutions. I dreaded for smog check every year because of the CEL due to misfires. Here's a list of what I have done order of time over the course of 3 years:
1. Spark plugs, wires, ignition rotors.
2. BG40 fuel injections cleaners...alot of bottles
3. Gas tank cap
4. Porsche belts (all three)
5. Oil change
6. Clutch and Flywheel

Granted that all the stuff are in need of replacement but the actual culprit is the flywheel. There's no record of the flywheel replacement on the last clutch job and since the clutch needs the replacement soon so I did both. The new clutch and flywheel really help with the heavy clutch and high catch point that I feel like the car just got extra 50hp. The shifting is amazing, not soft not hard, just perfect. Knock on wood but it's been at least 360 miles without the CEL. The new flywheel is not light weight, just OEM if you're curious. The clutch is SACHS from Jon EBS Racings GB. When they are functional, these car are awesome to drive. My lesson learnt for random misfires is to replace the flywheel when you do your clutch.

QN

IainM

Proj993

I've had CEL come on and off for a number of years and in the past this was not too problematic because the car would pass smog test without any problems once the light went off. During the last smog test a few days ago, the car failed because It was emitting 64PPM of HC with Max allowable being 54 at 15MPH. It barely passed at 25MPH measuring 36 with Max being 37. After driving around with added Techron and octane booster and replacing spark plugs, distributor caps, rotors and air filter had to wait for CEL to go off. This time it was a longer wait than usual. It eventually went off and the car passed easily with much reduced emissions for HC, CO and NO.

Misfires were showing for all 6 cylinders. The codes were cleared, CEL stayed off until the next day during a highway drive (steady speed, over 3000RPM, which seems to be typical) and then again went off that same day after a few more starts and stops.

I'm hoping to identify the CEL cause. I've read a lot of the threads on the subject and among the fixes mentioned are a new flywheel and a new ECU. Both very costly. How can one determine if the flywheel is the culprit? Used a Hantek 1008C oscilloscope and got the attached image. Unfortunately, this oscilloscope does not come with a library, so I have no idea if there's anything out of the ordinary. Can anyone help with that? The signal pick up point was the crank sensor. I should also mention one more symptom on my car. It has a slightly rough idle, which occasionally becomes rougher for a second and then less rough. CEL has never come on during idle or low speed/RPM drives.

This seems to be a very common problem with 993s. Low misfire count coded into the ECU is mentioned above. Is this simply an error on the part of Porsche engineers? Does anyone know if that's what has been corrected by this replacement ECU: https://www.sunsetporscheparts.com/o...bC1oNi1nYXM%3D

Its part number is: 993-618-604-JX, which it says replaces: 993-618-604-00.

IainM

Proj993

Spark plug wires were not replaced because they were visually fine and I had assumed they were replaced at the last major service (30,000 miles and 7 years ago). I checked my records just now and they were not mentioned in the work order. How often are they supposed to be replaced and is it possible that they may be the cause? Any other symptoms to look for to determine if they need to be replaced?

The reading is from the crankshaft sensor. In some settings regular breaks were showing up, which could have been the 0 degree marker. What should one look for to indicate faulty flywheel?