Mike J

Well I have been getting a dreaded CEL lately and its now time to start chasing it down. I thought I would ask the list sleuths some guidance on how to approach solving this problem.

Symptoms:

1 - occasional CEL code P0300 Random Misfire, P0306 Cyl 6 misfire, P0304 misfire, P0305 misfire....the cylinder appears to be random. Happens from once every couple of weeks to every 10 minutes. Reseting the code resets the CEL no problem.

2 - At idle the right exhaust has a **phut**phut** misfire sort of sound that is not steady. The left exhaust has it as well but much less. The sound does go away at times and it has a steady idle (mabye when cold, not enough evidence to conclude that). Of course it was steady last time my mechanic listened to it ;-)

3 - The engine when misfiring idles fairly rough...enough to shake the car a bit (not a lot but certainly not smooth).

4 - If the AC is on to bring up the idle the misfire sound lessens but does not go totally go away

5 - The car appears to pull fine up to redline.

Other facts:

1 - The car is a 96 993 Targa. The car as 75,000 miles.

2 - The misfire sound has been occurring for quite a while. I recently dropped the engine (do the clutch) and replace all the plugs..no difference. It was occurring before the drop so I doubt if that has something to do with it.

3 - The caps/rotors have 13,000 miles on them. Have not pulled them yet to see how worn they are.

4 - I cleaned the ISV with no effect.

5 - I do have the collapsed Varioram vacuum connector which I will fix. However my Varioram appears to be working anyways.

6 - I get a very very slight glow around the coils in the pitch dark. A previous post talked about this glow being a problem...but the glow is hardly visible. Is anyone willing to check their cars to see what is "normal"?

Rather than throw $$$ to a mechanic I figure I would give it a try. I figure its worth replacing the caps/rotors anyways and see what happens. If that does not work what's next?

Cheers,

Mike

flatair

Original wires?

Mike J

Yup, original wires. I talked to my mechanic about that...he told me that he has rarely replaced them. It seems from the list that its been more often than that. Looking at the plug wires is definitely a logical next step...especially the lower ones. The engine also appears to idle much better when cold...or is it the higher RPM's that is masking the rougher idle?

Thanks!

Mike

Ash

It looks like you're hunting down the most obvious things first. I'll pitch this in as something you might you want to investigate but do you still have the original Dual Mass Flywheel? My understanding is that when the DMF is failing and the rotation is not smooth, the crank sensor can interpret this as misfires. Do you hear any noises when your car is in neutral with the clutch pedal out? Do the noises disappear when you engage the clutch?

Todd B

Ash is right, My DMF was failing and causing misfire codes. It would only happen if the car was at the track and was hot and idling. Replaced the flywheel with a lightweight flywheel, and the problem was cured. Did this after replacing the plug wires, plugs, dist/cap and cleaning the ISV, which didn't fix the problem.

Todd

cabrio993

Mike,

I was having the exact same symptoms that you are describing, with daily CEL's that will go on and off. Scanning found multiple random misfires. Car would Idle OK and no CEL's will come on as long as I kept the AC On or if I would not let it idle to low after the car had been warmed up.

Found lower original wires cracked and some connectors that were not making good contact with the cables. I replaced the wires, dictributor caps, rotors, spark plugs and that was it. No more CEL's, no more *Phut* *Phut* idle either. The engine runs smooth now on idel and runs great too. Before, there were times were I would feel a hesitation sometimes under acceleration...not anymore.

My wires had only 40K miles but I guess this is due to age more than mileage. After 9 years, that rubber starts to dry up and break, specially the lower ones since they get wet all the time.

Hope this helps...

Sergio.

Mike J

Thanks Everyone!

I think I will wonder down the caps/rotors/wires route as the first path. While I do have a DMF in the car (second one...the PO replaced it) I doubt if a faulty DMF will cause the **Phut**Phut** sound on idle (could be wrong here of course!). My situation sounds more like Sergio's....especially if he had the *Phut* sound. Sonds pretty similar. I also noticed a few plug wires on the lower side are in not great shape. I will try to solve the bad idle as the first step.

I suppose I can check the resistance of the plug wires first. Caps/Rotors are in tomorrow, will do a quick swap to elminate that.

Too bad the parts on these cars are so expensive...or you would just keep swapping until it works! ;-)

Cheers,

Mike

kkim

Mike,

Just to let you know, if you go down the plug wire route, they are available individually so you don't have to buy the entire set if you can identify the bad ones.

good luck

Mike J

Thanks Kelly!

I already priced out the wires as a OEM-Compatiable set as well as individually. I usually deal with Weissach here in Vancouver (Rennlist sponsors, great prices and they are a 20 minute drive away) and a full set of clone wires is about $500CDN but each individual wire from Porsche is $80-$100CDN per wire. It makes sense to buy a whole set especially if you are looking for a bad wire. You can also buy it by the bank from Porsche as well.

Cheers,

Mike

viperbob

While you were doing the clutch, did you check the dual mass flywheel to see if it was still good? A lot of your symptoms could also be a bad DMF....

Mike J

Hi Bob,

I took a look at the DMF while I had the transmission off...from a visual inspection it looked fine. I do get a very very small **rattle** noise when I shut the engine off that goes away if the clutch is in...so the DMF does make a small noise. I am trying to maintain this car on the cheap so I did not do the LWF upgrade but instead replaced the clutch disk with a slightly used one from a fellow rennlister (thickness was very very close to new) to give me a couple of years before I pop for a LWF. Since I can drop my own engine to change the clutch the cost is parts and the cost of an alignment, so if I can't find the misfire problem replacing the clutch is on the list...

What is leading me down the plug wire route is that at idle the exhaust (especially the right one) makes a **phut* **phut** sound at random...sounding like a misfire. The tone is not steady. What does a failing or failed DMF do the idle of a car? I can understand the car shaking or vibrating but does it actually cause a misfire sound in the exhaust? Have you seen that?

Thanks for the help!

Mike

flatair

Sounds very similar to when my old 73 needed a new set of wires. "phut phut" could be heard both at the exhaust and in certain MFI stacks. Idled better when cold, got worse as it warmed up. Replaced cap, rotor, points, and oem wires with a nice set of Magencors and all was well. I think as the car heated up the old wires they expanded and made the problem wires worse. Funny thing about these engines - they're so balanced and smooth. When I got around to checking things out toward the end, I had a completely dead cylinder (clean plug). I noticed a power difference, but it actually still idled 'ok'.

Lorenfb

"What is leading me down the plug wire route is that at idle the exhaust (especially the right one) makes a **phut* **phut** sound at random...sounding like a misfire."

Yes, that's a misfire. You're "dropping" a cylinder. Remember you have a dual ignition
system which allows you to disconnect one at a time to troubleshoot the misfire.
The misfire may not only be ignition, but could also be fuel related or just a weak
cylinder. You need to do all the basics, but first start by disconnecting one ignition
system to determine if it's plug wires related.

Todd B

With my search for the misfires, I still had all of the original symptoms, after replacing the plugs, wires and caps/rotors. The car would start to stumble and shake, if the CEL wasn't already on it was about to come on. I also found if I ran the AC, I didn't have the problem. I don't remeber the sound, but it was not like a normal idle, you could hear the misfire as the car stumbled. This wasn't constant, but random. Finally replacing the DMF, solved the problem for me. The car ran fine, just would not have a smooth idle when warm. This was at ~52K miles.

Todd

Lorenfb

"Finally replacing the DMF, solved the problem for me."

If it's a true misfire versus a vibration causing the knock sensors to think it's
a misfire, then replacing the DMF WON"T solve the problem. Avoid the "shot-gun"
approach to troubleshooting, as it can be very costly, i.e. time & money.