BesideTheBox

Chasing down the cause for a cylinder 6 misfire.
Wondering if the erratic popping that I hear at idle after the engine is warmed up might give me some insight.
Any thoughts about what the popping is?
This is a stock '97 C4S with motorsound.
Apologies for the video part, it was dark in the garage when I got back from driving.

vincer77

Wired, sounds like someone cloth pinned a card on the cooling fan!

I was thinking at first a lean miss, but does not sound right. Did you get a cyl 6 misfire code? Is there one.

BesideTheBox

Heh. We liked to pin playing cards to our bike seat stays and front fork when we were kids!



Yes. I will get a cylinder 6 misfire code if I try to drive normally while the engine is warming up. I've had the code checked 4 times and it is always P0306, misfire on cylinder 6.

If I let the car idle for 10 minutes to let the car warm up before driving, I do get some hesitation, but I can avoid the CEL. This is my current workaround.

It is possible that this popping is not related to the P0306. I posted the audio hoping that someone might recognize what the noise is. Then I can try to see if there might be a relationship to the P0306 CEL.

kjr914

No expert, but could the miss be due to plugs/plug wires? Have you taken a look at the plug wires up under there? IIRC, #6 gets lots of heat and is the area (bottom right) that plug wires begin to degrade first due to the heat down there. With the engine running and a dark garage, look for sparking....and I've heard a squirt water bottle on mist might help make that show up. Easier with right rear tire off.

As well, I think #6 has one of the the hard spark plugs to pull and sometime the lower one doesn't get done when "all" plugs are changed.

BesideTheBox

<rant on>
You hit a hot button here. In diagnosing my P0306 CEL issue, I ended up changing all the plugs. Plug I-6 under the power steering pump was ancient. Absolutely horrible. I-6, I-5, and I-4 were absolutely not changed at the 60K service performed at 68K miles just prior to my purchase of the car, and doubtful they were changed at the 60K service performed at 60K! It looks to me like I-6 had 40K to 50K miles on it and I-4 and I-5 had maybe 30K on them. There is 77K on the car now. The other 9 plugs looked about right for 9K miles. Questionable service from an authorized Porsche dealer in WA state.
A picture of the I-6 plug can be seen at https://rennlist.com/forums/993-foru...-p0306.html#20
<rant off>

I have ohm'ed out the wires and did listen and inspect for arcing in complete darkness. So if it is the wires, it is something VERY subtle. I also used a spray mister. I couldn't see anything.

The caps and rotors are only 3.5K miles old and I checked them last week - ohm'ed out good and looked good as well.

On a positive note, today I found that the popping noise was eliminated (at idle, engine warm) when I ran on distributor I by itself. I'm going to recheck this tomorrow.

This brings up what I feel is a really intriguing question - Why would -adding- distributor II make the idle -worse-?

kjr914

Looked at that plug picture....WOW! Triangular in shape after all those miles...

Did you rebuild or replace that broken fuel injector in that thread? I'm expecting I'll find similar dirty injectors this summer when I get to that job.

Ah, a clue!!! hmmmm, can you run only on Distributor 2? Has your distributor belt ever been replaced? When was the last cap & rotor replacement on the car?

BesideTheBox

I pulled the injectors and had them cleaned by Ravenworks in Maple Grove here in MN. They test the injectors before cleaning and they were spraying just fine. The broken pintle cap and the grunge did not affect their operation. So in the end I found out that it was unnecessary to have them cleaned and not the source of this issue. But not a big expense IMO, at $120.


Additional testing on Sunday showed that I basically get the same idle / running behavior when running off of either distributor. Initially, I found that the popping was gone when running off a single distributor; didn't matter which one. And when running on both distributors, I had popping.

Then later, additional testing showed that I got popping when running on a single distributor. and maybe a little bit

IXLR8

Brad,

Some weeks ago, I recommended switching the lower ignition leads for cylinders 4 and 6. Did you ever try that?

Those ignition leads are identical in length and if you can switch them at the distributor cap (II) and at the spark plugs, that might be a test to see if you get a P0304 instead. If so, you just solved your issue.

The upper ignition leads for cylinders 4 and 6 might be harder to switch since their lengths are uneven, but you can give them a try.

Mike J

This could be a stretch but I have seen this once - the belt on the distributor slipped one notch - it showed up as a misfire and a slight hesitation at high RPM's and load. You can check it pretty easily - both distributor rotors should be at the same cylinder at the same time. That can lead to some weird misfires given the spread in the firing between the two distributors.

Cheers,

Mike

IXLR8

Mike, did it slip or was it installed off by a tooth?

I went over that with Brad yesterday via PM. I find it hard to believe that it can "jump" a tooth considering how little slack (none) there is in the belt. In any case, I asked him to check the alignment of the rotors to the distributor case marks. 20° per tooth, by the way.

Mike J

Yeah, it was weird, and I agree that I do not understand how it happend.

I rebuilt that distributor years ago, put a new belt in it, and the car ran for about 6 years fine, pulled like a train and no misfires. The owner started to get some misfires, and a whole pile of diagnostics was done to test it, including new caps/rotors/plugs, etc. It took quite a while to find it, a local shop was checking it out and found that the rotors were one belt notch out. They checked because they said they had seen something similar to another car.

So either i messed up and did it wrong at the beginning, but the car had about 20K miles on it since that happened and was running great, or the belt jumped, which is weird. I did not fix it in this case, but the shop reported that they moved it by one notch, and the car has been running great again in the last 6 months.

The running symptom was a low level misfire at high rpm pushing the car, no CEL's though. Just you can feel it was not running a full power. It started all of a sudden.

Unfortunately, in this case, I did not do the final repair, so i am going by the shops word, but the issue is gone... oh and they said the belt looked like it was in fine shape (it should be with only 20K miles on it).

cheers,

Mike

BesideTheBox

Alex - I did look at swapping II-6 and II-4 at the distributor. The II-6 wire will not reach to the II-4 spot. I will look again, and possibly unscrew part of the plug wire harness. Perhaps this will give me enough length. On the bottom end, if the boots at plugs II-4 and II-6 are not identical, I should be able to swap them on the wires.

Mike - I had looked at the rotors and they were pointing at the same cylinder. I will double check this again as well. I do not have misfire or hesitation at high RPM or load. I DO have hesitation if I drive away while the engine is warming up and this will lead to the P0306 CEL.

Thank you for the suggestions guys.

I had called the local Porsche dealer last week to see what they might be able to do for me. The service manager suggested right off the bat that my coil could be bad. I have ordered a Bosch coil to swap in and see if that is the issue. So either the coil is the problem or I will have a spare sitting on my shelf.

BesideTheBox

I ohm'd out the two coils yesterday.

Cold:
7.9K ohm from either screw-on post to the high voltage post
.5 ohm between the screw-on posts

Warm - a few minutes after running the engine:
8.5K ohm from either screw-on post to the high voltage post
.5 ohm between the screw on posts

I could actually see the resistance numbers dropping while I was measuring as the coils were cooling.

Mike J

Yeah. my idea is lame in this case I think.

You can always ask another Rennlister in the area if you can borrow a coil for a hour or so. Quick swap, sounds like you can duplicate the problem so it's easier to diagnose.

Cheers,

Mike

IXLR8

Brad, if it doesn't reach, don't bother. Trying to get to the hold-down screws is a pain. It was a thought.


Strange! According to my notes and this based on the original harness on my 993, the bottom spark plug caps are all identical.

Spark Plug Caps

Upper 1-2-3: 911 602 314 00, Beru 0 300 322 109, 3KΩ.
Upper 4-5-6: 911 602 313 00, Beru 0 300 322 108, 3KΩ.
Lower 1-2-3: 993 602 313 00, Beru 0 300 322 116, 3KΩ.
Lower 4-5-6: 993 602 313 00, Beru 0 300 322 116, 3KΩ.

You did make sure that the ignition leads are fully threaded into the caps.? Careful...owners have twisted the wires right off. Use some silicone spray under the lip of the rubber where the ignition lead enters to reduce friction.

I'm curious to see how/why a bad coil that should affect all cylinders, only affects your #6 cylinder.