Rough idle - which cylinder is it?

[iamtheari]

My 2004 Cayenne Turbo idles rough, sometimes. It seems like a single-cylinder misfire. I used a tap tempo app to approximate the frequency of the roughness, which manifests as what I can best describe as a pulsing roughly like you'd get with an annoying 21" subwoofer pulled up next to you at a stop light.

The pulses are random, but this morning there was a steady enough stream of them to count it out. I came up with 250bpm, and the idle speed was a bit under 600rpm at the time (the tach drops slightly with each pulse, to maybe the 500 point on the dial). As a guitarist, I probably tapped off-tempo and I am really getting exactly one misfire per two revs of the crankshaft.

That tells me it's a single-cylinder misfire. It is mostly random. It happens when the car is warm. I don't think it happens when it is cold. I only notice it when I get to the first stop sign on my way to work or home. It persists when I shift to neutral.

Research indicates coil pack failure or spark plug fouling (secondary to coil pack failure) are the likely causes. Are there others?

And, other than replacing all 8 coil packs and spark plugs, is there a way to track down which cylinder is not doing its job? I do not have a CEL. I do have a GoPoint BT1A and an iPhone to speak OBD-II to the car if that would help.

[island911]

I use the Durametric tool for diagnostics - it gives just about any info you could think of.

Else, you could take the cosmetic covers off the coilpacks, go around unplugging and repluging the leads. Each time it should run worse. ...until you unplug the one which isn't working.

Or you could do the same with the injector leads.

[Cajundaddy]

Sounds like a cracked/arcing coil pack. They usually are visibly easy to spot so I would start there by inspecting each with a good light and see if you find the black sheep. A 2004 with original coil packs and plugs is due. Good hunting!

[Nostril Cheese]

It's likely a coil pack. They fail all the time.

It's much easier to use Durametric or the PIWIS to determine which cylinder it is. Here's a pic showing the cylinder layout.

[Nostril Cheese]

One other thing to check are the bellows that come off the duct feeding the intake manifold. These can also cause a rough idle.

[iamtheari]

Thanks for the cylinder layout. Mirror image from what I would have guessed. And if I had it to do over, I would probably have bought a Durametric Professional instead of the 928-ecu-repair kit for my 964 plus the GoPoint BT1A.

It sounds like the coil packs all go bad if you give them enough time. My car has 130,000 miles. Other than the $350 price tag, is there any good reason not to just replace all 8 spark plugs and coil packs now? (Even with that and the other things I've done in the past 6 weeks, I'm still in the car for a good price.)

[Nostril Cheese]

Porsche is up to about the 6th or 7th design revision on the packs. Might not hurt to just replace all of them, however Id also check the obvious stuff for leaks (boots, hoses, etc..)

[iamtheari]

Any particular spots to check? Leaks before the throttle plate should not have an impact on idle in my (admittedly limited) understanding of how fuel-injected engines work. I have taken a couple of the hoses apart in my quest to inspect the water pump and they're all fastened together nice and tight as far as I can tell.

The "hard" stutter makes me think it's a misfire and not a vacuum leak or other idle-seeking behavior. But for $350 I'll spend some time checking for anything obvious.

[Nostril Cheese]

Did you open the cooling system up? If so, you'll need to bleed it. Like I said, check the bellows at the front of the engine for cracks.

[iamtheari]

Assuming that you mean the bellows on either side of the throttle body, just before the two paths come together, they are both in good condition.

I did open the cooling system up as far as replacing the expansion tank, but no further. I plan to flush and fill the coolant soon, as while the expansion tank was leaking I was forced to resort to coolants which nobody can say are incompatible but which may nevertheless be, and lots of distilled water, resulting in an antifreeze level that will not get me through the winter. But as it sits, there should not be any air in the lines.

[island911]

Originally Posted by Cajundaddy

Sounds like a cracked/arcing coil pack. They usually are visibly easy to spot so I would start there by inspecting each with a good light and see if you find the black sheep. ...

That's what I used to think. But I tore one apart. The cracking is not cracking of insulation. It's just a very thin cosmetic cover and sealing surface, as far as I could tell. No signs of arcing there either. I'm fairly certain that these things are failing internally.

Yeah, an 04 will definitely start needing new CP's by now. I've replaced all but one.

just another thought, don't sweat using the earlier dash numbered coil packs. Considering how long even the -00 lasted, and the MUCH better pricing on any of the superseded dashed CP's, the econ math is easy.

[iamtheari]

I replaced all the coils and spark plugs last night. Driver side took 1 hour. Passenger side took 2 hours and it was dark and frustrating by the time I finished. Had I done it in the garage or earlier in the day rather than outside after work, probably each side would have taken the same amount of time. One problem I ran into was the bolts that hold the coils in place on the passenger side did not want to go in. It's like the coil wasn't quite lined up perfectly, causing the bolts to drag. Or I cross-threaded two of them despite multiple attempts. And one managed to break off on me, so I get to try to extract what's left of it and wait for the replacement to arrive in a couple weeks. Not a huge deal, though. And the car does start up and run okay. Next time I actually drive it, I'll find out if the rough idle is fixed.

At least the Cayenne is just like my 964: An absolute joy to work on the engine, but an absolute terror to get to the engine enough to work on it.