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03 C4S #4 cyl misfire/rough idle...sometimes

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Looking for some guidance:

Check engine light comes on (no noticeable roughness, noises etc.), drive car to Indy, code reset and car runs fine.

Next day, all is well driving around, park the car for a while and starting up again there is a noticeable rough idle, car does not want to accelerate.

Limp the vehicle to indy. Code shows #4 cyl misfire.

Plugs are coming due, so are replaced. Same problem. Coil pack swapped. Same problem. Injector changed. Same problem. Periodically, the car runs fine again, Accelerates fine all through the gears, then when coming down towards idle (after driving for 30') it happens again. Upon further investigation, some oil noticed in AOS, so it gets replaced. Car runs better, but housing to the AOS gets cracked during install (brittle plastic) and car runs too lean. After replacing, car runs fine....for a while. Again same issue recurs. Cat's checked, still fine. Initial compression check came back a bit low, but leakdown was perfect.

It is very intermittent and only affects the car in lower rev range when it does happen. Stopping and restarting mostly clears the problem. Most mechanical issues should be permanent problems, yet the car drives like a dream between 'flare ups'.

Any ideas?

MAF, variocam, leads?


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This is a difficult problem indeed. I see you took the advice and replaced the injector. Sorry that didn't work out.

From here I would inspect the wiring, both visually and with an ohm meter. The impedance between the positive injector lead & coil pack lead to the DME should not differ dramatically from the rest of the non-misfiring cylinders. There should not be a short to ground. The connector should be inspected as well.

Have you boroscoped the cylinder?

Have you repeated the compression test with a squirt of oil in the cylinder (if the numbers on 4 are slightly less than the rest) ? (rings or <cough> slipped sleeve)

Have you tested intake vacuum (intake vacuum can tell a lot of stories)?

What does the plug in #4 look like compared to the rest?

Can you post some numbers from the DME at idle? Post everything you can, including fuel trim, maf readings, etc.

Can you post the compression and leak down numbers?

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If the engine is already equipped with variocam plus, check out the hydraulic tappets of the corresponding cylinder. It may be that one of the tappets remains blocked in the full lift possition.

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I concur......

recently changed an adjuster because of a sticky plunger.....

also just had a cayman with intermittent rough running.... boroscope revealed bank 2 cylinders scored to sh!t! ( the oil burning + knocking didnt bother him untill it started running rough ) go figure.

hate to give worse case scenarios but these are common realities with these engines.....

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Thanks for the replies.

I will pass along to my indy and report back. I'm not much of an engineering type and appreciate all the insight.

What I should have made a bit more clear is that while the problem occurs mostly at random, it pretty much only occurs when the engine rpm decelerates through 4000-3000 or at idle. Not at random rev ranges.

To me anything mechanical/wiring should behave more randomly so I'm guessing it should be along the lines of DME or ECU.

I'll report back as I hear more. Thanks again

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Troubleshooting continues. Next will be pinning out wiring harness.

Sales guy came by the shop with new type of British OBD reader for testing/sales. When hooked up to my car, with codes P300, P304 it suggested to replace DME.

So, hopefully we're in the final stretch. Update to come soon...I hope.

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After a significant amount more of troubleshooting, the electronic side of things has been fully checked and all deemed serviceable, so covers are coming off to look for a bad lifter or broken valve spring.

Hopefully more in the next few days....

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Thanks for the update, I hope it's something simple in the head. Did you boroscope before pulling the cover?

I believe he did, early on, didn't find any evidence of anything obvious gone wrong. Getting the cover off will be final step. This whole thing has been fairly frustrating so far for both the mechanic and myself due to very odd nature of the fault.

The fact that there are no spare parts in our area (Porsche dealer seems to carry nothing on the shelves) leads to additional wait times for everything. That being said, I'm extremely happy with my indy for not throwing parts at it randomly in search for the fix and using due diligence. While it took a bit longer, he helped me save good money.

While everything else on the car is near perfect, I'm seriously considering selling it when it's repaired. Hard to go without your primary vehicle for over a month and I can't afford to wait for parts all the time if something breaks....

Love the car and got a tough decision to make.

Will keep you posted when I find out more.

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I'm not sure where you are located, but in the USA and UK there are many many options aside from the local dealer, who offer much better pricing and availability than what I've read of most other places around the world. GL.

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umm i know that you guys are already into taking apart the motor... but I just had a situation with another brand of vehicle where a whole bunch of work was done to a vehicle to solve a missfire... and it came back with a missfire still... turned out additional to everything the same cylinder that originally had the miss, ironically got a BAD NEW spark plug causing a intermittent miss.......

I know that someone mentioned how the plug looks on the questionable cylinder but i didnt see a response... I know your tech will hate you especially for all the forum advice :) but just make sure the BASIC basics are covered.....


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I believe the OP started a thread over on RL.com and one of the steps they went through was new plugs, and possibly swapping plugs between banks as well. I could be mixing him up with someone else though.

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Thanks for all replies.

Finishing touches to the car today: it did turn out to be a stuck lifter. As the cover was off, replaced all of them to avoid a repeat on that side anyway. Indy said that other than the lifter, the engine looks extremely good with 80000kms on it.

All work was done with the engine in the car. Overall I got a new AOS, 1 new injector, new plugs, the lifters and a few extra parts that did not survive the removal (brittle plastic). I did not receive the bill yet, so can't say what total cost it will be. I'm guessing it wont be cheap.

Again thanks for all the input, certainly learned a lot over the past few months.

Edited by KrisKringle
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