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Misfire when wet

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Hi all,About two weeks ago I got the car washed at a touchless place near my house I've been to many times before. On the way home at a stoplight it started idling rough (at first I thought I might have blown an engine mount). The engine was shaking the whole vehicle. Under power, it sounded like the clatter of a diesel. Through a check engine light briefly. I got home parked it, tried it the next day, ran perfect.A few days ago I was driving in the pouring rain from Madison to Milwaukee (about 80 miles). After about 60 miles of running great, it suddenly started running rough, and the CEL came on. I got off the interstate and parked it for about 15 min. Started it up again, seemed to run fine, but CEL still on. I got home parked it, and haven't started it since. I plugged in my Durametric and got codes for misfire on cylinders 1,3,5. Car has 76,000 miles, I replaced the plugs (but not coils) at about 60,000. I believe 1,3,5 are all one side of the engine, correct? I do have an IMS guardian, it's no where near going off (it gets progressively louder with more debris) so no one say IMS!Any ideas?Thanks in advance and thanks if you read everything.

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I would guess coils too although hard to explain why all from bank 1 fail the same time. If anything wrong with bank 1 timing, you should see other cam deviation/ sensor codes too. How are your cam timing readings from Durametric?

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As Ahsai mentioned the coils packs are very common. When the heat shields are removed you can often see the plastic covering split and cracked which is a good indication that its time for replacement. Also moisture causing a missfire is a good sign as well.


Edited by EleCTriCT
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Coils was my thought, but it seems weird that all of a sudden 3 of them would go bad.

They probably already were bad; the multiple moisture exposures just pointed that fact out. We often find coil packs with small cracks and often electrical tacking marks as well, so this is nothing unusual.

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  • 4 years later...

Reviving an old thread here. I'm replacing the coils in my 2003 C4S and I'm seeing this part number as the correct part on Sunset:  997 602 104 00. Is it true that Beru are the OEM parts? Are any aftermarket coils worth the $ savings over original Porsche parts? 

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Go Porsche numbers for the coils.  Seems this is a classic moisture caused coil failure.  Not cheap, but not horrible.  Change them all out, with new plugs too.

Good luck.  And, sorry to say, please don't go through the carwash.  Too much moisture can get into the engine compartment.  And carwash tracks can rip up the soft underbelly of the beast.

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there is a TSB for these coils, 996/997. they all crack, updated version are thicker, and bolts length are longer, beru is the oem manufacturer for 996 and part of 997. Theoretically they are same as genuine ones. original parts number is still available, but only for no6 cylinder on turbo. you have to order new version for most 996/997. genuine one you have to order bolts as well. aftermarket come with the bolts in box.  

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