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CEL - Random Misfires '07 C4S


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Here's the scenario..new plugs and battery over the winter (60K service) and last month, I started having some random misfire CELs. Silly me didn't replace the coils when the plugs were done and I just figured it was that. The salty mess I drive in is pretty rough on the coils, I thought. Coils replaced and problem went away for a couple of weeks. I filled up with a tank of premium in WI (it's cheaper there) at a Mobil station but I'm worried that it might have still be the winter mix and it warmed up quickly around here. My daily driver is now seeing a lot less driving since I'm back to commuting on the train and the car sat for a week or two with only a few miles driven.Now the misfires are back. I've narrowed down when it happens. It's only on a cold engine. The tach goes up to 1200 when first started and engine is running smooth. Then after a minute or two, it drops down to 800 RPM and things get rough. It will even stall sometimes if you don't blip the accelerator. CEL will go blinky and then solid if it's enough misfires. I had random ones and ones on cylinders 4, 5 and 6. The CEL doesn't continue to blink.Once the car is warmed up, she runs as smooth as ever. It's only in this initial cold period that things are rough.I've just filled with another tank of premium, run it down, added Techron and filled up again (went to a more well trafficked Shell station).

Here are the original codes:

P0300P0306P0304P0305P0300P0306P0304These we're thrown over a couple of days anytime I let the engine idle after a cold start. I just disconnected the MAF to see what would happen. Ran really rough for the first minute or so at idle and then completely smoothed out. It threw these two codes, which I assume are due to the MAF being disconnected:P1090P1175 (I couldn't find any info on this code).

I pulled the MAF and used a can of cleaner on it. Carefully reinserted and cleaned the electrical connections. Took it out for a short jaunt and didn't get any additional codes but now the car is stalling at low idle.

Thoughts on what to try next?

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The MAF codes are normal when you remove it.

Have the codes always been on one bank only (P304, P305, P306)?

If so, then you need to look at the electrical connections on the cam adjustment solenoid and if they are okay then check the hall sensor. Sometimes they throw their own faults and sometimes not.

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I tried figuring it out from the part diagrams to no avail. :) Would you be able to point me to a diagram on how to find them? Also, I noticed this weekend when the weather has been warm (>80 degrees) the engine is running a bit rough on start up but not nearly enough to cause the misfire.

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  • 2 months later...

Update! Finally got the car over to my indy mechanic. He was able to root cause my misfire problems. Note my misfires were always on one bank, cylinders 4,5,6. He talked to a Porsche tech and he first thought it was a cracked cylinder! That would have been painful! But one thing mine noticed was that the compression was high on all three cylinders. If it had been a crack, only one (or however many) would be showing a different pressure. That led them to suspect the Variable Valve Lift Control (Part: 997-105-301-00 Solenoid for Variable Cam Adjuster). Sure enough, they swapped it to the other bank and saw the behavior follow to that side. Ended up replacing that part and all is well. The part was only $130 but it was 8 hours of labor to debug and install.

It was going bad just enough so the failure was only showing at low RPM. It cleared up once the car was warm or at a higher RPM.

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