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My 2008 Cayenne S begin running very roughly so I plugged in my OBD scan tool and got a "Cylinder Misfire" on cylinder 8 and a pending "Cylinder Misfire" on cylinder 1. I replaced both the ignition coils and plugs less than a year ago.

I pulled the coil and plug on cylinder 8 and the plug as totally fouled. I didn't have a replacement plug handy so I cleaned it as well as I could and replaced it. That didn't work, still got the "Misfire" error codes.

I ordered a new set of Bosch FGR5NQE04 plugs and installed them. All the plugs with the exception of cylinder 8, which was again totally fouled, and cylinder 1 which was dirty, sooty black, but not fouled, looked normal.

The car started normally with no cylinder Misfire error codes.

I don't think it is normal for modern spark plugs to fail.

My concern is that the valve cover gaskets may be failing and is allowing oil to get to these end of the block spark plugs. Perhaps the fact that cylinder 1 and 8 are at the end of the block may be a place where the valve cover may start to fail?

There was a comment in the Pelican Parts tech article on replacing ignition coils about a plug getting fouled due to a valve cover gasket leak.

Has anyone had this issue before? Is it reasonable to assume that the valve cover gaskets are failing and to proactively replace them? Or, should I just drive the car and see if I get a "Cylinder Misfire " error again?

Does anyone know the the typical mileage or age of a Cayenne when the valve cover gaskets should be replaced?

The car is a 2008 Cayenne S with about 148,000 miles on it.

Thanks,
 

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I have a 2012 S Hybrid that experienced a cylinder misfire error code (engine vibration, reduced power). Original owner 202k miles on the odometer at the time of the problem. Cylinder 2 plug was heavily fouled. Other plugs were dirty but ok. Book maintenance is plugs every 80k miles, but I’m told by my independent Porsche specialist shop that a technical service bulletin has updated that to every 40k miles for engines with forced induction that have over 100k miles (the hybrids are supercharged vs turbos for other models). Cylinder 2 had some bore scoring which they said is the culprit in my case. Was told to change my plugs more regularly and just drive it until it dies. Car only fetched $4k offers on trade-in so I kept it. I’m only using it on local drives now anyway so not really putting much mileage on it.
 

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My 2008 Cayenne S begin running very roughly so I plugged in my OBD scan tool and got a "Cylinder Misfire" on cylinder 8 and a pending "Cylinder Misfire" on cylinder 1. I replaced both the ignition coils and plugs less than a year ago.

I pulled the coil and plug on cylinder 8 and the plug as totally fouled. I didn't have a replacement plug handy so I cleaned it as well as I could and replaced it. That didn't work, still got the "Misfire" error codes.

I ordered a new set of Bosch FGR5NQE04 plugs and installed them. All the plugs with the exception of cylinder 8, which was again totally fouled, and cylinder 1 which was dirty, sooty black, but not fouled, looked normal.

The car started normally with no cylinder Misfire error codes.

I don't think it is normal for modern spark plugs to fail.

My concern is that the valve cover gaskets may be failing and is allowing oil to get to these end of the block spark plugs. Perhaps the fact that cylinder 1 and 8 are at the end of the block may be a place where the valve cover may start to fail?

There was a comment in the Pelican Parts tech article on replacing ignition coils about a plug getting fouled due to a valve cover gasket leak.

Has anyone had this issue before? Is it reasonable to assume that the valve cover gaskets are failing and to proactively replace them? Or, should I just drive the car and see if I get a "Cylinder Misfire " error again?

Does anyone know the the typical mileage or age of a Cayenne when the valve cover gaskets should be replaced?

The car is a 2008 Cayenne S with about 148,000 miles on it.

Thanks,
While the plugs 1 and 8 are at the ends of each head, they're at opposite ends. When you pulled the coils and plugs, was there any oil in the sparkplug well? If not - you can discount your first guess.

When you say plug #8 is "fouled" - can we get a more detailed description, or even better a photo?

Cylinder #1 sounds as if it's running rich, might be a sign of a leaking injector. #8 sounds like bore-scoring, which wouldn't be a surprise given the mileage on your Cayenne. I'd do a compression test on all cylinders. They should be +- 5PSI of each other. Any outliers indicate a problem.

Without at least a compression test you're guessing what's wrong. That's the minimum diagnostics needed at this point.
 
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