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I have a 2003 C4S with the X51 package that I purchased last November. I use the car as my daily driver so I've put about 11k on it since purchasing. The engine has always run a bit rough, like an old V8 with a bigger cam than stock. Kind of a smooth - shudder - shudder - smooth cadence. The motor pulls hard but I feel the shudder even free rev-ing the engine and trying to hold a steady RPM. 

 

At idle, when it's doing the shudder thing, the RPM's jump imperceptibly on the tach but I can see them bounce through the OBD reader. The plugs and coils have been replaced for other reasons and I've used Liqui-Moly injector and valve cleaner several times with no effect. There are no codes in the ECU, no misfire or cam or injector codes at all.

 

My local Indy repair shop thinks it may be the vario-cam not fully disengaging. Oil's been changed twice in my ownership but I've not used any lubrication modifier or anything like Sea-Foam to mess with the oil.

 

Any ideas? The vario-cam thing seems likely but before I spend that kind of time or cash I'd like to be sure.

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I would check the MAF values.  Often, when a MAF is either very dirty, or on its way out, rough idle is a common complaint.  The VarioCam system can be fully tested with a Porsche scan tool like the Durametric system (which can also check the MAF values); I would test that before even thinking about replacing any components in the system as they are all rather $.

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