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Rough Idle and Exhaust Cough/Sputter

  • 1998
  • 911 Carrera S
71 000
San Francisco, 

I have a 1998 Carrera S that exhibits a rough idle and sputter/cough sound coming from the passenger-side tailpipe once the car is warm.  There have never been any CEL errors. 

From the interior of the car, when it is idling roughly, I can hear a loose rattle, possibly coming from the transmission/flywheel/clutch region. There is also a bit of a rattle coming from this same area (briefly) when turning off the car. 

The car has exhibited this behavior from its purchase 8-years ago (34K miles) to now (70K miles). To my knowledge, the clutch and dual mass flywheel are original. The spark plugs were changed at 60K miles -- no change in the sputtering/rough idle behavior. The ignition wires are in good shape. ISV is operating normally, no vacuum issues. 

Thoughts on what might be wrong, how to properly diagnose and then fix?

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