I am having problems with misfires only on drivers side at RPMs over 5000. Dealership shows misfire codes, secondary cat readings seem to be OK and similar to each other. Having trouble diagnosing problem.

My '04 S4 with 85,000 miles has been starting somewhat hard, has to be pushed very hard to get over 5000 or so RPMs, gets misfires on 5,6,7,8 but only at high RPM while pushing harder on the accelerator. MIL flashes for a moment when this happens, then light stays off until you try to push high RPMs again. Misfires are more on 6, and 8, but all on drivers side. Passenger side is fine. My gut tells me bad cat. Any ideas? I did have a coil problem with 5 and 8 in January and the exhaust smelled bad, too rich at that time. Light was flashing at that time. Replaced all 8 coils, its been fine until now. Could I have cooked cats and took til now to see it?

I am tempted to replace entire exhaust with APR, but hate to spent the money if it will not fix my problem. Please help!

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if the engine is out of tune up; excess fuel entering exhaust; oil or anti-freeze entering exhaust; bad spark plugs or spark plug wires; bad 02 sensor.. those are some of things which my cause the catalytic converter to fail.. so i suggests that you check on those things and inform us what other things did you find out.. good luck

 

Is it on every cylinder, or bank? It might be the EGT sensors, or failing O2's.

 

Guys, thanks for responses. Yesterday, I dropped the driver's side down pipe. When disconnected from the cat back system, I started car, very little exhaust was coming out the drivers side downpipe. Lots of air coming out passenger side downpipe. I dropped driver's side, found primary cat looked fine. Blew compressed air in both ends and through O2 sensor holes. Turns out the secondary cat was almost completely clogged. Temp repair for now, new exhaust on order. Even with the clog removed sensors are up line, so engine light does not come on, and car runs great.

 

Good to hear. How was your gas mileage with it clogged?