A friend of mine has a 2000 S4 Tip that runs great off idle and on the highway, etc. But every once in awhile dosent want to rev up while cold, let it sit and warm up and it's fine. I think it's a misfire, but it's not setting any codes. I checked boost leaks there are none, the plugs look fairly new, and compression is about 180psi across the board. Any ideas?

 

do you know if it is a misfire or could the throttle actuator be a problem.

 

Check for fault codes perhaps?

 

I would have guessed a sensor, maybe coolant or MAF but I don't think they would affect it when it's cold.Pretty sure only in closed loop would they have an effect.. That's strange but it's probably something simple. Sorry, no help here.

 

That's the thing, there are no fault codes present which is what is completely throwing me because it runs horrible when it acts up and then completely clears up in a few minutes.It almost has to be something with the closed loop fuel injection, but ususally that would point to an o2 sensor and I'd think it should set a fault code.

 

does he have aftermarket software in the car?

 

 

My thoughts are MAF & coolant temp sensor.

 

Here a good question, are the tip/6spd maf's any different? I have a spare maf for my 6spd that I could try and swap in and I might even have my old coolant temp sensor.

 

Same, if they are Bosh. Same if they are both Hitachi.