Does anyone know the most common problem associated with a P0171 error code on a 2003 A4 turbo(frontrak)???.

I am thinking O2 sensor, (pre cat)..... Or MAF.. !!
Any direction is appreciated... I cannot find anything physical (i.e. loose intake hose, vaccum leak).

 

Vacuum leak.

 

Try and see if you can easily remove the oil filler cap with the engine running and let us know the result.

 

I can remove the cap and the engine continues to run. No change in engine behavior. Of course you here the engine crank case flutter and feel the crank case pulses if you place your hand over the oil cap hole. I have verifiedthat none of the hoses are looseanywhere. (ie. turbo inlet, engine case breather, Evap. system).

 

I did notice that when the engine is running (idling) I can hear crank case pulsescoming from the breather/filter air box. I traced a small rubber hose going into the airbox from what looks to be coming from the back of the engine. Maybe even the back of the valve cover. If I pinch the small hose going into the air box, the noise goes away. This is all done after the engine has had sufficient time to warm up. Could this be my problem?? Could these pulses of air going into the air box be screwing with the MAF and giving me this error code?? Also, if I borrow a code reader and erase the codes, when I start the car it will run rough for awhile at idle, then work itself smooth, then the check engine light comes back on after awhile...