The car is an A4 Avant which has done 90,000 miles and has just developed a flat spot at 3000rpm, below this speed all seems well but as the revs rise after the flat spot the engine does not seem as brisk as it once was. Does anyone have any suggestions about the possible cause? Air and fuel filter changed 10,000 miles ago.

All input appreciated.

 

check boost, thats the point that turbo should start spooling on gas and on diesel should be runnig full at that point.

any black or blue smoke?