I have a 1999 A4 1.8T with a stage 1 APR ECU upgrade. The car runs fine except for when it is under near to full boost when it stumbles and misses until you let you foot off. The ECU never throws a code though so I don't know what the the trouble could be. Does anyone have any ideas?

 

I had a very similar problem (but no upgrade). Took it to the garage and no codes came off so they tested the pressure on the fuel pump which ewas pushing out 2.5 bar instead of the required 3.5 bar. They suggested changin the pump but didn't know if it would fix it so i left it.

About a day or two later the indicators stopped working then the elctric windows so i had the alternator/battery tested (the battery warning light had not come on). Turns out the alternator had gone so had that replaced and bobs your uncle, full power and no more stutter. Turns out the fuel pump was the first symptom of a weakenning battery. The light didn't come on because the battery was a good oone and well charged so took a long time to run down.

Alternator is alot cheaper than a fuel pump.