My son has just upgraded from a battered Lupo to a smart 2000 Audi A4 Avant 1.9 TDI auto. It starts and runs very well, with no engine management lights showing. However, when he tries to put his foot to the floor, it only revs so far and then seems to lose power, trying again repeatedly in a very jerky manner until the throttle is reduced. The car can be coaxed to higher revs and illegal speeds by using softer throttle settings, but this is a blight on an otherwise excellent car.
I have changed the fuel filter and the EGR valve looks new. The EML does not illuminate no matter how many times I initiate the problem. Simon Silvie There is a strong possibility that the problem is not with the engine, but with the ECU control for the gearbox. Even though no EML is illuminated, there may be a code within the gearbox ECU which can be read off and which could pinpoint the problem.
If the fault is related to the ECU, then one element which will fail without leaving a code is the mass air flow (MAF) sensor. A quick test can be made by driving the vehicle with the MAF plug disconnected. This will throw up a code, but if the vehicle drives without the problem, this will confirm the MAF sensor is at fault. The plug should not be disconnected for more than a short test-drive.
Another possibility is that the throttle position sensor is faulty and not throwing up a code. This can be checked using diagnostic equipment.
The gearbox ECU may be the problem.