My car is stock and when I bought it the car did not feel like it boosted.

I changed the N75 valve to one I bought of Ebay and the car did boost and was very fast but I started getting boost overpressure codes.

So thinking the problem could be with the EBay N75 valve I bought a valve from the dealer and I'm back to feeling like I have no boost.

So basically with the Ebay valve it will overboost and with the genuine Audi valve It feels like no boost.

I have removed the cat and checked that the wastegate valve is intact. The car has no codes unless I put in the Ebay N75 valve in which case I get boost overpressure codes. It did previously have a code for low cat efficiency but I put on a new aftermarket cat which fixed that.

I have vcds but I don't have a boost gauge.

The car is a 1999 A4 1.8t Quattro with 160K miles auto. No other problems from the engine.

Any ideas on where to start looking at this problem?

Thanks,

Andrew