b5 1.8t quattro major issues *PLEASE HELP*

[bean_machine88]

hey there, I know this ain't a passat question but as their the same engine and chassis and what not I was thinking you might have some knowledge onwhat's going on, my friend has a x plate Audi a4 1.8t quattro, 180bhp model, at the moment car is not full boosting, and ain't throwing up any engine management faults, but on that note the car won't let us connect to the ecu to run a diagnostic test, so far we have changed intake pipe all vacuum pipes n75 valve recirc valve and so on tried running the car unplugging the maf car runs majorly worse, unplugged n75 valve plug car run exactly the same with only half boost, don't seem to be in limp mode as car runs and drives fine still hitting high speed just not full boosting, any information will be great on what steps I should take next, thank you