So my car boosts to only about 3-4psi, which is atleast half of what it should be making stock. I read somewhere that the n75 could be the problem and that you could check it by removing the hose going from the n75 to the wastegate.
So I did that and went for a ride. Slowly started giving it gas and it boosted way up to 9 psi, didn't try going any higher just in case. Figured the n75 was at fault so I ordered a new one, installed it, yet my boost still sits at 3-4psi.
Also tried just disconnecting the wires from the n75 (old one, haven't tried it with the new one if that makes a difference) and it barely has any boost in it, around 1 psi, 2 if i'm lucky.

What else could be the problem?

edit: So, I disconnected the wires from the new n75 and went for a ride, boost was the same as when I had the wires connected, except it dropped down to <1 psi at redline. Makes me think this is an electrical problem. Checked the connections and they seemed fine. Also ordered a vag-com so that I can finally check the codes and see what it gives me.

 

So I scanned the car with vag-com today and got the following codes:
17967 - P1559 - Throttle body (j338): fault in basic setting.
17973 - P1565 - ?
16500 - P0116 - ?
16518 - P0134 - Oxygen (Lambda) sensor b1 s1: no activity
17978 - P1570 - ?
01262 - ?
Scanned it with unregistered vcds-lite so it didn't give me the explanation on what some of the faults are, just the codes, neither did it let me clear the codes.
Thinking the pre-cat o2 sensor is bad, anything else I should check before I go out and buy a new sensor?

 

coolant temp sensor?: 16500/P0116/000278 - Ross-Tech Wiki

and a new key? dunno what that second to last one is: 17978/P1570/005488 - Ross-Tech Wiki

The first two seem to think your throttle body is all carboned up, can you take it out and clean it sometime soon? I think on your 1.8's it's not too bad.

Last one thinks you have a short somewhere: 01262 - Ross-Tech Wiki

 

Managed to clear 4 of the codes, the 2 throttle body codes stayed, so I'm gonna take a look at it soon. After a 20km test drive none of the other ones came back. Probably gonna have to drive her around for more and then check again.

Could the last code (01262) be from the old n75 which I swapped since I did not clear the codes when I swapped it? The whole wire back to the ecu seems fine, nothing broken or anything, connector was also looking good, pulled on the wires at the connector a bit and it did move, but it moved the connector as whole so I'm guessing it's fine. Or can the short be inside of the ecu (casing) itself?
Also, is it possible for my car to be in limp mode due to either the sensors or the throttle body, and cause low boost that way?

 

I'll be honest, I have a 2.8 so I know just about nothing about the turbos but I don't think your car can go into limp mode due to a bad throttle body. I think it has to be the boost issues.

Have you pulled off the ecu cover and looked inside? Was there any water? Did you ever have the car chipped?

 

As far as I know the car hasn't been chipped. Will try pulling the ecu cover off tomorrow when I get back from work.

 

Haven't gotten around to check the throttle body yet, been raining alot here and I've been busy with work.
Anyways, coming home from work today, I noticed my boost gauge was sitting on 0. No vaccuum, no boost, nothing. I could still hear the DV though but the gauge didn't move at all. Drove around for a while to see if I could figure out what was wrong with it. After a while, the vaccuum started coming back, however, when I released the gas pedal the vaccuum pressure didn't drop down instantly, it went down very slowly. Drove around some more, and now the boost came back too. Could this mean there's a vaccuum leak somewhere and it magically fixed itself?

Running out of ideas now, will probably take it to a shop next week and see what they say about it unless I figure something out.