I recently changed the leaky valve cover gasket; also changed the cracked vaccum hose for the fuel pressure regulator. Now the vaccum is good and normal at idle around 20-21inHg; however i am having some boost problem. the car has GIAC chip, evo motorsports intake, upgraded DV, APR bipipe, catback with custom 3" piping, turbos are recently replaced from Audi dealer.

stock mode:the car peaks at 10-11 psi for the second I gas it, it drops so fast to around 5 psi for a while and if i gas it more it goes to 10psi again and back to 5psi or so.

GIAC mode: the car peaks at 20psi when i gas it, it drops to like 10psi or so for a while and then back to like 15psi and then taper down.

The boost just doesnt want to stay. I know it will always taper down but i dont think the boost will drop that fast?
is it because my N75 valve is going bad? if so will it trigger the CEL because I dont have the CEL on.
I will try to get a video camera to record the boost action
thanks for any input.

 

here is a link to the video that i just took.
i held steady on the gas pedal on 4th gear, notice the boost pattern...
http://www.lamsight.com/test/boost_pattern.mpg
thanks

 

Your boost is doing that when you are steady on the gas?

You NEED to log block 115 with Vag-com & see what the requested boost is. You might be getting a little throttle cut, have a misfire, bad N75, vacuum leak, dirty MAF, etc...

Also check for any stored codes.

 

Ok i did a research and came across to this thread from S4 Dan, that issue is somewhat similar to mine.
I already bought a MBC and im thinking to installed it standalone instead of parallel to the N75, so that i can eliminate the faulty N75.
I know that i will just have to connect the two lines from turbo and wastegate and leaving the recirculate hose alone plugged to the N75, but do i also UNPLUG the electrical plug onto the N75? or do i just leave it alone as well?
i just have the ebay cable with shareware ver of vag-com so i cant really do data logging. I also plan to purchase the TCD from vast if needed to.

as far as for the dirty MAF, i think i should unplug the MAF and see if it will do any better? is that right to test the MAF?

another question about the vag-com software, I recently scanned for codes and i was trying to clear the stored codes but it said i need the "professional interface" or sth like that, i already have the 409.1 ver, so what ver will be the "professional interface" program?

thanks

 

it seems running more steady after i put in the MBC standalone. it still spikes at ~20psi and back down to 15-16psi consistent. so i think i have a faulty N75.

however i still have problems with vag-com that it doesnt allow me to clear DTCs... it prompted "Professional Grade Interface required"

anyways thanksa lot 2kS4

 

Or the was engine was requesting less boost for a reason like 2k said?

 

i have these stored codes.

17584 -P1176 -O2 Correction Behind Catalyst,B1 Limit Attained
17956 -P1548 -Boost Pressure Control Valve Open
18058 -P1650 -Data Bus Powertrain Missing message fr.instrument panel ECU

so i guess i really had a faulty N75. what about the rest of the two codes? howdo i fix them?

 

Yea, it sounds like a failing N75.

17584 can be a failing O2, or some leaking injectors. My bet is the o2.

Have you disconnected the gauge cluster?

 

Oh, sell the eBay POS, & PURCHASE A REAL VAG-COM! It will pay for it's self! You need logging, & be abel to clear codes!

 

17584 can be a failing O2, or some leaking injectors. My bet is the o2.

so is it the O2 sensor located after the cat? does B1 mean Bank1 (driver side) ?

thanks