Boost tailing off too early and goin too low!

[s2tommy]

Hi I am after alittle help again! After yesterday on the rolling road I found out that the cars boost was tailing off too early and goes too low!! I should get 2.3 bar and tail off to 2 bar to red line! I am gettin the 2.3 bar but coming right down to 1.4 - 1.5 bar
What to check for?? So far I have done,

Carried out full! Boost leak test. All ok no leaks at all
Scanned ecu no fault codes at all
Removed DV an checked, ok

Inline next this afternoon is
Intake pipe off an check to see if these any turbo play!
Waste gate off and check over
And when friend arrives later borrow his n75 for a test

Anyone got any ideas what else to look for?? Cheers tom

[Rusty]

Based on how modified your car is, you'll need to provide as much information as possible.

I have a few questions - what WG are you running, what WGFV, who mapped the car and have you made any changes to the car since it was mapped?

[prj]

I mapped it, and actually opened the map right now, and it used to hold 2.1-2.2 bar to redline. It's mapped to 2.3 taper 2.15.

[Rocco]

99% wastegate worn,more if your wastegate spring is harder than the stock one...and produce like 1.4bar mechanical boost

[Jamo]

On my car I had this and it was the wastegate, I managed to get round it with a 4 port solenoid and controlling top and the bottom chamber, eventually I moved to a non Audigate.

I know however Doug doesn't think it's the wastegate.

The other thing have you checked you vacuum and pcv system for leaks?

[s2tommy]

Based on how modified your car is, you'll need to provide as much information as possible.

I have a few questions - what WG are you running, what WGFV, who mapped the car and have you made any changes to the car since it was mapped?

I am running a stock waste gate and a stock n75
Yes waste gate is many years old an n75 about 2 maybe 3 years old
I had striped cleaned an checked over waste gate if not 2years ago but won't rule it out!
As anything is possible with these cars!! Lol

[Rusty]

Coolio, always worth getting the full picture up to focus peoples' responses.

WG does indeed sound likely, just make sure that if you get a replacement, you get the same bore, as there are at least 2 sizes, as Newsh found out.

[s2tommy]

Right I have strip her down and removed the turbo intake hose, there is no play in an out on the turbo but there is up an down movement!! About 1-2mm but not enough for the comp wheel to touch the housing! Would this cause the boost tailing off??

[s2tommy]

So I have spoke to universal turbos where my unit was brought from, the bloke said coz its a jernel bearing there will be slight play up an till the oil gets in there! Which I through. There is still good clearance from blades to housing so I think the turbo is ok??!! But not sure??

I have removed the waste gate completely and striped it right down! I can't see anythin wrong with it!
Found valve to move nice an freely once removed the spring
Diaphragm is perfect, and quite clean still from last strip down
The valve is seating good and seat looks all good no cracks founds!

Now there is abit of clearance where the valve slides through the housing, But take it this clearance gets smaller as it gets hotter?? Maybe wrong?!

So when people say waste gate what does go wrong with these?

[prj]

Guys, when you are suggesting things, don't forget that this car has run properly and made the required boost in the past.
So it is not a fundamental flaw of something not operating at all - I have logs of this car making more than 2 bar boost to redline each and every time.

Tommy, I pm-ed you two things to check with VAG-COM... might want to do that before you take the whole car apart.

[A80Avant]

Wastegate piston seems correct to me.it needs to have some play to accommodate heat expansion. I had one rebuilt to very high tolerances and ended up sticking and keeping the valve open.

[Rusty]

In terms of the wastegate's part in boost control, under target boost is caused by too much boost getting in to the lower chamber from the N75 or the spring exerting a lower pre-load on the diaphragm than it once did (unseated or broken). A split diaphragm would see more boost than expected.

Have you re-fitted the N75 recently? Might be back to front...

[prj]

Btw, I think the throttle is not opening fully or the altitude sensor is shot.

[s2tommy]

Btw, I think the throttle is not opening fully or the altitude sensor is shot.

I have not forgot theses checks just got to wait till later when a friend comes over with vag com.

[Jamo]

If the throttle isn't fully open how do you get max boost at the start? my understanding of the ecu was unless it got the code for WOT that it wouldn't fully boost