Torn wastegate membrane theory?

[A80Avant]

Can someone explain in simple words why is it that we say that if the wastegate's membrane gets torn, the boost creeps at a high level?

I was thinking... If i have a tear on the membrane, then the bottom and top chamber are equalised, but the top chamber vents to atmosphere, and therefore any attempt from the N75 valve to pressurise the system is failing...? Hence, i should only be seeing spring pressure in that case?

what am i thinking wrong?

[prj]

No, because there is no positive pressure in the bottom chamber to open the wastegate.
Boost pressure in the bottom chamber needs to overcome spring pressure to open the wastegate.

If you have a leak in the membrane, then you never build enough pressure in the lower chamber to overcome the spring pressure (due to leak), so wastegate stays shut.

It is effectively the same as disconnecting the hose from the bottom chamber of the wastegate.

[driftin jo]

Pressure from the N75 opens the valve, if it is torn then the pressure leaks from bottom to top and out and thus cannot act to open the valve.

[A80Avant]

I see...

So in that case what would be the problem if pressure is not building up at all and you get something like 0.3bar gradually increasing to 1.5bar at the red line, while the turbo is intact and free spinning(ie not damaged)?

Looks like i have some issue with that area and boost is no longer building up. Could it be that the wastegate may have been stuck open?

[driftin jo]

N75 is most likely dead.

[A80Avant]

3 of them????

[prj]

Dump valve/BOV dead or boost control solenoid not working.

Yes, possible that WG is leaking also.

[A80Avant]

trouble is that WG is was recently rebuilt with a new guide and a valve to correct leaks.

its funny cause the sound of the turbo has changed completely. Where i used to hear from 80kpa the turbo whistling, now it is silent, as if there is nothing helping it to build pressure anymore.

if i use a 1.9bar spring, i should expect to see 0.9bar at spool rpm and then that hold until redline.

however now i see 0.3bar at low rpm and reaches 1.5bar towards the redline!

what's going on??

visibly, no evidence of a large torn hose to cause a boost leak

while doing some runs i heard a couple of times a whoosh, which lasted a sec, and at that point i got a pressure drop, and then after that i got that boost issue i describe above..

[Jamo]

DV or Wastegate, but also make sure there is no blockage in the exhaust, IIRC a collasped cat causes these symptons also

[A80Avant]

damn, those were also checked 3 months ago!

would that increase in boost towards the end of the rev range be an indication of an exhaust problem? is that representing an increased backpressure?

[A80Avant]

Also, would a blocked exhaust also cause higher EGTs on the pulls? cause i am not seeing that on the runs i did yesterday...

For dump valves i have 2 APR units so ill check them and their vaccum lines.

[prj]

Yeah blocked exhaust would cause EGT's to skyrocket.
Fit blanks instead of dump valve(s) temporarily and try.

[A80Avant]

Thanks. I'll try and get a blank or 2 to test. Egt and mixture seemed spot on on normal driving so I suspect a leak is happening only during boost...?

sent using Android

[Rusty]

If the hose to the top of the DV is not connected properly, then they might leak. Similarly, if you're running a pistom DV, then make sure that the boost is acting radially, not axially.

[prj]

Well you are using speed-density based fuelling, so a boost leak won't affect your mixture or timing, the car will just not drive properly.

Obviously do a full boost leak pressure test, but the way you are describing it, seems more like something is wrong with boost control or dump valves, rather than an all out leak.