Unusual Boost Leaks ?

[Martin F]

I'm looking for peoples experiences of unusual boost leaks.

I'm battling with one on my Rev1 Turbo engine, in that I can drive and boost fine up until 14psi (boost controller set for this) and then all of a sudden there is a quiet pop and boost drops back to around 4psi. I then change gear and boost will still be stuck at around 4psi, but if I stop accelerating and then give it a couple of seconds and accelerate again boost will start to build again OK to 14psi.

This all started last year when I noticed on a track day that I was losing power at certain points, but I ended up distracted by damaged blades on my CT26 turbo and a significant boost leak, which turned out to be one of the vacuum barbs on the plenum being jettisoned. Turbo was replaced with a CT20B and the vacuum barb replaced and used to feed a boost gauge until now when i’m left with a car that drives quite well apart from this annoying boost issue.

Things I have done to try and resolve:

• Because the issue is reminiscent of a wastegate being stuck open I have tried three different down pipes in total, but also confirmed that wastegate is not staying open by use of a camera on it when driving. Currently have a Blitz cast downpipe fitted.
• Eliminated recirc valve by testing but also fitting a known working one.
• Replaced all couplers with silicone versions, although no obvious signs of problems with OEM rubber couplers.
• Last year I was using stock boost control and now I am using a Greddy Profec boost controller because of the CT20B.
• Pressure tested intake between turbo inlet and just before throttle body and no leaks.
• Pressure testing intake at entry to throttle body and into the rest of the system. A couple of small leaks spotted (throttle gasket and one chaffed hose) and rectified.


Because boost is building to 14psi and then something is opening\happening and this is dropping back to 4psi it must be something significant, but i’ll be damned if I can find it.

So wandering what experience you guys have of any unusual boost leaks ?

[AlunJ]

Sounds like you're hitting the boost cutoff limit rather than experiencing an air leak. I see no mention of any mods fitted that would raise that cutoff point above its stock level.

[Martin F]

Apologies, I should have said there is a Greddy FCD fitted.

It's not like fuel cut off as the revs continue to rise, just a lot slower as the gauge is showing only 3 or 4psi.

[AlunJ]

Hmm. Yeah the FCD pretty much eliminates that possibility.

Sounds strange. You've pretty much eliminated any significant air leaks by the checks you've already done. Stock boost pipework?

[Martin F]

Yep, is stock pipework, apart from turbo intake which is now a silicone one, but I had same issue on stock turbo intake pipe as well.

It is perplexing and I seem to be running out of ideas.

[Martin F]

So today I noticed something curious when pressurising the intake, the cam cover is also pressurising (the vent hose from the throttle body to the cam cover is disconnected). Roughly I have around half the boost pressure building in the cam cover, i.e. if I pressurise the intake to 18psi, i see 9psi in the cam cover. When pressure is removed then the intake pressure decreases first and the cam cover pressure does not decrease until the intake pressure reaches the same level.

This doesn't seem right to me, but wondering if anybody has ever noticed something similar ?

[jimi]

Have you done a compression test?

[Martin F]

Yep, apologies I meant to add that to the above post:

1 - 179psi
2 - 180psi
3 - 179psi
4 - 180psi

So all good.

[AlunJ]

This doesn't seem right to me, but wondering if anybody has ever noticed something similar ?

Are you seeing that cam cover pressure when carrying out a pressure test from the throttle body, or from a pre-turbo position, or both?

[Martin F]

I'm pressurising from the input to the turbo, but I have the PCV vent from the cam cover disconnected.

[jimi]

Leakage past the valve guides?

[Martin F]

Yeah, this is what i'm wondering. My next job will be to perform a leak down test.

[Martin F]

So the results of my leak test are in:

1 - 20%
2 - 30%
3 - 30%
4 - 30%

So at least I know my engine is in good health even if the plot thickens with regards to the boost leak.