I'm getting a deep rumbling hiss in the intake pipe that feeds the turbos (big cast pipe right after the MAF). I don't believe the hiss is coming from the divertors. It's coming from deeper inside.

Does this likely point to noise coming from the oil tank? More specifically, a failed check-valve on top of the plenum that keeps boost from getting to the oil tank?

Any good fixes for that one or do I need a whole new plenum?

 

It's not what you think it is.
Disconnect the black plastic line from the pre turbo piling on the left side ( US drivers side ) and you will see no more sound in the intake. It's your check valve on top that's causing the issue.

 

I'm familiar with that line you're talking about and I've had to change that check valve in the past. I don't think it's that one.

To clarify, I'm pressure testing through the turbos. I've taken off that black plastic pipe feeding the turbo compressors, capped one turbo off, and am feeding compressed air through the other turbo compressor intake.

That small line from the top of the motor to the intake is hanging, no leaks there.

With a stethoscope, I do hear noise within the oil tank which is why I'm suspecting it's coming from the intake plenum to oil tank line?

 

Do it from the Lower boost hoses and it will be gone. Your air is going into the catch can etc. I bet your oil cap has air coming out too. So skip the turbos do it like I said and it will be ok.
http://youtu.be/mv_2gHLTTbg

 

Thanks for the pointers, I'll give it a try.

 

New 996tt owner and glad I found this thread. Will be ordering a leak test kit from Mark but wondering what size compressor I'll need. Can someone link me to one they recommend? Many thanks I'm a real noob when it comes to this!

 

Any air compressor will do, but what I end up using all the time instead of cranking on my compressor, is my all in one battery jumper. It has a built it air pump, which is perfect for the job and not expensive at all. Just plug it into the wall or cigarette lighter, I think it was around $90 new.

 

Markski, you're a godsend to this community

 

I recommend the largest air tank you can reasonably get. Having the compressor running all the time makes it hard to hear the leaks. Or get a really long hose and put the compressor far away.

 

I've got to do mine again after finding the FPR vac line off the car ran awesome and idled perfect. For some reason now I am only getting .8 bar of boost instead of 1.2 still idling were it should also still getting the secondary air codes.
Leaking DVs maybe?

 

That's actually a really good idea! Pretty sure I also need a new battery as I wasn't able to start it last night after sitting for about 5 days. I can see the battery jumper coming in real handy as I have read these cars kill batteries. Wondering if something like this would be powerful enough to jump the car as well as have enough compressed air to do a boost leak test.

 

That looks like a good one and definitely more bells and whistles than mine. If it's able to pump air in a tire, it will definitely do the job.

 

Your expertise is spot on once more. I took the elbow line off the top of the plenum distributor that runs to the oil tank. Indeed the check valve within the plenum is just fine as there is no leak there. The air is coming from the oil tank. And yes, air does come out of the oil cap.

Can you explain a bit more how the air gets there? Is it via the compressor cover -> through the CHRA -> to oil tank? Is this typical of GT28 turbos?

I still need to get the right cap to bypass the turbo and test from the lower boost hoses.

 

After some research, the Garrett BB turbo's by design, does have leakage from the cover through the CHRA. The turbo's have dynamic seals that come into play during use. Also, the pressures are largely at the edges of the compressor cover and not at the center of the housing.

Wonder if the stock journal bearing turbo's don't exhibit this behavior as much.

 

I just leak tested my car equipped with GT28 and I did not have air in the oil tank, my system held pressure very well.