There's a check valve in the top of the plenum on the 12mm hose that connects to the oil tank. Makes sense, no reason to pressurize the oil vent lines.
Mine has lost it's checkiness.
Is there any reason I can't use a 12mm check valve external, inline? I can see no reason why not. If so, anyone have a source, as all I can find are rather chinzy looking. I have found 3/8, 5/16, but no 12mm (7/16 would be usable, too).

 

Are talking about the #16 check valve as shown below? A common failure point, causes significant boost leak when bad, and a bit of a PITA to replace.

No reason you can't use a suitable generic check valve, but last time I checked there was nothing identified as a better check valve that was readily available. If you find one, let us know.

Part 996.110.135.70, about $25.

 

Not the famous #16, there is a check valve actually in the top of the plenum:

The fitting with an arrow contains a check valve. This check valve blocks boost pressure going to the oil tank, which if not blocked pressurizes the crankcase and the turbo oil drain boxes.

 

See pic.......is that yellow / black check valve stock? Does not look like the IPD comes with a check valve in the housing....... The other two ports have factory valves connected, yes?

 

That yellow/black check valve is not stock and it's exactly what I am looking for. All I can find are 3/8 and that one looks to be 7/16 which would be perfect.
I wonder where they got it?

 

OK, found it!

The right-angle version is a 191-611-933-F brake booster vacuum check valve for a Porsche 930, a straight version is a 191-611-933 (no suffix) for many air-cooled VW's. Both are 12mm (7/16).

 

Nice!

 

I just check my plenum check valve last night and I also found it was leaking slightly.
I cleaned successfully with MAF cleaner (not brake clean).
I put MAF cleaner in hose opening, than air compressor nozzle with about 50 psi of air and modest flow (not full). Proper check function returned after first cleaning. I did 2 more cleanings just to sure.

I inspected the diaphragm and it appeared perfect other than a build up which caused the leakage, so I cleaned it. I also found my #16 valve bad, inspected it and found it needed replacement.

I am new to the turbo engines and just started taking apart mine, you would be wise to take my advise with a grain of salt.

 

Curious, what issues are caused by IPD omitting the check valve in question? Maybe I've missed them before, but IPD has been selling their 996TT for a long time now, and this is the first mention of a "missing" check valve I've seen.

 

i have the ipd and i suspect that what the OP is calling a check valve might not be one in the way that the #16 is(?) ( i'm asking, not telling ).

my ipd simply has the fittings atop their cast plenum - much like the OP's pics on the oem plastic plenum.

 

The IPD external check valve or OEM internal check valve keeps boost from pressurizing the oil tank and oil vent system, which is essentially the turbo oil drain boxes & the crankcase. If the oil vent system gets pressurized it can pressurize the crankcase with boost pressure below the pistons increasing blowby. Boost could also either push oil from the turbo drain boxes into the turbo scavenge pump or could aerate & pressurize the oil in the turbo drain boxes, making leaks turbo seals behave even worse. Also because the oil tank is vented into the aluminum intake plenum downstream of the MAF it can act as a silent and hard to find boost leak.

 

Looking at T Kono's picture: The top left hose goes to the #16 check valve, the bottom left hose goes to the brake vacuum venturi.
The right-hand hose goes through a yellow/black check valve to the oil tank. At the oil tank there's a "T" connection and the hoses that go to the left & right eventually attach to the turbo drain boxes.
On the oil tank the metal pipe with the hex fitting on the left is the output from the engine crankcase scavenge pump. Hiding behind that is the "?" shaped crankcase vent hose which connects the air space of the oil tank to the top of the crankcase. And not in the picture but just above the "?" shaped hose is another vent hose that's connected to the aluminum intake plenum to vent the oil tank.
So with boost on there's a leak path through that check valve, through the oil tank, and into the aluminum intake plenum that runs along the left side of the engine near the fuel filter. The check valve ought to prevent airflow from the TB plenum to the oil tank while in boost and allow airflow from the oil tank to the TB plenum when not in boost.

 

thanks for clarifying that. never knew and thought it was simply the cast housing on the ipd!

 

I believe I still have a bit of a boost leak and I think it's coming from the plenum. Not really trying to spend $400 for a new plenum..did this solve the issue @theprf ?

The $10 VW option looks enticing.

 

Yes it's been working well.
You can easily test the check valve in the plenum.
Set up for a boost leak test and apply pressure as usual. Remove the oil filler cap. There should be no air escaping.