Beezupra

This Q feels like asking for a miracle, but man I really need somebody to come through!

I've had reason to suspect a mild boost leak on my 991.2 TTS. I've been teaming with pfbz to boost leak test both our cars as in other recent thread. I'm pressurizing between one intercooler inlet to the other. I am convinced that there are no leaks at any typical location, ie IC, Y-pipe, TB boot, plenum, small vac lines to diverter valves etc.

I can get pressure momentarily up to around 22 psi, but it bleeds off rapidly down to about 5 psi then finally holds pretty well. Between 15 and 22 psi, a hollow hissing sound of air moving through a pipe is faintly heard, better towards the R rear wheel well than in the engine area. The same sound is most loudly heard at the open fuel cap. Closing the cap decreases how easy it is to hear, but doesn't change the movement of air. As it bleeds down under 15 psi, I believe the air pressure is still being lost along the same path, but there isn't enough air movement to make the sound.

I believe that the only vacuum lines that I am pressurizing that could be the origin of this air, are the two coming off the back of the T plenum piece. The sound of air moving into the gas tank must be related to some sort of fuel vapor return line. It seems to me this would be caused by some component with a bad check valve, that lies in-line between the Plenum and the fuel vapor return system.

One could wonder if this observation is normal, and just proves that this model can't be pressure-tested with the TB connected, but I just watched pfbz hold pressure well with nothing clamped except the one larger hose on the top of the Y-pipe, and similarly dgreen78 reports successfully holding pressure w TB connected as well. Those two cars are both 991.1, while mine is a .2, but seems doubtful that would matter.

So the bottom line Q is, how do I next go about identifying what component fault would let boost leak into the fuel tank?

pfbz

I am still learning the 991 'plumbing' but know the 996 Turbos quite well, so I often try to map what I'm seeing on the 991 Turbos to how things work on 996 Turbos. Very similar functions, somewhat different implementations. Just for reference, it has to be something similar to this setup on the 996 Turbo.

18 Connects to the plenum.
16 is a one-way check valve that often fails. It prevents plenum boost pressure from back-feeding into the tank vent and EGR return.
19 drops down to the turbo inlet for EGR gas recirculation. The hard line has another built-in check valve.
14 is the gas evap purge vent, with a a solenoid to turn it on and off.

The pressure leak into the gas tank might be another red herring (we have found a few of those, haven't we) if the ECU 'thinks' it's ok to vent fuel vapors into the manifold and has some solenoid open and there isn't a mechanical check valve that shuts under plenum pressure that it connects through...




There are a few connections on the back of the 991 plenum. Only one has a check valve built in. Does the fuel fuel tank evap canister route to this nipple or another one??

pfbz

Well, looks like Beezupra and I are working this issue on our own, but I found the parts diagram for the 991 Turbo that is essentially the same as the 996 diagram I posted earlier.

Nearly identical from a functional perspective. Once change is that on the 991 Turbo, they combined several different separate pieces into a single part number that is all 'glued' together at the joints. Fewer failure points, but more of a hassle to replace if something goes wrong. I've annotated the 991 diagram with additional letters for reference.

You can see the T-connector, #16 check valve, and the plenum connector on the 996 diagram as 15,16,17,18 and on the 991 diagram as C, B, A. It is that #16 check valve that goes bad frequently on the 996 Turbos.

The return from the t-connector to the turbo inlet is 996 #19, 991 D, E.

The fuel tank evap solenoid and pipe is 996 #14, 991 F, G

Again, all the separate parts 13-17 on the 996 are combined into a single part on the 991...

BMS

From what I remember the check valve was on the plenum itself. I don't honestly remember to well as it's been a long time since I've had my car or else I'd be physically poking around trying to help you guys

-Payam

pfbz

The plenum does have a built-in check valve on one of the ports, but that connects to line 1 in the diagram.
The plenum port that connects to 16 in the diagram does NOT have a built in check valve, but the line itself does.