My 996 Turbo only makes 0.5 bar max boost and after working on it for many hours the problem still can’t be identified. All the previous posts on this topic seem to suggest that a solid 0.5 bar max means the car is in limp mode and not a massive boost leak. According to the in dash boost gauge the peak boost never goes over 0.5 bar, it just rises smoothly to 0.5 and stays there until redline with no fluctuations.

Background: The car has been flashed and always made 0.9 – 1.0 bar before digging into the engine bay to refresh a number of parts as preventative maintenance. I replaced the diverters with Bosch 710N units, new N75 valve, silicone F hose, FR6LDC plugs, 997 coil packs, intercooler o-rings, one-way check valve (996-110-135-70), fuel filter and added hose clamps to all the accessible vacuum hoses and fittings. I didn’t do a boost leak test before all the maintenance work but by all accounts everything was tight since the car pulled hard and smooth and the boost never oscillated after reaching 0.9 or 1.0 bar.

After all the new parts were installed I did boost leak tests from the rubber intake boot after the MAF (as per the FSM) and then tried at the turbo compressor inlets by capping off one side and pressurizing the other. No matter which position I added pressure to the system didn’t want to hold much over 10psi and it would leak down to approx. 6psi after approx ten seconds. I smoked tested the system under pressure and there was no trace of smoke leaking anywhere. Then I used soapy water and wasn’t able to find any leaks. When the system is pressurized I can hear some faint air hissing sounds but it mostly sounds like air leaking past the valves into the combustion chamber and it isn't very loud. After a few hours of repeating the same process it really seemed like I was doing the test wrong or there were truly no leaks despite the fact that the system didn’t really hold much pressure. Did I miss something? How are other people holding 20psi for extended periods of time? These are the threads I referenced for information.

https://www.6speedonline.com/forums/...cs-inside.html

https://www.6speedonline.com/forums/...ig-w-pics.html

On the first test drive after reassembling everything with the new parts the car failed to go past 0.5 bar so I installed a new Bosch MAF but that made no difference. I have scanned the car with Durametric and no codes have ever been recorded. I also unplugged the battery a number of times and left it unhooked overnight but that didn’t help.

If the wiring to the N75 valve is bad would it trip a fault code in the ECU? Could the ECU be in some kind of limp mode that disconnecting the battery and scanning the ECU isn't resetting?

This problem is very annoying so any suggestions or tips would be greatly appreciated.

 

Boost leak I think. Could you be losing pressure at the ICs? O rings?

 

No - the entire intake system and intercooler circuit will pressurize to over 10psi and every part of the system that is accessible has been checked for leaks by ear, with smoke and with soapy water - 100% sealed. The faint hissing air noise is coming from deep within, maybe behind the TB plenum area but it sounds more like from the intake manifold area, hence by guess that air is leaking past the open valves.

 

you need to hold at least 20psi for a good 20-30+ seconds
If you're not, you have a leak period.
There's many common places for leaks if you search around.
I've had a few leaks here and there. The biggest one for me was TB oring...It lost pressure like no ones biz....

 

If you are blocking 1 side of turbos and pressurizing the other side, you should hear absolutely no hissing

 

I read through all the pages of the boost leaking testing threads but some of the info isn't very clear and different people seem to use different methods.

If I cap/seal one turbo inlet and pressurize the other turbo inlet while leaving everything on top of the engine as-is, i.e. the aibox and MAF, TB, etc are you saying the system should hold 20psi for at least 30 seconds and not have any hissing noise? I read some people mentioning capping off the TB and one of the rubber hoses that attaches pre-TB. Is this not required when pressurizing from the turbos?

 

you need to pressure test to 20 psi and it must hold on its own for seconds...
furthermore, disconnect the battery and see if the issue goes away for a while. if so it may be in the tune if you have a tune and or of you changed something your not disclosing...

 

that's what a cracked check valve which sits atop the plenum sounds like, and will easily account for this kind of leak. it's a possibility.

 

cap 1 turbo inlet(I do driver side as easier to pressurize air into passenger side due to the way the hoses are curved...anyways) and pressurize the other side.
Do not touch anything else leave airbox etc etc all alone.
pressurize to 20psi....no need to overdue it to 25psi+ etc as you don't want to blow anything(you're not boosting past 20psi anyways).
Watch to see if pressure drops or hear any hissing.
any loss in psi or hissing means you have a leak

 

exactly

 

I was really hoping the fix would be something other than a boost leak but since there is no way my car will hold over 15psi for more than a couple of seconds there must be a leak. At the risk of beating a dead horse, if I pressure test at the turbos I don't need to do anything up to, correct? Nothing else needs to be capped off?

Anyone want to guess what might be leaking that isn't easily accessible? I say this because everything that can easily be checked (visible hoses, diverters, F hose, intercooler connections, etc) has been checked and the hissing air sound is coming from closer to the firewall >>>>> I think

When I did the pressure + smoke test after the MAF area (a la FSM) no smoke was visible in the engine bay but when I removed the oil filler cap a ton of smoke came out. Is this normal?

In this pic I circled the Venturi pipe and the three way connection on top of the intake T plenum. Do either of these have a one-way check valve that could go bad? I already replaced the one-way check valve on the driver's side of the T plenum.

 

i am not familiar with the smoke test so I can't comment on that....

when my TB gasket oring was leaking, it was bad. It was leaking at the bottom and It took forever to get it up to 20psi and it would instantly drop to under 10psi....yet i could not pinpoint where the leak was coming from....sorta sounded like it was coming from the back(firewall) like you're describing....it's worth a shot to change it if it's the original. soap test wouldn't work at this location as the leak was at the bottom....bad area to spray soap water

the 2 areas you circled are also somewhat common area where it can leak...markski will know more about those 2 areas....

 

just contact markski for the boost leak tester caps...it's well worth the money if you don't have a set. It's metal so it'll last forever and easy to use.

 

I've said it once, i'll say it again ..and even though i might well be wrong. i believe you have a cracked check valve that is hissing. thats what they do.

its on top of the plenum at your pics. there are two iirc? i would replace both ( and did ) it cured my issue which shared your symptoms.

 

As preventative maintenance during this round of work I already replaced the one-way check valve (996-110-135-70) that sits to the left of the three-way connection on the T plenum. Is there another check valve that I missed? FWIW, both the original check valve and the new one only allowed air to pass in one direction.