Can't boost higher than ~ 12 psi, yes I searched, need help!
 

Yes I have searched, as well as consulted a few experts, including Lucky from ARD & a good friend who is a tech for Audi. My car will not boost beyond about 12 psi. I am running a 17 psi ARD tune. No matter what I do, I can't get past this 12 psi. I have tried a lot and so far I have only got an extra psi or two, how I got to ~12. I was trapped at closer to 10 psi for a while. All of this has happened over the past few weeks. I need to find the solution, and I do not want to just throw expensive parts at it as a method of trouble shooting. I need ways of testing & troubleshooting. I will try to give the short version of the facts, I hope someone can shed some light.

Car is a 96 855 T5, ~130k miles. Idles perfect with vacuum around -22 at idle in park or neutral. The boost holds steady, just not at the right psi, so I doubt its a leak. But I have changed a ton of vac lines & the coupler from the turbo to the pipe that goes over the top of the engine with a silicone one, as well as any clamps that looked questionable. But again, boost is steady, but not at the right psi.

Before getting the ARD ecu, I added a MBC and upped the boost to about 10 psi, and the car felt good and as it should have. I then got the ARD ecu with the 17 psi Green tune. I installed that and upped the boost on the MBC to about 15 psi. At this point, I was hitting 15 or so, but with a rapid fall of to about 10 psi. Switched back to stock BCS and it spiked at ~11-12 psi and dropped still to the ~9-10 psi. This is where I have been stuck.

So I searched and spoke to Lucky trying to figure out what was causing this. Possibilities were: a bad BCS, a bad wastegate actuator, or the cat being clogged and causing a massive flow restriction. The MBC now will not go higher than ~12-13 psi either, even with the BCS capped, so its not the BCS. I also did a test Lucky recommended of tying the wastegate shut, with the actuator rod off, to see if it would even want to boost higher then, and it still didn't. This was what made me think the issue was “after” the turbo, and this is when I removed the cat.. I also saw a post about the wastegate flapper not sitting correct, so I checked that too.

Not only do I need a solution, but why was I able to hit 15 psi for a bit and now can't even do that with a tied wastegate??!??

Oh, and sound wise, it doesn't make any noises that would indicate a blown CHRA. With no cat & a cone filter, I would hear it, I have heard blown turbos before. And also speaking of sound, it doesn't even sound like its trying to build more than 10-12 psi, as far as spool sound. So I really doubt a leak. Also way too steady of a hold (at the wrong boost) to be a leak. In my experience, leaks are more erratic.

When I first got my car the vacuum lines to the pressure control were reversed and I had to always keep an eye on my pressure gauge so it would stay below 16 inches. After I changed the lines back to the way they were supposed to be it controlled at stock pressures. You could try changing the vac lines around.

Thanks, but I have checked the lines as well as reversed them. Also, as mentioned above, I tested with the wastegate completely tied shut. That eliminates all boost control, MBC or factory, and it still would not boost higher. Its definitely nothing to do with boost controllers.

Even though I inspected the diaphragm, is it possible the CBV spring is weak and venting off at 10-12 psi? The diaphragm definitely has no tears, I took it apart and checked. But it is the original spring..

Turbo could be worn out as well.

I haven't checked but is there a way to blank off the waste gate for testing?
How are you measuring the pressure? What about cracked hoses etc that could be blowing out at around that pressure? If you were getting higher pressures earlier I don't think the turbo is bad but with the other work a cracked hose or loose clamp could be possible.

Wouldn't that either have noises or smoke? And wouldn't it be erratic boost as opposed to hitting this ceiling like I have?

I did test the wastegate itself to also test if it was the actuator. I disconnected the actuator rod from the flapper and used thick metal wire to tie the flapper in the closed position. It still would not boost higher. This eliminates all boost control and the wastegate actuator.

Starting to think maybe the CBV spring since its original. I'd love to block it off totally just for testing, but I'm not buying the plate just to test.

I have seen situations where things leak at about the same pressure every time although this is in commercial HVAC situations. I'm just saying it might be worth looking into. I worked in pneumatic controls for years and had times where hoses would blow open at a certain pressure every time but below that pressure it would all work fine.