This is on an S4 TII engine, stock turbo, Megasquirt EMS.

Anyway, I was driving it yesterday playing with the boost control. I just got it working reliably at 12 psi a few days ago. I park the car for about 30 min, come back out to drive home, and she starts up and runs fine, is idling at the normal 800 RPM, but when I floor it, she absolutely refuses to build boost. You can hear what sounds like a very large boost leak, and manifold pressures dont build above atmospheric. I pull over to check for leaks, cant find any, but still she's not holding any boost.

I really dont think its a disconnected vacuum line, b/c for it to be big enough to bleed off 12psi of boost, it would definitely have affected where the engine was idling. I use the stock idle valve for closed-loop idle speed control, and the signal that it was getting was the same as any other time, so it was still open the same amount, meaning that for the same idle speed, the same amount of air was getting into the engine through the throttle and idle valve, so no extra was leaking in thru a disconnected vac line after the throttle. The only vac lines i have pre-throttle are the hose that feeds the idle valve, the vac line to the wastegate and the crank case breather, with the idle valve feed being the largest. This has blown off before at stock boost, and i was still able to build 2 PSI though, so the intermittent leak needs to be larger than this.

The only thing i can think that it could be would be a broken BOV, since it vents to atmosphere (I'm running speed-density air measurement, so no worries with AFM), but only when there's a positive pressure differential.

Any other ideas what it could be?

 

BOV may not hold pressure. The do fail from time to time.

My TII had an issue with the tank on the IC separating from the fins on boost. Would seal well enough to hold vacuum in the engine but under pressure greater than 2psi it would separate and bleed off. Took me a while to find it too!!!

 

use a pressure tester to find out, stop guessing

 

If you think it's the BOV, just plug it for one run. The world won't end if it's blocked off for a few minutes.

 

Or leak down test the engine. 2" cap on the turbo inlet, (Not on your intake pipe) Pump air into it and you'll see.

 

BOV is fine, one of the FMIC couplings came off, but from the angle looking down at it next to the light it still appeared on.