If the solenoid vent is clogged by EGR soot from the intake and/or the electrical part is not working, it will continue to boost as high as it can on the fuel (accelerator) you give it.

I've run in the mountains with 20+psi boost *once* before. That engine is still in use but it really hurt my idle quality. I'd bet it scored my pistons/cylinder walls and/or burned the rings. It runs great under load and when it's warm, but when the coolant is cold I get misfires, smoke, and it won't idle well below 800rpm. It's been well over a year since that happened, but I'm looking to replace it with a used engine soon.