4JB1-TC Boost Pressure drop.

[4JB1TCR]

Hi,

I have an Isuzu with a 4JB1 Turbo Intercooled engine. Ever since I put it in (about 1000k ago) It has had a boost pressure leak. I have tried everything I can think of, checked every hose, and every joint. Yesterday I finally managed to get a boost gauge, so now I can see what is happening. It builds boost fine up to 10PSI, but the minute it hits 10, or a tad more, it immediately drops to 5PSI, and the drop is audible as though it were venting to the atmosphere. I have tried everything I can think of. I blocked off the little "petcock" thingy on the intake manifold, I disconnected the pressure hose to the wastegate. Nothing solves it, and I can't find any leaking hoses or anything. Is there anything I am overlooking? The symptoms are consistent, it always boosts to 10 psi, then immediately drops to 5.I'm going crazy, because I have a long trip on Tuesday morning, and it really looses power when the pressure drops.
Thanks in advance.

[geeves]

sticky wastegate?
Try blocking the little hose between turbo and wastegate