if you can rev it as high as you want in neutral but you are cutting out around 4k than it sounds like a boost leak. check all of your couplers, piping, intercooler. if you can't see or find anything than you will have to use or make a boost leak tester (2 pieces of pvc pipe, 90 degree bend, cap and compressor tap.) and test it that way.

 

I noticed something else that may be of use. Without a load on the engine (in neutral) there is no problem revving the engine. However, with a load (in gear) the engine cuts back in the 4k to 5500 rpm range. What's interesting, though, is that this anomoly happens even when the turbocharger is not engaged any more than just cruising down the road at 55 and not accelerating. What I did was left it in first gear and when I hit 3500 I just accelerated, ever so slowly, up to 4k so that the turbo wasn't increasing boost. It took about a minute for the acceleration and was ever so slight that it would seem to indicate that it is not related to the turbo, else it should happen at any rpm where the boost reaches a certain level. Does this seem logical to you, or am I off base here?

 

i duno dude... i still say it sounds like a boost leak but that wouldn't make since if the turbo wasn't spooling when u were accelerating than that would count that problem out.

 

Found the problem and it was an easy fix. The electrical connector was not securely attached to the vehicle speed sensor on top of the transmission. Once locked down, the speedo started working properly again and the engine doesn't miss a beat. Go figure...