When I get under full throttle my boost should stop at 15psi. but now it keeps going past that. It doesnt want to stay at 15psi. What could this be?

 

could be a bad bad/sticking boost controller or the wastegate isnt opening the way it should.

 

or your internal wastegate is just too small to bypass that much exhaust.
External wastegate might be required which will need to have a flange custom mounted in the exhaust manifold.

 

Well this just recently happened. The car used run fine, then it started doing this. If my wastegate was messed up, could my car still run normal with full boost? or would it run bad? I was having trouble putting the wastegate actuator in the right place. I dont know if I have it set wrong.

 

It could be the wastegate not opening all the way/sticking, or the vacuum signal line could have a leak or is collapsing like it did on me.

 

I had this happen on two turbo vehicles...the vacuum line to the wastegate is either loose, has a hole in it, is cracked, or is not there. Find it, replace it and be happy again.

 

speaking of boost and i know this has been asnwered before, but with an intercooler kit how much boost can the stock turbo safely and reliably hold up to?

 

You're talking about a 280zxt right? I ask that because you have a 350z as your avatar... 280zxt w/ an IC can safely go up to about 12 psi. After that the injectors just can't spray enough to keep the a/f ratio correct.

 

I will check out those vacuum lines, then I will check that boost controller. If the wastegate is sticking how do I fix? Do I need a new wastegate or do I have to open it up?

 

Well as of right now I fixed it, I hope! I just moved the actuator a little each time both ways until it held boost, which it does now. Thanks for your help.