today i tried to see how much boost i could get, so i just hopped on it and stayed on to about 140 (kms). and the highest my boost gauge neddle would go is 3/4 of the way. isnt it suppossed to all the way? im sure id get alot more power if it would, i need a new coupler, would that affect the performance though (its not really that bad).
Your coupler won't cause that kind of boost loss. It usually will only cause some noise at idle. I'd bet more on your boost control solonoid. Cheap to change, too. If that'* not it, it could be worn rotors on the SC itself, but I doubt it. That SC should outlast the entire drivetrain.
So how much boost should it show? Mine will only get to the yellow section on 20-30 degree days. In the 70-90 degree range it'* more like a little over halfway between 0 and 10. Is this normal? The car runs ok, I drive it to work and on trips but it'* not really my speed machine. If there is a way I can get a little more boost that would be good. The GN runs 22 lbs boost so I drive it if needed.
Like Fuddy said though, it would be a near total failure to throw a code. We've had a few go bad like yours (same or similar symptoms) with no SES light. That'* just the first thing that comes to mind with what you're experiencing. The other possibilities aren't as likely. Some of those would be a vacuum leak, faulty SC outlet gasket, worn rotors, slipping belt, etc.