Just wondering if anyone else's scoob does this mine is a 93 wrx and sometimes it runs about a bar then i will drive it and it struggles to do half a bar dont know why someone suggested a bad air intake so ive changed the filter for a green stuff cone but hasnt made a difference help would be greatly appreciated as a rs turbo nearly had me last night.

 

Maybe the wastegate actuator is not closing fully ?

 

Excessive knock will make the ECU panic & release control of the actuator, so it will only boost to half a bar.
It's a good idea to find out why you are getting the knock!

 

my 93 wrx started to do this due to a sticking boost solenoid. I connected the black plugs under the dash and got a fault code (45 I think) I cleaned it out with brake cleaner and boost went to 0.6 (from 0.5) I was then told to check the intercooler hoses as I should have been seeing 0.7-0.8 bar as standard. I removed the intercooler and found the black plastic hose from the turbo to the intercooler had split at the "flexible" section. I replaced it with Samco ones and now it's always a constant 0.7-0.8 bar boost

 

EXCESSIVE KNOCK DO YOU MEAN FROM BOTTOM END I THOUGHT ABOUT THAT YOU CAN DRIVE IT FOR HALF HOUR AND ITS FINE THEN IT JUST CUTS BOOST TO HALF AND STAYS LIKE THAT TILL YOU STOP THE CAR FOR ABOUT 20 MINS THEN IT RETURNS BACK TO NORMAL ITS LIKE SOMEBODY IS WINDING A BLEED VALVE IN ITS REALLY GETTING ME DOWN

 

check your MAF too..

 

0.5 bar is the limp home boost so check for error codes!! That will tell you if one of the sensors etc is playing up. If something is failing (maf/boost solenoid etc) then it will be reducing the boost as a precaution. Mine used to do this on a regular basis before I sorted out the solenoid/pipe leak