Having left the battery disconnected for 2 days to reset the ECU I had Friday afternoon off to fix the boost issues.
So I reconnected the battery and went out for a drive – car still only boosted to the start of the end of the broken yellow/red section.
To check that the car could actually hold boost I disconnected the wastegate pipe for the APC solenoid valve. I gave the car a couple of short bursts and the boost gauge went merrily past the red.
Having done that I reconnected the pipe tothe APC and drove on – only to find I now had lots of boost – needle past horizontal boost.
I worried that maybe the wastegate had stuck closed. I went home and swapped the APC valve for my old valve which wasn’t holding boost. True to form it only boosted to the end of the yellow.
I then replaced it with my new valve and carried on. It was still producing worrying amounts of boost.
I gave it a run through the rev-range in 2nd gear. This culminated in a loud bang, smoke out the back and the engine dying.
I coasted to a halt outside someones house and looked under the bonnet to see if my engine was still there. Thankfully it was and no bits were missing. I gave it a few minutes, and it started fine.
Drove home gently and gave John at WMSS a call. He suggested sticking wastegate or faulty APC valve. A suggestion was made to unplug the APC electrical connector and that should produce base boost. I did this, and the car boosted to the end of the yellow (still felt fairly quick).
From these problems the likely explanation seems to be a faulty APC solenoid valve – I am not sure what else could cause the symptons.