I'm only getting around 1.5 bar instead of almost 2 on the original gauge. Sometimes, not often though, after having had the engine switched off for about 10mins I'm back to full boost but only for a minute or two then returns to 1.5.

I've checked for vacuum leaks, loose clamps, electric plug fittings etc without success. In the past with vacuum leaks I'd only get about 1.2bar so this higher boost suggests it's not a vacuum leak. I thought of the collapsed exhaust pipe but those times my boost had short periods of recovery negates that.

Could it be an electrical problem in the engine bay. Maybe 10 minutes of heat soak momentarily fixes an electrical problem. If I remove the knock sensor will the simple resistance test confirm it's broken.

What could I try now as I'm stumped. It was going well for the first 2000k's after a few years of being laid up.

 

Stock setup?

 

No. It's got Autothority stage 1 chips, shimmed wastegate and SFR headers. Otherwise stock 86 model.

 

I believe Knock Detection limits Boost to 1.2 Bar, but full Boost to restricted boost and back like a switch after shutoff sounds like KLR Safety ... bad gas and poor gauge accuracy?

 

I thought about bad petrol but new fuel hasn't fixed it.

There's full boost when I clamp the wastegate hose. Disconnecting the CV doesn't change the 1.5 that comes up on the gauge.

Would the knock sensor be bypassed with the clamped wastegate hose and the full boost I get?

 

Pinching the wastegate hose gives the computers no say in the resulting boost.
Per your test, the wastegate and exhaust are OK.
Most likely you have vacuum leak going to the CV valve, dead CV valve, or a problem with your KLR.
Next thing I'd do is make a 'blink code' tester and see what the built in diagnostics say.
Follow the 'test fault code' link
http://members.rennlist.com/951_racerx/

Some good info here:
http://www.clarks-garage.com/951faq.htm

 

Brilliant. That's the best guide I "couldn't find" for doing the blink testing.

I'll make the LED tester and let you know how I get on. Thanks.

 

Well I get 2-2. So that's either low or no signal from the knock sensor or a faulty k/cp (KLR). Hope it's the first. Given the symptom of short term full boost after left standing for 10 mins or so that my prime suspect.

I need to extract it, check the resistance and refit with only 9nm torque. I didn't use a tension wrench when I refitted it recently.

 

They're baack. The missing horses have been corralled and it feels great.

Boost's hitting 2 bar, the highest it's been since I got the car back on the road a month ago.

Turned out the knock sensor had come loose. When I put the engine back in I must have only done the bolt up finger tight and it backed out.

Thanks guys for your advice. Great site.