[Dan schultz]

I would first look for any intake leaks from the compressor discharge through the whole system including the sonic nozzle and the hose going to the pre cooler.

Compression checks like you have done, 55 should still be ok if there is not significant burning of the exhaust valve or play in the guides.

Check the exhaust by pressurizing from the tailpipe with your shop vac then using soapy water to check for leaks, bubbles are ok, but blowing bubbles away not good. I have a new Acorn rear exhaust and had to eliminate the springs.

Unfortunately if those things are ok, time to dig further, looking at the waste gate and turbo. You can take the tailpipe off and check the play of the turbine shaft. If there is significant play up and down of the shaft that is not good. Cessna does not object to using a front turbo on the rear engine as up to S/N 98 they used the same one. Not approval from them, depends on how your IA looks at that. My opinion is that it is a minor alteration and no other approval needed, however, I am not your IA.

The wastegate when closed may have significant erosion on the butterfly valve causing blow by.

And then of course it could just be the controller, or the waste gate actuator may have picked up some trash from the oil, take the two hose fittings loose and blow it out both ways.

The maintenance manual has good troubleshooting on the system.

Dan
N67S
Finally got mine flying after 2.5 years, flew it about 12 hours the last few days.