Before over reacting to a loss of manifold pressure at altitude, consider minor sources for leaks before replacing expensive components. Also consider the phenomenon of Bootstrapping. Cessna defines bootstrapping as "the unstable manifold pressure condition that occurs when the wastegate closes under high altitude and low RPM operation." In other words, it is the RPM below which the engine will no longer maintain cruise manifold pressure at altitude. This problem is frequently noticed when the pilot tries to operate the engine at low cruise rpm. Doing so reduces the available exhaust gases required to spin the turbocharger. Increasing rpm by as little as 25 rpm may be enough to restore desired manifold pressure.
Applicable Cessna Service Manuals outline a test normally entitled "Turbocharger Operational Flight Check Procedure" which allows the pilot and mechanic to determine if the engine is operating properly. Should the engine fail this test, many times the problem is a result of exhaust and / or induction leaks, not a faulty component. Normally, an inspection as outlined in AD71-09-07 R1 and AD75-23-08 R5, a simple leak check, and a repair of leaks can solve this problem. Such will save the aircraft operator from needlessly replacing costly components.
To inspect the exhaust system:
A. Plug overboard exhaust and wastegate overboard pipes (when applicable) with suitable plugs and/or tape.
B. Using a compression test adapter, pressurize a cylinder on the exhaust stroke with shop air. The exhaust valve should be open with the piston at the bottom of its travel.
Caution around the propeller while the exhaust system is pressurized.
C. Use a soapy water solution to inspect the entire exhaust system for leaks. Pay particular attention to weld areas in the turbo wye duct area, couplings, and turbocharger housing. Any leakage will be detected by soap bubbles and is cause for further inspection. Some very slight leakage is normal around sealless slip joints and exhaust risers. When testing
sealless slip joints and exhaust risers, if the soapy water solution is being blown away (instead of bubbling) then the leak is excessive. The part needs to be inspected further.
D. Remove all plugs and tape at the conclusion of the test procedure.
To inspect the induction system:
A. Remove the engine induction air filter and tape over the turbo scroll area to prevent air loss.