Manifold Pressure
Induction and Exhaust Leak Checks
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 AD 71-09-07 R1 and AD 2000-01-16, 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:
Plug overboard exhaust and wastegate overboard pipes (when applicable) with suitable plugs and/or tape.
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. Use caution around the propeller while the exhaust system is pressurized.
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.
Remove all plugs and tape at the conclusion of the test procedure.
To inspect the Induction system:
Remove the engine induction air filter and tape over the turbo scroll area to prevent air loss.
Using a compression test adapter, pressurize cylinder on the intake stroke with shop air. The intake valve should be open with the piston at the bottom of its travel. Use extreme caution around propeller while induction system is pressurized.
Use a soapy water solution to inspect the entire induction system for leaks. Pay particular attention to all intake manifold hoses, throttle body to intercooler connections, magneto pressurization (if applicable), air reference lines, and intercooler. The intake system should be leak free. Any sign of leakage should be inspected further. A nominal amount of leakage may be present at the butterfly shaft on the throttle body. Pay particular attention to the intake manifold hose body, in addition to the clamp sealing area. These hoses have a tendency to deteriorate due to heat from the exhaust system. Many times they can be found to be leaking through the middle of the hose body. Examine induction flex elbows closely for cracks or holes that have been chafed into them. Intercooler to throttle body “O” rings will also deteriorate due to heat. Such deterioration can cause leakage. Any leakage noted in these areas requires replacement of the hoses or “O” rings.
Leaking alternate air doors and transition ducts should be inspected. They can be a source of hot engine compartment air being ingested into the engine instead of the cooler outside air. Hot air can cause a decrease in performance very similar to an induction or exhaust leak.
Remove all tape. Reinstall the filter at the conclusion of your test procedure. Should these simple tests prove the exhaust and intake systems to be secure, then further troubleshooting is appropriate. The aircraft mechanic can proceed to determine which major component needs replacing.