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Engine overboosts. High manifold pressure 
1. Pressure relief valve fails to open or exhaust bypass valve fails to
open.
2. The pressure relief valve may fail to open if the relief valve aneroid
bellows fails or the relief valve is out of calibration.
3. If the exhaust bypass valve, which is normally open, fails to open,
this must be due to either a hydraulic problem which prevents relief of
the oil pressure which actuates the valve to close, or a mechanical problem
which interferes with opening. This can be caused by (a) restriction in
the oil line from the exhaust bypass valve actuator to the controller(s),
(b) poppet-type exhaust bypass valve, if the poppet is stuck in the closed
position, (c) binding of the linkage from the actuator to the butterfly
valve. If the butterfly itself is stuck, perhaps from carbon deposits,
soak in Mouse Milk (d) packing in actuator jammed piston in the closed
position (e) Internal springs jammed or broken, (f) controller located
between the actuator and the engine crankcase which is suppose to relieve
oil pressure to open the bypass valve does not do so.
4. In some systems, a safeguard against overboost is the rate controller,
or the rate section of a rate/absolute dual controller, which opens the
exhaust bypass valve when there is an excessive rate of increase in compressor
discharge pressure, as during too-rapid throttle advance. If the exhaust
bypass valve has failed to open, the trouble may point to a malfunctioning
rate controller.
5. Lycoming hose from wastegate actuator to controller kinked. This
is a short hose that can kink underneath the Firesleeve and not be externally
visible.
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