Exhaust valve seals worn
If the seals on an exhaust valve are worn, a seal of the combustion chamber may not be made. This can cause a loss of compression, or permit fluids like oil and coolant to enter the combustion chamber.
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Exhaust Blows Blue Smoke
If there is blue smoke come out of the tailpipe, this is a sign that oil is entering the combustion chamber and being burnt with the fuel and air.
See burning oil.
Loss Engine Power
If the engine is struggling or stalling, there may be leak in the combustion chambers caused by worn seals. The exhaust valve seals the combustion chamber during ignition. If the seal is not tight, the fuel and air can escape during the compression stroke. Upon ignition, less of the fuel is available to be burnt, reducing the power of the engine.