What's the main engine valves failure?

tapping the valves at the timing

wall door seal failure

not using recommended engine oils

Starting problem, due to air fuel mixture leakage from intake valve seat, (Lake of compression) especially in cold weather. Also if inlet / exhaust valve not adjusted properly, that can cause a serious damage of engine ( can hit to piston top ) result, engine sized etc.

Less power

High fuel consemption

engine smoke, high consumption oil, loss power

Engine valves wear failure, valve face recession, fatigue failure, thermal fatigue, errossion or corrossion of valves, carbon deposite on the valves

An exact diagnosis of the reason for a failure can never fully be assertained until disassembly and inspection of the unit, the main symptoms of a valve failure will be a lack of compression in the engine and possibly related audible indators resulting from physical damage to the valve grear train.

common engine valve failure is thermostate

Cylinder power imbalance. If leak occurs on the intake side, it causes intake flow issues with other cylinders. Burnt valves (due to escaping exhaust gases during the combustion cycle)

oil pump , air filter , oil filter. fuel filter 

Bent Valves,  Burnt Valves