Piston rings worn
The piston rings provide a seal in the combustion chamber. If the rings wear out, oil can leak into the combustion chamber and be ignited with the fuel. The engine power may also be reduced as the breached seal will allow the fuel/air mixture to leak past the piston.
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Exhaust Blows Blue Smoke
The exhaust blowing blue smoke is a sign of burning oil. If the piston rings are worn, as oil circulated through then engine and on to the piston, it can slip into the combustion chamber. Upon ignition, the oil will burn with the fuel, producing a blue smoke in the exhaust.
Loss of Engine Power
Worn piston rings can reduce engine power by failing to seal the combustion chamber. The combustion chamber compresses the fuel air mixture prior to ignition. This allows the fuel to be burnt and expand, causing the piston to travel downwards. The rings keep the compressed fuel and air in the chamber. If the fuel/air mixture is leaking past the rings, there will be less power on the power stroke.
Additionally, this can lead to stalling.
Lack of Oil
If oil is not reaching the piston rings, the friction against the cylinder can wear the rings down during engine operation. The rings need a film of oil to reduce the amount of friction.