Valve Stem Seal
A small seal placed on the valve stem to prevent engine oil in the cylinder head from leaking down into the combustion chamber.
A valve stem seal is a small rubber or fluoroelastomer seal that fits over the stem of each engine valve in the cylinder head. Its primary purpose is to regulate the amount of lubricating oil that passes down the valve guide to lubricate the valve stem. It prevents excess oil in the top of the cylinder head from leaking down into the intake and exhaust ports and entering the combustion chamber.
Because the cylinder head valvetrain is bathed in engine oil, a seal is necessary to keep this oil out of the engine cylinders. If a valve stem seal fails, it will allow oil to seep down the valve guide when the engine is running or when it sits idle. This oil is then burned during the combustion process, creating distinctive symptoms.
The classic symptom of worn valve stem seals is a puff of blue-grey smoke from the exhaust tailpipe immediately upon starting the vehicle after it has been parked for several hours. This happens because oil slowly drips past the worn seals and pools on the back of the valves or in the cylinder while the vehicle is off. Other symptoms include oil consumption, carbon buildup on the valves and spark plugs (leading to misfires), and catalytic converter deterioration.
Replacing valve stem seals is a labor-intensive repair that can sometimes be performed without removing the cylinder head by using compressed air to keep the valves closed while the springs are removed. Otherwise, the cylinder head must be removed and sent to a machine shop.