Valve Stem Seal – Function And Failure Symptoms

The purpose of the valve stem seal is to prevent the oil from the cylinder head entering the combustion chamber.

Valve stem seals play a critical role in controlling valve lubrication as well as oil consumption.

If the valve stem seals are worn, the guides may be either starved for lubrication or flooded with oil.

Valve Stem Seal
Valve Stem Seal

If the valve stem seals are:

  • Worn
  • Cracked
  • Missing
  • Broken
  • Improperly installed

Valve Stem Seal-This will cause the engine to suck oil down the guides and into the cylinders.

The engine may still have good compression, but will use a lot of oil.

High operating temperatures cause lower grade materials such as nitrile to harden and become brittle over time.

Eventually, this can lead to cracking, loss of oil control and valve stem seal failure.

When seals lose their ability to control the oil that enters the guide, they can cause a variety of problems.

Valve Stem Seal-Failure Symptoms

With A Cold Engine

One of the most noticeable signs of worn valve stem seals will be just after a cold engine start. Especially if the vehicle has been sitting for any length of time or even overnight. The top of the cylinder head will be coated with residual oil that was pumped up earlier during running operation. The seal has also cooled during non operation, which causes it to contract and leave a small gap. When the engine first starts up, residual oil gets sucked down through the bad seal and into the combustion chamber. A large cloud of blue-white smoke will be seen exiting the tailpipe just after start-up. The burning smoke will disappear during cruising or highway speed.

Idle and Stop and Go Driving

Bad valve stem seals will show themselves during prolonged idling at stop signs or stop lights in congested city conditions. When the vehicle sits at idle for prolonged periods, high levels of vacuum at the intake manifold result. The high vacuum attracts oil in the heads to congregate around the valve stems. Upon acceleration, the oil gets sucked past the seal and down through the valve guide. Huge clouds of blue-white smoke exit the tailpipe after each acceleration from a stop. The burning smoke will disappear during cruising or highway speed.

Off-Throttle Braking

Evidence of valve seals being compromised will also show up during off-throttle braking. More so when descending a steep downgrade where the accelerator pedal remains static. High manifold vacuum, coupled with the downward slant of the engine, oil collects toward the front of the valve cover. Upon pushing the accelerator after a long coast, burned oil will exit the tailpipe in large amounts. After that the smoke will stop again.

Spark Plug Fouling

Oil Fouled Spark Plugs
Oil Fouled Spark Plugs

Spark plug fouling may occur as oil ash builds up on the plug’s electrodes. The accumulation of heavy, oily carbon deposits on the backs of the intake valves may cause hesitation and misfires. As carbon deposits build up compression may increase to the point where it causes engine-damaging detonation and/or pre ignition problems.

Oil Consumption

Increased oil consumption due to worn or leaky valve stem seals will also increase hydrocarbon (HC) emissions in the exhaust. Oil burning can also damage the catalytic converter because phosphorus in motor oil contaminates the catalyst. If oil is fouling the spark plugs, misfiring can cause HC emissions to soar. As a result, unburned fuel passes into the exhaust.

This may damage the converter because unburned fuel in the exhaust makes the converter operating temperature soar. The converter may overheat where the substrate breaks down or melts creating a restriction or blockage in the exhaust. Bad valve stem seals will cause excessive oil consumption. In an otherwise normal engine, bad seals will cause a loss of oil.

Excessive Smoke

Blue Smoke
Blue Smoke

If the valve stem seals have deteriorated enough, the blue-white exhaust smoke will last longer after start-up and acceleration. Yet the smoke will eventually disappear after long engine operation or during periods of hot weather. Bad valve seals nearly always show an intermittent problem of oil burning. Worn piston rings and valve guides will smoke during all times of engine operation and never disappear.

Valve Stem Seal-Conclusion

Debris from deteriorating valve stem seals is another concern that can cause additional problems inside an engine. Pieces of the seal may clog oil passages starving lifters or rockers for lubrication. Valve seals normally fail due to a clearance problem between the valve and the valve guide in the cylinder head. When loose, the valve is able to move side to side in the bore and wears out the seal.

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