The purpose of the valve stem seal, is to prevent the oil from the cylinder head, entering the combustion chamber.
So, the valve stem seal, plays a critical role in, controlling valve lubrication, as well as oil consumption.
But, if the valve stem seal is worn, the valve guides may be either, starved for lubrication or flooded with oil.
Different than other seals in the engine, valve stem seals play an important role in lubrication. Because, valve stem seals allow a controlled amount of oil, to lubricate the valve stem.
But, if the valve stem seal is:
- Worn
- Cracked
- Missing
- Broken
- Improperly installed
Then, this will cause the engine, to suck oil down the guides and into the cylinders.
So, 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.
Seal Choices
So, there are two basic stem seal designs – deflector and positive seals. Depending on the application and the design
of the seal, the material used may be nitrile, polyacrylate, fluoroelastomer, silicone, nylon or Teflon.
Nitrile is one of the least expensive materials. And, has been used for many years, in umbrella or deflector type seals for older pushrod engines. Nitrile’s temperature range is -40º to 250º F. It can withstand intermittent operating temperatures of up to 300º F. However, that is usually good enough for intake valve seals, but not exhaust valve seals.
Valve Stem Seal, Failure Symptoms:
With A Cold Engine
One of the most common 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 exhaust pipe just after start up. But, the burning smoke will disappear, during cruising or highway speed.
Idle and Stop and Go Driving
Bad valve stem seals, will show themselves during, long idling at stop signs or stop lights, in busy city conditions. When the vehicle sits at idle for long periods, high levels of vacuum at the intake manifold result. The high vacuum, causes oil in the heads, to gather 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 exhaust pipe, after each acceleration from a stop. But, the burning oil 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 exhaust pipe in large amounts. After that, the smoke will stop again.
Spark Plug Fouling
So, as oil ash builds up on the plug’s electrodes, spark plug fouling may occur. 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 and cause, 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 also, cause a loss of oil.
Excessive Smoke
If the valve stem seals have deteriorated enough, the 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. On the other hand, worn piston rings and valve guides will smoke, all the time.
Conclusion
So, debris from deteriorating valve stem seals, is another concern, that can cause added problems inside an engine. As a result, 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. So, when loose, the valve is able to move, side to side in the bore and wears out the seal.
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