Valve Stem Seals – Basic Function, Seal Types, Failure Symptoms

Valve Stem Seals - Basic Function, Seal Types, Failure Symptoms
Valve Stem Seals - Basic Function, Seal Types, Failure Symptoms

Valve stem seals, control the lubrication of the valve stem, as it moves up and down in the valve guide.

By specific design, all valve stem seals are made to include, a precisely controlled leak.

But, if too little oil flows past the valve stem seals, the valve stem and valve guides will wear. However, if too much oil flows past the valve stem and valve guides oil consumption increases.

Furthermore, faulty valve stem seals, can cause a buildup of carbon, leading to valve, seat and guide damage.

Types Of Valve Stem Seals
Types Of Valve Stem Seals

Consequently, as the engine wears oil consumption or smoke, would be the first sign of a problem.

Types Of Valve Stem Seals

Valve stem seals, come in two basic types, deflector seals ( also call umbrella seals ) and positive seals.

Deflector seals ( also call umbrella seals )

Deflector seals ( also call umbrella seals )
Deflector seals ( also call umbrella seals )

Umbrella or Deflector seals, like the name implies, deflect oil away from the valve stem. Also, they move with the valve stem, to shield the valve guide from excess oil. Deflector seals are a simple and effective design, and are easy to install.

Positive seals

Positive Type Valve Stem Seals
Positive Type Valve Stem Seals

You will see positive type valve stem seals on most late model engines. Consequently, because of better emissions and oil control. A positive valve stem seal, provides a tighter seal, which reduces the amount of oil that enters the guides. This minimizes oil consumption and hydrocarbon emissions. Also, helps to keep intake vacuum high, for better idle quality. Worn valve guides and seals, will allow air to leak. As a result, causing a lean misfire and a rough idle.

So, overhead camshaft engines (OHC), require positive valves seals. Because, a positive seal, will prevent oil from flooding the guides.

Unlike an umbrella seal, a positive seal does not move. The seal does not actually make, direct contact with the stem. But, actually rides on a thin film of oil, creating a hydrodynamic seal. This allows a small amount of oil, to slip past the seal to lubricate the guide. For this reason, a precise fit is extremely important, to get accurate oil metering.

Emission Control Regulations

Increasingly stringent emissions regulations, make it imperative, that oil not enter the combustion chamber.

Diagnosis Valve Stem Seal Failure Symptoms, Under Different Conditions

When valve stem seals begin to wear or fail, they produce some obvious and unique symptoms:

Cold Engine

One sure-fire way to tell if you have a faulty valve seal, is to perform a cold engine test. Let your vehicle sit overnight. The top of the head, will have some oil left over, from the last time you drove. The oil will, slowly seep past the valve seal. And, at some point, it will end up in the combustion chamber. Consequently, smoke out the exhaust, is the end result.

Idling And Or Stop And Go Driving

Bad valve seals, will show themselves during, prolonged idling at stop signs or stop lights in congested conditions. When the vehicle sits at idle for prolonged periods, high levels of vacuum at the intake manifold result. Because, the throttle valve remains closed.

Huge clouds of blue-white smoke exit the tailpipe
Huge clouds of blue-white smoke exit the tailpipe

The high vacuum attracts oil in the heads, to congregate around the valve stems. Then, problems happen during acceleration. Firstly, the oil passes by the worn seal. Next, it passes by the valve guide. Huge clouds of blue-white smoke exit the tailpipe, after each acceleration from a stop. Consequently, the burning smoke will disappear, during cruising or highway speed.

Excessive Oil Consumption

Bad valve stem seals will cause, excessive oil consumption. In an otherwise normal engine, with good compression, rings and valve guides, bad seals will cause, a loss of oil.

Excessive Smoke

If the valve seals have deteriorated enough, the blue 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.

Blue Tailpipe Smoke
Blue Tailpipe Smoke

Bad valve seals nearly always show, an intermittent problem of oil burning. However, worn piston rings and valve guides will smoke, during all times of engine operation and never disappear.

Off-Throttle Braking Or Coasting

During off-throttle braking, worn valve stem seals, will show their ugly face. Especially, when descending a steep downgrade, where the accelerator pedal remains static. Also, with the creation of high intake manifold vacuum, coupled with the downward slant of the engine.

As a result, oil collects toward the front of the valve cover over the head. Upon pushing the accelerator after a long coast, burned oil will exit the tailpipe in copious amounts. The engine will continue to burn the oil longer in this case. But, it will still be a temporary condition, until finally the smoking stops, under normal cruise.

Lack Of Acceleration Power

The final indicator of a poor valve seal, is a lack of acceleration power. You can also perform, a compression test to see if this is the case. A higher level of compression, will indicate, that it’s a valve seal problem. However, a low level of compression, will indicate a piston ring problem.


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

Finally, with worn or incorrectly installed valve seals the:

  • The valve guides, may be starved for lubrication.
  • The valve guides, may be flooded with oil.

Either way, the engine is going to have problems.

Thank You !