Symptoms of Worn or Failed Valve Stem Seals

Modify: Aug. 23, 2021

Valves regulate the amount of fuel and air mixture allowed to burn in the cylinder. While the valves have guides or sleeves to prevent combustion gases from passing through, seals on top of the valves prevent oil in the valve cover from being drawn into the engine. The seal is usually made of high-strength rubber and is mounted on top of the valve stem inside the small collar. When valve seals begin to wear or fail, they can produce some distinct and unique symptoms.

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1.Lack of acceleration

The ultimate 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. Higher levels of compression will indicate that this is a valve seal problem. However, low levels of compression indicate a problem with the piston rings.

2.Oil Consumption

Poor valve seals can lead to excessive oil consumption. In otherwise normal engines with good compression, rings and valve guides, poor seals can lead to oil loss, which can be detected on the dipstick. By accurately recording the oil level on the dipstick, a significant reduction in the amount of oil due to oil burning with the fuel will be detected. If no oil leak is found on the engine to compensate for the loss, a poor seal will be confirmed.

3.Stop-and-go driving

In crowded urban conditions, poor valve seals will be evident when idling for long periods of time at stop signs or stop lights. When the vehicle is idling for long periods of time, the vacuum at the intake manifold can be high as the throttle remains closed. The high vacuum attracts the oil in the head to collect around the valve stem. When accelerating, the oil is drawn into the corroded seals and down through the valve guides to burn in the exhaust. Each time you stop accelerating, the exhaust emits a large amount of blue-white smoke. The burning fumes disappear at cruise or highway speeds.

4.Cold Engine

One of the most obvious signs of worn or cracked valve stem seals will be after a cold engine start. If the vehicle sits for an extended period of time or even overnight, the top of the head inside the valve cover will be coated with residual oil that was pumped out earlier in the run. The rubber valve seals also cool during non-operation, causing them to shrink and leaving small gaps. When the engine is first started, residual oil is drawn into the combustion chamber through the poor seal. Just after starting, a large cloud of blue-white smoke is seen exiting the exhaust pipe.

5.Excessive Smoke

Blue-white exhaust smoke will persist longer after starting and acceleration if the valve seals have deteriorated to a certain point. However, the smoke will eventually disappear after long engine runs or during hot weather. Bad valve seals almost always exhibit intermittent oil burning problems, while worn piston rings and valve guides will smoke all the time the engine is running and never go away.

6.Non-Throttle Braking

During non-throttle braking, especially on steep descents where the accelerator pedal is held stationary, signs of valve seal damage can occur. As vacuum is created in the intake manifold and with the downward tilt of the engine, oil collects in the front of the valve cover above the head. As the gas pedal is pushed after a long coast, a significant amount of the burned oil will exit the exhaust. In this case, the engine will continue to burn oil longer, but it will remain in a temporary state until it eventually stops smoking under normal cruising.

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