Valves regulate the amount of fuel and air mixture allowed in the cylinders for combustion. While the valves have guides or sleeves to keep combustion gases from passing through them, the seals on the top of the valves keep oil in the valve cover from being sucked down into the engine. Seals, typically made of high-strength rubber, fit over the top of the valve stem inside a small collar. When valve seals begin to wear or fail they produce some obvious and unique symptoms.
One of the most noticeable signs of worn or cracked valve stem seals will be just after a cold engine start. If the vehicle has been sitting for any length of time or even overnight, the top of the head inside the valve cover will be coated with residual oil that was pumped up earlier during running operation. The rubber valve seal has also cooled during nonoperation, 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.
Idle and Stop and Go Driving
Bad valve seals will show themselves during prolonged idling at stop signs or traffic lights in congested city 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. The high vacuum attracts oil in the heads to congregate around the valve stems. Upon acceleration, the oil gets sucked past the eroding seal and down through the valve guide, where it burns in the exhaust. 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.
Evidence of valve seals being compromised will show up during off-throttle braking, especially when descending a steep downgrade where the accelerator pedal remains static. With the creation of high intake manifold vacuum, coupled with the downward slant of the engine, oil collects toward the front of the valve cover over the head. Upon pushing the accelerator after a long coast, burnt 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.
Bad valve 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 that can be detected on the oil dipstick. By keeping an accurate record of oil level on the dipstick, a noticeable oil reduction due to the oil being burnt along with the fuel will be discovered. Bad seals will be confirmed if no oil leaks can be found on the engine to account for the loss.
If the valve 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, whereas worn piston rings and valve guides will smoke during all times of engine operation and never disappear.