The duty of the valves is to regulate the amount of fuel and air mixture allowed in the cylinders for proper combustion. While the valves are made with guides or sleeves to keep combustion gases from escaping through them, the seals on the top of the valves keep oil in the valve cover from being sucked down into the engine. Seals are made of high-strength rubber and fit over the top of the valve stem just inside a small collar. When the valve seals begin to wear or fail they do produce some obvious and unique signs like;
- One of the most conspicuous signs of worn or cracked valve stem seals will be seen just after a cold engine start. If the engine has been inactive for an overnight, the top of the head inside the valve cover may get coated with residual oil that was pumped up earlier during running operation. The rubber valve seal had also cooled during inactivity, which causes it to contract and give a small clearance. When the engine first starts up, the residual oil gets sucked down through the bad seal clearance and into the combustion chamber as a result large cloud of blue-white smoke will be seen coming out of the tailpipe just after start-up.
IDLE AND STOP
- Bad valve seals will normally show themselves through prolonged period of idling at stop signs or traffic lights. When the car sits at idle for long periods, high levels of vacuum at the intake manifold happens because the throttle valve remains closed. The high vacuum attracts oil in the heads to collet around the valve stems. When the car is accelerated, the oil gets sucked past the eroding seal and down through the valve guide, where it burns in the exhaust. Great clouds of blue-white smoke is sent out of the the tailpipe after each acceleration from any stop. The burning smoke will disappear during continuous speed or during highway speed.
- Signs of valve seals being worn will show up when there is an off-throttle braking, especially when descending a steep down path where the accelerator pedal remains static and unused. In this situation with the creation of high intake manifold vacuum, coupled with the downward slant of the engine, oil collets toward the front of the valve cover over the head. Upon pushing the accelerator after a long drive without acceleration, burned oil will go out of the tailpipe in huge amounts. The engine will continue to burn the oil longer in this situation, but it will still be a temporary situation until finally the smoking stops under normal driving.
EXCESSIVE OIL CONSUMPTION
- Bad valve seals will cause excessive oil consumption. In a normal engine with good compression, rings and valve guides, bad seals will cause a loss of oil that can be detected by the use of the oil dipstick, thus by keeping an accurate record of oil level on the dipstick, a significant oil reduction due to the oil being burnt along with the fuel will be discovered. Bad seals will be confirmed if there are no oil leaks can be found on the engine.
LONGER LASTING EXCESSIVE SMOKE
- When the valve seals have deteriorated enough, the blue-white exhaust smoke will be seen longer after start-up and speeding. But the smoke will eventually disappear after long engine activities or during periods of hot weather. Bad valve seals wills always show and problem of oil burning, but worn piston rings and valve guides will smoke during all times of engine operation and never stop.