Valves are essential in regulating the amount of air and fuel mixture allowed in the cylinders for combustion. It is true that the sleeves or guides on the valves keep the combustion gases from passing through them. However, the seals on the top of the valves keep the oil in the cover from getting sucked down into the engine. Typically, the seals are made from heavy-duty rubber, and they fit over the valve’s top inside a small collar. When they start to wear out or fail, they begin to show some evident and distinct symptoms.
Starting up a Cold Engine
You will notice the most common sign of cracked or worn valve stem seals just after starting a cold engine. If your car has been sitting overnight or over a length of time, residual oil pumped up earlier will start to collect on top of the head inside the valve cover. While the car is idle, the rubber valve seal will also start to cool, causing it to contract and leave a small gap.