Valve stem seals are made of high-strength rubber and as the name suggests, sit around the inlet and exhust valve stems. The seal’s purpose is to keep oil located within the valve cover from being pulled into the combustion chamber. over time, valve stem seals wear out and start to allow oil into the combustion chamber. Clouds of blue-white smoke coming out of your exhaust is a tell-tale sign that your valve stem seals are failing. These clouds are caused when the oil leaking down into the engine cylinders from the valve cover is ignited. They often form after the car has sat overnight or when the car has been ticking-over for long periods. Excessive oil usage is another potential sign that you need new valve stem oil seals.