Valve stem seals are located on the intake and exhaust valves. A valve regulates the amount of fuel and air mixture that is allowed within the cylinders for combustion. Valves have guides or sleeves to keep combustion gases from passing through. A seal on the top of the valve keeps oil in the valve cover from being submerged into the engine. Seals are usually made of high concentration rubber and fit over the top of the valve stem inside of the small neck.
When valves are functioning correctly they allow some oil to pass and lubricate the valve stem and guide. They also prevent larger amounts of oil from seeping into the combustion chamber and causing excessive amounts of oil to burn. When the valve stem seals get older and the rubber ages, the valve stem seals can’t provide the proper seal between the seal and stem. This starts allowing oil to collect on the valve stems. Over time the excess burning oil causes valve carbon build up.
The added oil consumption and excess smoke aren’t the only issues. It’s very bad for the environment. If the problem isn’t fixed it can also decrease your catalytic converter life, foul spark plugs and cause engine light and emission system issues..