What Would Cause Oil to Get on Spark Plugs?

Oil on the ceramic portion of the spark plug is from an external source. If the oil is on the electrode only, there is an internal problem. This must be located not only to stop the leak, but to protect the spark plug wires as well. Hot oil will cause the spark plug wire boot to swell and soften. The high voltage ignition systems used in late model vehicles can blow a hole through the boot, resulting in a short against the engine block and may even allow water to pool in the spark plug well.

Non-Overhead Cam Engines

The majority of non-overhead cam conventional engines will have the spark plugs installed in the side of the cylinder head. The oil on the spark plugs is due to a leak in the valve cover gasket. Replacing the damaged gasket with a new valve cover gasket will cure this situation.

While the valve cover is removed, check the oil return holes for obstructions or severe sludge caused by a lack of timely oil changes. If the return holes become blocked, the oil level in the valve cover will rise and promote a leak.

Overhead Camshaft and Hemispherical Head Engines

Most overhead camshaft engines and those with hemispherical heads have the spark plugs located in the center of the cylinder head. The spark plugs are in the middle of the valve cover, which makes it necessary to isolate them from the oil in the valve cover. This is accomplished by placing a metal tube around the spark plug hole in the valve cover that is on both ends by gaskets. The top gasket seals the tube to the top of the valve cover and the lower gasket seals the tube to the cylinder head spark plug well.

Hot oil causes the lower spark plug well gasket to swell over time, which will eventually allow oil to leak into the well. Adding to the problem is the fact that the spark plug wells are not vented and will accumulate condensation due to the varying temperatures in the engine. Once the condensed water mixes with the oil, it cannot evaporate quickly. When the oil permeates the spark plug boot long enough to cause it to soften, the high voltage burns a hole through the boot and short circuits into the water and oil mixture, causing a misfire.

The correction for this is to install a new valve cover gasket along with the round lower spark plug well seals. Do not attempt to remove the valve cover or plugs until the engine is totally cold. Make sure to clean the spark plug well of all debris before removing the spark plug to prevent anything from falling into the cylinder.

Turbo Charged Engines

Turbo-charged engines further aggravate the situation due to the increased cylinder pressures associated with forced induction. This increased pressure results in increased blow-by past the piston rings and valve guides resulting in higher than normal crankcase pressures. The increased pressure transfers into the valve cover as well, making it more difficult for the valve cover gasket and spark plug well gaskets to maintain a good seal.

Oil on the Spark Plug Electrode

Oil-fouled spark plug electrodes are indicative of worn or broken piston rings or valve stem seals. The valve seals are located on the valve stems above the valve guides. Their purpose is to prevent valve train oil from draining down the valve stem, through the valve guide and into the cylinder head.

This can be corrected by disassembling the valve train and installing new valve seals. Replacing valve stem seals is a difficult process that is better left to a professional technician.

Piston rings are a different matter. If they are cracked, worn or broken a major overhaul is in order.