What causes oil to go into spark plugs?

Oil getting into spark plugs is a common issue that can cause problems with engine performance and drivability. There are several potential causes for oil making its way into the spark plug wells and fouling the spark plugs that need to be examined.

Worn valve guide seals

One of the most common causes of oil getting into spark plug wells is worn valve guide seals. The valve guide seals help retain oil in the valve train area and keep oil from seeping down into the combustion chamber. When these seals get worn or start to fail, oil can begin leaking past the seal and make its way down into the cylinder and spark plug well.

On overhead cam engines, the valve guide seals are located right above the spark plugs, so when they leak, the oil can drip directly onto the spark plugs. On overhead valve engines, the valve guides are located higher up in the cylinder head, but leaking oil can still seep past the valve seats and down the spark plug wells.

Replacing worn valve guide seals is the proper repair for leaking seals. The cylinder head will need to be removed on an overhead cam engine to gain access to the valve guide seals. On overhead valve engines, a valve job procedure can be performed with the cylinder head still on the engine in some cases. New seals will be installed during this valve repair work.

Worn piston rings

Worn or damaged piston rings are another source of oil getting into the combustion chamber. The main function of the piston rings is to seal the combustion chamber and prevent oil from getting past the rings from the crankcase into the combustion area. When the rings get worn, warp, or break, they lose their sealing capability and allow oil to be thrown past the rings as the piston goes up and down.

Once oil gets past the rings into the combustion chamber, it can make its way down the spark plug wells and foul the plugs. Replacing worn piston rings and honing the cylinder bores during an engine overhaul is the correct repair for addressing this issue.

Excessive oil consumption

If an engine has an issue that is causing it to consume or burn too much oil, this can also allow oil to readily get past the piston rings and down into the spark plugs. Some common causes include:

  • Worn piston rings
  • Damaged or sticking oil control rings
  • Worn valve guides
  • Clogged PCV system
  • Excessive blow-by

Finding the root cause of excessive oil consumption such as worn internal engine components and fixing that problem will take care of the issue of oil getting into places it shouldn’t, such as the spark plugs.

Incorrect spark plug heat range

Running spark plugs with an incorrect heat range for a particular engine can also lead to oil fouling of the plugs. The heat range of a spark plug refers to its ability to transfer heat from the tip to the plug body and cylinder head.

If the plug is too hot for the application, the tip temperature will get very high during combustion and allow oil to more readily carbonize and foul the plug. Running a cooler heat range plug suitable for the engine will help minimize this carbon fouling issue.

Inadequate engine warm up

Letting an engine idle for an extended period without proper warm up can also contribute to spark plug oil fouling in some cases. When an engine is first started cold, the piston rings have not expanded and seated to the cylinder walls to form a tight seal. This allows more blow-by past the rings during cold idle conditions.

Extended idling to warm up the engine allows more oil blow-by getting into the combustion chamber, where it can then easily make its way into the spark plugs wells. It’s best to drive the vehicle gently after starting to get the engine up to normal operating temperature quicker.

Damaged dipstick/filler tube

On older engines, a damaged or missing engine dipstick tube can be a cause of oil getting directly into the engine and spark plugs. The dipstick tube provides a path for air from the valve cover to get pulled into the intake manifold and combustion chambers.

If the dipstick tube is damaged or missing, oil can travel down this path directly into the intake and cylinders. Installing a new filler tube properly sealed to the valve cover and intake is the repair.

Clogged crankcase ventaliation system

The positive crankcase ventilation (PCV) system is designed to draw vapors and blow-by gases out of the crankcase and route them back into the intake to be burned. A clogged PCV system can cause excessive pressure build up inside the crankcase.

This increased pressure can force oil past the seals and rings into the combustion chamber. Cleaning out the PCV components and passages will allow the system to properly vent the crankcase again.

Excessive engine sludge

A build up of thick oil sludge inside the engine can also contribute to oil getting past seals, rings, and into the combustion chamber. Heavy sludge can block oil drain back passages as well as clog the PCV system.

Oil sludge problems are most common on engines that have gone too long between oil changes or used cheap low quality oils. Performing engine flushes and switching to the proper quality oil can clear out sludge build up inside the engine.

Conclusion

Oil fouling of spark plugs is most commonly caused by worn valve guide seals, piston rings, and other engine components subject to high pressures during combustion. This allows oil to leak past worn seals and by the piston rings into the combustion chamber where it can foul the plugs.

Other issues like incorrect spark plug heat range, poor engine warm up, and problems like sludge build up can also contribute. By examining all potential causes and making the necessary repairs, oil getting into spark plug wells and fouling them can be resolved.