How To Diagnose and Fix Oil On Spark Plugs

​​The spark plug is the "pacemaker" of the car engine. It ignites the fuel-air mixture at exactly the right time so that the pressure of the explosion pushes the piston down, which in turn causes this component to rotate the crankshaft. A defective spark plug noticeably and directly reduces the performance of the engine.

How often should you check spark plugs?

​​As a rule, spark plugs should be replaced every 30,000 to 60,000 kilometers. But now there are also spark plugs that last up to 100,000 kilometers. Many spark plug manufacturers recommend checking the function of the spark plugs once a year.

Symptoms of defective spark plugs

A fault in the ignition system can manifest itself in the following ways:

  • The engine starts poorly or not at all.
  • The engine has slight misfiring while driving.
  • The engine lacks power.

The ignition system is also the first choice for troubleshooting these symptoms because it is easily accessible and easy to repair.

Visible signs of spark plug problems

  • Heavy oiling: Worn piston rings or valve stem seals.
  • Contacts fused: Overheated engine, fuel mixture too lean
  • Shaft discolored brown on the hexagon: the candle is not seated correctly, the gas mixture can escape and ignite outside the combustion chamber
  • Spot welds or dimples along with flash point: Candle overheated or overaged
  • charred black: fuel-air mixture too rich
  • Ceramic body cracked: candle overaged

How to fix oil on spark plugs

You should be particularly sensitive to an oily spark plug. This indicates that too much oil is entering the combustion chamber. If the mixture is far too rich, the candle will not only be black and sooty but also oily and wet. 

An oily spark plug could also indicate a defective shaft seal. If the shaft sealing ring is leaking, the engine draws the oil into the combustion chamber - the candle becomes oily and the engine smokes. If you have such a spark plug, you should stop driving and take care of the problem immediately

With a bit of luck, the reason can be simple: The oil quantity could be too large. In this case, changing the oil correctly can help. 

The engine vent could also be clogged. This can also be easily cleaned or replaced. Before cleaning, you should pre-clean the spark plugs with a rag. The actual cleaning of the electrode should be done with a fine wire brush. Clean each spark plug neatly and carefully, then reinstall. Do this very carefully.

In most cases, however, the cause of an oily spark plug is the wear on the piston rings or valve stem seals. You should research some compression tests and consider whether repairs are still worthwhile.  


If a spark plug has problems generating sparks, the engine will no longer run optimally. This is often noticed by a loss of performance and increased fuel consumption. In many cars, the check engine light coming on while driving can also indicate a defective spark plug.