Spark plug deposits

Spark plugs are, in a sense, witnesses to combustion in the engine. The appearance of the spark plugs (deposits) shows whether the engine is operating optimally. Before inspection, the engine must be thoroughly warmed up on a country road or highway. Checking after a short distance may lead to incorrect conclusions. Look at the tip of the insulator cone with the center electrode and the side electrodes.

The tip of the insulator nose is grey or light brown: the fuel injection system is properly adjusted, and the engine runs economically.

Heavy deposits: the cause may be additives in the engine oil or fuel, or increased oil consumption. Consider changing the brand of oil or fuel.

Black sooty deposits: the spark plug does not reach its self-cleaning temperature due to frequent short-distance trips; incorrect heat range.

The tip of the insulator nose is brown: ignition timing is too advanced, electronic ignition timing control is not functioning, or the knock sensor is faulty.

Melted traces on the center and side electrodes: pre-ignition caused by combustion chamber deposits, overheated injectors, incorrect ignition timing, ignition timing offset, a faulty knock sensor, or engine overheating.

Cracked insulator nose, visible in the initial stage as a fine crack: detonation (knock) caused by poor fuel quality, incorrectly adjusted ignition, ignition timing offset, a faulty knock sensor, insufficient engine cooling, or a lean mixture due to unmetered air entering the system.

Oil film on the electrodes and inner surface of the spark plug: worn piston rings, valve guides, or valve stem seals.

If the spark plug surface shows none of the above signs, but the engine still starts slowly or runs unevenly, the cause may still lie in the spark plugs. Invisible cracks in the ceramic insulator may fill with condensed fuel during cold starts, causing spark leakage. Spark plugs may fail during operation even though the spark is visible when the plug is removed.