Plain English Meaning

Spark plugs that have gotten dirty enough on the inside tip that they can no longer spark properly, so the engine runs rough or loses power.

Full Definition

Spark plugs whose firing tips have become coated with deposits — typically lead, carbon, or oil — that prevent them from producing a strong, consistent spark. Fouled plugs misfire or fail to fire at all, causing rough engine running, power loss, and a noticeable drop during the magneto check.

Context Anchor

Seen in piston-engine ignition discussions, especially during engine start, taxi, run-up, or when an engine runs rough.

Derivation

‘Foul’ comes from Old English fūl, meaning dirty or unclean. A fouled plug is simply a plug that has become dirtied to the point it no longer works correctly — the everyday meaning carries directly.