A bad spark plug can make a lawn mower hard to start, cause repeated stalling, produce uneven engine performance, or prevent the engine from running at all. However, those symptoms can also come from stale fuel, a clogged air filter, a restricted carburetor, a damaged ignition coil, or another problem in the starting system. The…

A cracked spark plug can cause a misfire, rough idle, hard starting, poor acceleration, or an engine that suddenly runs badly. However, those symptoms can also come from a damaged ignition coil, fuel problem, vacuum leak, incorrect plug installation, or low engine compression, so the symptom alone does not prove the plug is cracked. The…

A spark plug gap that is too wide can cause hard starting, engine misfires, hesitation under acceleration, rough running, reduced fuel economy, and a flashing or illuminated check-engine light. The ignition system may not produce enough voltage to jump the excessive gap consistently, especially under heavy load, high cylinder pressure, or damp conditions. The difficulty…

Oil on a spark plug usually means engine oil is entering the spark plug well or the combustion chamber, and the location tells you which type of repair may be needed. Oil in the plug well often points to a valve cover gasket or spark plug tube seal, while oil on the firing end can…

Bad spark plugs can cause hard starting, rough idling, engine misfires, poor acceleration, reduced fuel economy, and an illuminated check engine light. However, those same symptoms can come from ignition coils, fuel delivery problems, vacuum leaks, sensors, or low engine compression, so replacing plugs without diagnosis is not always the right fix. Most drivers are…

Worn spark plugs can cause hard starting, rough idling, misfires, sluggish acceleration, reduced fuel economy, and a check engine light. Those symptoms do not prove the plugs are worn, because ignition coils, fuel delivery, vacuum leaks, and engine problems can produce similar behavior. Replacing plugs at the first sign of a problem can prevent a…

A failing spark plug can cause a rough idle, hard starting, weak acceleration, reduced fuel economy, or an illuminated check engine light. However, those symptoms can also come from a bad ignition coil, injector, vacuum leak, low compression, or another engine problem, so replacing plugs based on one symptom alone can waste money. The small…

A worn spark plug can cause hard starting, rough idling, misfires, weak acceleration, reduced fuel economy, or a check engine light. However, those same symptoms can come from ignition coils, fuel delivery problems, vacuum leaks, low compression, or sensor faults, so replacing plugs without diagnosis is not always the right fix. A rushed plug change…

A spark plug misfire occurs when one or more cylinders fail to burn the air-fuel mixture correctly. Common symptoms include a flashing or steady check engine light, rough idling, hesitation during acceleration, reduced power, hard starting, poor fuel economy, and a noticeable shaking or popping sensation. The spark plug is often the first suspect because…

A loose spark plug can cause a rough idle, misfire, hesitation, ticking or puffing noises, reduced power, and a flashing or steady check-engine light. In severe cases, combustion gases can damage the cylinder-head threads or eject the plug, so continued driving is not always safe. You may notice the problem after a tune-up, while accelerating…