Which is a sign of a faulty ignition coil?

A faulty ignition coil can lead to several noticeable symptoms in an engine's performance, all of which are common indicators that something is wrong.

Unstable RPM occurs because the ignition coil is responsible for supplying the correct voltage to the spark plugs. A malfunctioning coil may produce inconsistent spark, leading to poor combustion and erratic engine speed. This instability can be particularly evident at idle or during acceleration.

Foul spark plugs are another critical sign. When the ignition coil fails, it can cause incomplete combustion due to inadequate spark energy. This incomplete combustion often results in a buildup of carbon deposits on the spark plugs, which can be observed as fouling. Dirty or worn plugs can further contribute to poor engine performance.

Increased fuel consumption is a consequence of inefficient combustion. If the ignition coil is faulty and the spark plugs are not firing correctly, the engine may run rich, consuming more fuel than necessary. This results in poorer fuel economy as the engine struggles to maintain power and efficiency.

Considering all these symptoms together, it is clear that a faulty ignition coil affects the engine in multiple ways, making "all of the above" the most comprehensive answer. Each of these symptoms reflects the critical role ignition coils play in ensuring smooth engine operation and optimal fuel efficiency.