Ignition coils are an integral part of a car's ignition system. Without the coils, the engine could not create the necessary sparks of heat to explode the combination of fuel and air pumped into the engine cylinders. In order to do this job properly, the ignition system must work very fast with incredible accuracy. This puts an ignition coil under constant stress.
The ignition coil is actually two coils, or sometimes a system of several coils, one for each cylinder. They create magnetic fields and then immediately collapse them to force a bolt of very high voltage into the spark plugs. The spark plugs use this voltage to make the spark that lights the fuel, but the voltage must travel through the coils first. The coils are made out of wound copper wire, with the primary coil consisting of 100 to 150 coils of wire and the secondary, smaller coil made from up to 30,000 coils of fine copper wire.
Because of the high voltage that is constantly forced through the ignition coils to create the magnetic fields and power the spark plugs, it is very common for ignition coils to run hot, especially after they are used for a long time. The coils should quickly cool, however, because they are designed to dissipate heat quickly and avoid damage to the ignition system.
Oil is pumped into the ignition system to help cool it. It is important that this oil is present and stays clean for the coils to work properly. If for some reason the oil becomes thick with dirt or other particles, it may not do a good job of cooling the coils, and that can create too much heat in the coil compartment.
Ignition systems are built to last a long time, but they do not last forever. Eventually, the insulation used on the wires wears down and begins to crack. This can cause the ignition coil to short out completely. Before then, however, it also may cause the coils to run hot, as electricity is released through the worn parts of the coil as heat.
If the wires on the ignition coils touch, they will cause a feedback loop and immediately overheat an ignition coil. This will typically destroy the coil entirely, but it also may create a large amount heat as it burns out. If the ignition coil is hot and no longer works, this is a possibility. If the ignition coil is hot but still works, however, the heat is probably caused by something else.