Ignition coil is overheating what is the cause of this?

Overheating ignition coils can be due to a number of causes. The symptoms are always the same though: poor or non-existent hot engine starting and rough running and misfiring whilst running.

The first is simply due to age: if it's traditional "wet" coil filled with transformer oil, then the oil can either leak out or simply break down with age. If so, the fix is simply to replace it and there are often modern compatible dry resin coils which do away with the oil filling completely. Dry resin coils can also fail with age or hard use, but it's far less common than with wet coils. A failing coil which usually works perfectly well be over-stressed if the vehicle is left idling for long periods on a hot day, such as during summer traffic jams. In such conditions, the coil will usually recover if the engine is turned off allowing the coil to cool down, but the damage caused by overheating is cumulative and the coil will become increasingly prone to overheating.

The second most common cause is a poorly tuned engine (if the vehicle is equipped with a distributor). If the ignition timing is incorrect or if the points gap has lessened due to wear then the coil can be energised for too long and will overheat. Note that the points gap and distributor timing are related: if the points are replaced or re-gapped, then the timing MUST be checked and reset both statically (engine not running, turned over by hand) and dynamically (engine running). Incorrect timing can also be down to a faulty or non-functional ignition advance and ****** mechanism. If you do not have ignition contact points, then there may be a fault with the electronic ignition module within the distributor.

If the vehicle has completely electronic ignition with no distributor then it's possible for faulty components to cause overheating coil. Most incorporate a variable dwell feature which prevents the coil from overheating when the engine is idling for extended periods. But I suspect this does not apply to your vehicle as you refer to a singular coil rather than the multiple (dry resin) coil packs mounted directly atop the spark plugs as per modern fully electronic ignitions. In any case, diagnosis of a fault with this set up requires electronic diagnostic tools.

I hope that my reply has been of assistance to you; if so please take a moment to rate my answer. If there's anything I've written which is unclear then please add a comment asking me for further clarification.