خرابی عایق کویل در حالت گرم

An ignition coil whose internal insulation has degraded, breaking down under heat and load so the spark finds an easier path to earth than the plug gap. The coil works cold and fails hot, which is the whole of its diagnostic character.

So the complaint is a misfire that appears after twenty minutes and disappears overnight. The vehicle arrives running perfectly, every static test passes, and the resistance measurements are within specification — because resistance measured cold says nothing about insulation that fails at operating temperature under thousands of volts.

Load and heat are needed to reproduce it, so testing at idle in a cool workshop will not. A visible crack or tracking marks on the coil body confirm it — look for a thin carbonised line down the insulator. Oil in the plug well accelerates it by attacking the boot, so check the well before fitting a replacement, since a new coil into an oily well fails the same way.