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Water standing in the recess a spark plug sits in, on an engine where the plugs are down deep tubes in the cam cover. It reaches there through a perished cam cover gasket, a blocked scuttle drain letting the plenum fill, or simply a hose aimed into the engine bay — and once in, it has nowhere to go.

The immediate effect is a misfire that follows the weather, because the spark takes the easy path through the water instead of jumping the gap. On an engine using ion sensing the effect is subtler and worse: the plug is also a measuring instrument, and water corrupts the tiny currents it reads long before the misfire is obvious.

The tell is a coil boot that comes out wet or corroded, and a fault that is worse after rain or a wash. Drying the wells is the immediate fix and the gasket is the real one — leave the leak and the new coil follows the old one.