looks like the high tension circuit is taking an easy way to earth, through all the wetness. I would be inclined to dry the engine off completely, secure the expansion tank cap and then see what it does. A good way to see any ignition faults is after dark. Have a look at the engine whilst it is dark and you will see any HT leaks to earth. Good luck !

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1993 190E 2.0L (Euro.) sold
1991. 500 SEL (Euro.)
1991. 300 CE (Euro.) sold
1993. 500 E (U.S. spec.)