Use a shop vac to pull water out of the plug wells. I had this prob once and continue to power wash the motors of other older mercs but am careful not to spray directly on the electronic devices and wiring bundles, fuse and relays etc....

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1981 300TD
2005 C55
2004 E500
1999 ML430
1992 400E
1998 C43
2001 SL500
1988 300SEL
1986 190E 16V
1991 190E 2.6