I do all of my motors, including my 93 M104 W124.
NEVER use a high pressure hose...that will allow water to penetrate areas that would normally not encounter moisture. Then you get erratic performance, all sorts of dash lights coming on, error codes galore, etc, etc. and worst case, ruin one or more expensive engine management components. Also, the high pressure can knock leads off terminals, loosen vacuum hoses, and even break delicate parts, so use care and watch areas closely as you hit them with the hose.
Use a garden hose and low to moderate pressure. Low pressure if you are spraying on or near electrical components, and moderate pressure near the far reaches of the engine where most of the gunk resides...lower cylinder head and block area, and ancillary components that are attached in those locations.
Take into consideration the engine configuration when you decide to deep clean. In many cases the spark plug sockets are likely areas for water to "pool", causing much grief until the water is sucked away, or evaporates. Other components have shapes that can hold water as well, so make sure you go around and blot those areas dry with a towel.
Actually, I do most of my engine cleaning at the car wash bays using low pressure. Washing off dirt and rain residue after a long trip is fine at home, but I have a problem with letting engine cleaner, oil, and other caustic petroleum-based liquids trickle down my driveway into the city drainage systems. Car washes by law have a built-in reclamation system, so the runoff from the car wash does not get into the regular sewer system.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car
Last edited by G-Benz; 12-06-2001 at 05:40 PM.