I just wanted to clarify about 'feeling the valve cover' and radiator hose. When the temp gets cold (and my cold is 15F or lower) the valve cover or hoses do NOT feel warm. Maybe if it is plugged in at temps above 40F it may feel warm but mine has never felt warm. The cold wicks the heat from the edges of the engine away pretty quickly.

If the block is cold I can almost always hear the sizzle when I first plug it in.

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2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD