I plugged in my 95 E300D last night here in Chicago with our 3 degree low last night. The car was plugged in all night but this morning even though the car started right away, the engine sounded like it had not been heated at all and my temperature gauge took forever to rise (didn't get instant heat like i thought i would). Is there a fuse I can check to see if it is working or do those cold temperatures overcome eveb a block heater?

Should be able to put your hand on the engine and see if it is warm. You also should be able to hear the heater sizzle. In very cold temperatures, the gauge won't move much. No fuse on the block heater that I know of. Cords do go bad.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

At 3F, I disagree that the engine will feel warm to the touch via the block heater. Mine doesn't. Now if it is 32F, then it will feel warm. The cold wicks the heat from the edges pretty quickly at low temps.

I replaced the cord this season on mine as the cord did go bad.

Like Kerry said, you should hear a sizzle upon plugging in the block heater. That is how I've always judged whether the block heater was working.

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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