Block heater is an electric resistance heater and will take several hours to soak the block. I think they are designed to work overnight. I have no experience only general knowledge of what I have read - Alabama winters are pretty mild and I keep my 240D in the garage (have not used the heater).

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1976 240D
1987 560SL
2007 E320 Bluetec
1998 C280 (now son’s car)
1982 240D Manual - Sold