Can having weak coolant effect the efficiency of a block heater?

I checked my coolant and it's only good to -25*C!!!!! it sat for two days at -30*C and colder. The block heater was less than effective during those two days, I'm lucky my block didn't crack, although, I'm sure the block heater at least prevented the coolant from freezing....

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RIP: 80 300SD
RIP: 79 450SEL
2002 E430 4matic (212,000km)
2002 ML500 'sport'

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