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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2002 E430 4matic (212,000km)
2002 ML500 'sport'

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I don't think it would have any effect at all. Coolant simply raises the boiling temperature and lowers the freezing temperature. To my knowledge it has no effect on the abiity of the liquid to absorb heat.

Here's an interesting aside: you know the pink coolant used in RV pipes? The stuff freezes! However, it's value is found in the fact that it does not expand when it freezes.

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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

Water is a better conductor of heat than anti-freeze. It would (theoretically) heat the engine better if you had no anti-freeze at all, but obviously that is a bad idea for other reasons.

Perhaps your thermostat is stuck open and you're losing some heat to the radiator?

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