I haven't been driving my 240D because of the alternator belt, but I started it the other night when it was around 10, just to see what would happen. It started up easy, one glow cycle, without much cranking ... a little stumbly idle but smoothed out. Not bad considering it wouldn't start at all in 50 degrees before I put the fast glow system in. My 300D has started in as low as -7 F, never used a block heater on either. Those temps are ambient ... keep in mind wind chill doesn't affect a car ... other than the fact that your frozen hands my not be able to turn the key ...

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1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)