Hi Stimpy,
Don't know the long-term answer - haven't tried it. I'd glow it longer before starting. A minute at 20F isn't too long. Neither is a minute and a half, if your hand can take it. My series plugs can take it - they're older than some forum members.

My technique: glow the snot out of it, start then advance engine speed with foot throttle just as I let go of the gorilla knob. Then crank the fast-idle knob until it takes over from the foot throttle. Adjust that fast-idle knob (under the hood) so that it is "just" off at slow idle - you'll then get good rpms at fast idle.

With a diesel you want to hang onto the starter longer than you do when starting a gas engine. How long depends on how cold - maybe a third to half second at your temperatures.

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daBenz - 1970 220D