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I have a 99 F350 powerstroke..first start in morning can only happen if it has been plugged in for 30 min. Lowest temp here is about 45 degrees. New batteries, alternator and glow plugs. It also smokes white heavy at start up. After first start up no problems all day long.
It sounds like your glow plug relay has gone south on you. If the truck won't start at all unless it's been plugged in, and still blows a lot of white smoke at startup, that's what I'd replace. Fortunately, it's an inexpensive and very accessible part.
ive started my truck as lot as 10F without plugging it in and ive heard of people starting it a bit lower as well. Itll shake, rattle roll and smoke some that cold, but itll start. Good luck.