All glow plugs test a little below 1 ohm (when disconnected from the others). Plugs get voltage when the key is turned. I think they are OK.

Here is my new interesting update though: I left the car out in the cold the other night, no heater or charger, and started it the next morning. Probably low 20s overnight, maybe colder. One glow plug cycle and boom, fire and running. This is probably 7 hours later. OK.........

So, I leave it in a garage that day, overnight, with my new little magnetic heater on it. Come back and it won't start. Probably 24 hours later.

So, I am poking around under the hood and notice my secondary filter has lots of air in it..... I prime my stupid old leaky primer pump and nothing happens. I jiggle the filter and tilt it and fuel dribbles in it from the line. So, with fuel in it, I try again. One glow cycle and boom, it starts.

Fuel delivery: It's where I'm at now. I will try changing out both filters (I got 'em sitting on the shelf already, just in case kinda thing) and inspect all the fuel lines (especially the rubber ones that go into the tank).

Anything else I should look for here? This sounds like a VERY likely cause for the no-start.

And lastly, what relation might the cold have to this? Any?

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'79 240D 4 speed manual 105k miles