I've heard of the battery getting cold and not wanting to put out the rated amps.
Maybe, too high viscosity oil. Easy to check.
Weak glow plugs. Not so easy.
Gelled fuel. Easy to check.

I'm thinking probably cold battery and weak glow plugs.

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Codifex
1981 240D ChinaBlue (Got her running with a donor engine.)
1983 300DTurbo w/sunroof.
1984 300TD manual sunroof. (Electrical Gremlins)