i have to chime in on the side of something wrong with the glow plugs. Please avoid the ether though in the meanwhile! I also can't recommend Bosch glowplugs enough. I hate the champions.

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1978 Mercedes 240D
1981 Mercedes 240D
1982 Mercedes 240D

next step is to use a block heater. I have seen a man use a rolled up news paper, no he did not hit the engine, he lit the paper and held it in front of the intake to heat the air going in. Yes it did work.