For the WD40 to ignite, you need a healthy starter above 180 rpm and an engine with good compression. Just think of it this way, your injectors can atomize fuel much better than a WD40 spray can. WD40 is expensive $5 a can. I'd think you'd be much better off priming the system and let the injectors supply the fuel. Replace those glow plugs with Bosch Duraterms and also replace that starter, then your starting troubles may be over.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked