Is it spinning over, but not starting?

Take the Fuel Cap off and see if you here a vacuum when you do. If so, the tank vent may be clogged and the fuel is not getting pulled up to the IP. This is a common problem with cars that have set up for a while.

Is the prefilter clear or does it look dark? May have clogged filters.

Any fuel leaks? If so, it may also be pulling air into the system.

Touch the resistor wire between the first two glow plugs (cyl.1 and cyl.2) very quickly after glowing the plugs. Do not grab it, but rather brush your finger across it very fast. Is it hot, if not you have a glow problem. If it is, you may still have one but it's less likely.

These are just a few things that are easy to check.

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85 300CD
83 300TD
78 240D (daughter)