First glow plugs; I check plugs with an amp meter plugged between the battery and each GP socket on the relay output connector. Replace the 80 amp fuse. They often develop a hairline crack which you can't see till you take it out.

Valves are the next most likely culpret. Set them COLD after the car has sat overnight. Use a new valve cover gasket.

While setting valves, I'd check timing chain stretch. If the valves are getting to 4 or more degrees delayed, that can affect cold starts too.

IP timing would be the next issue, but only if it's way (6 degrees) off.

After these, I'd assume low compression and just use a block heater. I live at 6000 feet elevation, so cold starting ability is an issue here. I've been learning and working on (3) 300D's the last 3 years.

Bruce McCreary
(2) '85 300Ds, '83 300CD
Snowflake, AZ