I just purchased a 1982 300sd for $800. The body on it is awesome, decent paint, no rust anywhere, dent free and clear title. The front seats are ok (slight tear in the leather of the passenger) although the back seat leather is quite dry and torn on top. I have fixed all of the power windows, yarded a left turn-signal and windshield wiper linkage for it, and adjusted the valves.

The trouble is that starting it is somewhat difficult. The previous owner said the glow plug relay failed and he replaced it with a button/relay set-up. So hold the button for 10sec or so and give it a crank. It requires about 5 seconds of cranking to start, at which point it emits a quantity of grey smoke. He said it used to start fine before he ran it out of fuel because the gauge isn't perfectly accurate.

The engine seems ok, great power, no smoke except when I floor it and when it starts. The tea-kettle test on the oil-cap doesn't move a bit and there is no oil in the air filter. As I mentioned above, I adjusted the valves which helped a bit with the starting problem. So I am wondering what I can do to fix the starting issue. Could IP timing play into this at all? Glow plugs?

I just hope it isn't compression

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'84 190d 2.2 - Silver
Converted to 5-Speed
'84 190d 2.2 - Blue
Factory 5-Speed

'76 300D (W115)
Sold, found again in less auspicious circumstances

If the vendor is telling the truth. The only thing that he might have changed when he ran out of fuel was using the primer pump to get fuel back up.
See it it leaks fuel when you pump it. If so it could be effecting things by system sucking air in. Since it is cheap change it out in that case if leaking.
May not be the problem but he would have not touched anything else normally if the change in starting took place as he says. Also maybe he did not run out of fuel exactly. Leave the fuel filler cap a little loose for a couple of days to eliminate the possibility of an obstructed tank vent. That can produce symptoms of running out of fuel easily. Or hard to start. Fuel tank becomes a vacuum vessel and its hard for lift pump to overcome it and draw fuel if at all.