My shop has been working on a John Deere M-Gator with a hot start problem. It starts fine when it's cold, but if you drive it a while and then shut it off, it doesn't like to restart. We changed the air filter, replaced the batteries, and cleaned the fuel filter. I'm looking for ideas.

More information:

John Deere M-Gator, 3 cylinder Yanmar diesel engine. Glow plugs appear to work during cold starts. The engine has about 800 hours on it. The old air filter was several hundred hours past due for a change. The thing has had at least two oil changes in its life, but is probably still on the original oil filter. (Parts availability SUCKS over here). The engine may have been run without an air filter in very dusty conditions at one time. Cranking speed seems ok. It sometimes blows gray or black smoke when cranking. The engine has plenty of power once started. I do not have to ability to do a compression test (no tester). I'm thinking the motor is toast, but I'm hoping someone will come up with a fix I hadn't thought of.

Edit: Can't...fix...typo...in title...

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83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
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