FWIW, idle should be 750 +/- 50 once warmed up. I usually get my idle up to almost 1000 until it warms up then I can release the idle up. This takes much longer when it's cold out and it also runs worse when it's cold out - until it warms up. If you're uncertain of the maintenance history you might take a look at the valve adjustment and change the fuel filter. These are two areas that are easy to address and can make a big difference if they've been neglected for a while. The fuel filter is easy but a little pricey compared to a gasser. The valve adjustment might not cost you anything but an afternoon wrenching. All the information is available in the manuals that are available here online. If you haven't already checked out RockAuto.com for parts you should do that. They take a few days to show up but they have good prices and they have the correct parts for these trucks. Another thing you might check is ensure the glow plugs are working and they're all good. If they're all not working it might explain some of the smoke you see. Many of these trucks have converted the glow plug circuit from the stock timer to a push button in the cab. Before I found that I had issues with mine I was having starting problems and a lot of smoke once I got it started with a shot of gasoline. I installed the button and that solved the problem until I repaired it back to stock. I could hear the relays clicking and I waited until they un-clicked (?) before trying to start. I assumed they were working - they weren't. HTH