Sounds like your idle is set too low (something must have slipped), I'm not sure how to adjust it on your car though. Might try a search for "idle adjustment" or something similar. Not sure what is causing the smoke however....

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1985 300D Turbo ~225k
2000 F350 (Powerstroke) 4X4, SWB, CC, SRW, 6spd ~148k
1999 International 4900, DT466e (250hp/660 ft/lbs), Allison MD3060 ~73k