1. If a diesel smokes, under any conditions other than full throttle or high load, something is wrong. Generally, too much fuel is being injected. At idle, or normal operating conditions found in the mid-west of the U.S. and Canada, diesels shouldn't smoke.
2. Check engine oil level--ensure it's filled to the top line and that the oil and filter have been recently changed. Since this often affects fuel pressure (Ford), begin by ensuring it's properly filled.
3. Black smoke is usually a signal that there's too much fuel, not enough air, or injector pump timing is off. One of the most common causes is an air inlet restriction. The cause may be a dirty air filter, a collapsed intake hose, or an exhaust restriction.
4. Check air filter--ensure it has been changed recently (every 5,000 miles).
5. Check fuel injectors. Leaking injectors create an overly rich air/fuel mixture and cause black smoke. Take the valve cover off and look at the exhaust part of the injector, when it is running. Each time the injector fires, you should see oil exhausting from it. If you see oil coming out, then the injector is good. When replacing a bad injector, take the glow plug out and crank the engine to get any oil out of the cylinder. If you don't, you could hydro-lock and bend a rod.
6. Check turbo for carbon build-up and bent turbine blades. This ties into exhaust restriction.
7. Purge fuel filter or fuel separator of water. Do this at least monthly--daily, if necessary. Water can be a big problem with diesels. Watch for and treat bio-organisms.
8. Look into a commercial diesel fuel additive, which is available at most large truck stops. Try and find one that is pure biodiesel, intended to be mixed in the tank at a B20 or less rate. Look for the words "naturally produced oil" or "biodiesel" on the side of the can listing ingredients. Properly mixed, it will reduce the amount of smoke an older, worn engine produces.
9. Once you isolate and fix the problem, use full-synthetic diesel heavy duty engine oil. Installing a by-pass oil filter makes using top-quality synthetics very cost effective, when integrated with a used oil analysis (UOA) program. All engines run better on synthetics, particularly turbo diesels. Besides significantly improved engine life, turbo life, performance, and gas mileage, synthetics promote easier starting, by allowing increased cranking speed. This last factor is particularly important in cold weather. Synthetics are also easier on batteries and starter motors. Anyone who has a turbo and doesn't use full-synthetic diesel HDEO is asking for trouble.