Diesel enthusiasts often refer to their beloved powerplants by a title once considered more of an epithet than anything else. The motor's "oil burner" moniker was once well-earned, as legions of trucks and trains would pour through the countryside spewing massive clouds of evil-looking black smoke. The newest generation of clean diesel engines hasn't quite eliminated the smoking, but excessive smoke remains a bane of the malfunctioning oil burner.
Diesels are fuel-throttled engines, meaning that rpm is controlled solely by the amount of fuel that goes into the engine: The more fuel goes in, the more power the engine will make within the constraints of the oxygen present. The black smoke itself is actually aeresolized carbon, and it's what diesel fuel looks like when it gets really hot but doesn't have enough oxygen to combust. Put another way, smoke is what happens when you have too much fuel and not enough air to burn it.
Turbocharger boost leaks and turbo malfunctions are probably the greatest contributors to excess engine smoke. Turbocharging only really became common practice in the 1960s and 70s; turbos drastically increased power by shoving more oxygen into the motor and allowing it to burn more of the fuel. Cleaner emissions were an important -- if unintended -- side effect to turbocharging. Modern diesels have grown dependent on the turbocharger to the point that they cannot function properly without it; any interruption in boost delivery will stress your engine out to the point that it may, once again, take up smoking.
Compressing air makes it hotter, and hot air -- by volume -- contains less oxygen than cool air. Many newer diesels use intercoolers, which are radiators used to cool the incoming air charge after the turbo has compressed it. This one is more about a solution than a problem, since intercooler malfunction is unlikely unless there's something physically blocking airflow through it. However, the addition of an intercooler or a larger intercooler will effectively increase the amount of air going into your engine. This can help a smoke-prone diesel to run a bit cooler and cleaner overall.
Modern diesels are renowned for their ability to make massive power with little more than a new computer program and a tweak to the turbo's boost curve. Injecting massive amounts of extra fuel was once the go-to approach for aftermarket programmers, a strategy that did make more power, but resulted in massive clouds of black smoke and destructively high exhaust gas temperatures (EGTs). Modern diesel tuners have gotten a lot smarter in recent years, and many have introduced updated programs that optimize the engine's efficiency through injection timing rather than simply pouring more fuel in and calling it a day.
Not all excess fuel introduction is intentional; leaking or stuck-open injectors can inject more fuel than the engine needs, thus causing it to smoke. Since diesel fuel is basically oil, the engine is more than capable of burning any engine oil that makes its way into the cylinders. Leaking valve seals are unlikely, since boost pressure will force oil up into the valvetrain, but turbo oil seal leaks are common for older turbos and will indeed cause smoking. If a seal leaks, then you might want to try checking the oil drainback line before tearing the turbo apart. Drainback lines often clog with carbon and sludge, forcing oil pressure in the turbo to shove lubricant past the shaft seal and into the engine.
