Diesel Engine Troubleshooting: Black Smoke

With the modern advances in diesel engine technology, there should be no reason to ever see smoke coming from a diesel engine. You might see a small puff of smoke under acceleration (because of turbocharger lag) in older engines. But the days of diesel big rigs being characterized by belching black smoke clouds are gone and won’t be coming back.

That means if you do see smoke, it’s a sign that something is wrong. It could be a simple issue that’s pulling the combustion of the fuel away from an optimal state. Or it could be a big flashing warning sign preceeding a catastrophic death for your diesel engine. So, if something’s wrong, as a diesel owner you want to do something about it before it costs you more than it needs to.

Black diesel smoke means something is causing incomplete combustion of the diesel fuel. The smoke is made up of soot particles – partially-burned carbon particles. Full combustion of the fuel would give you carbon dioxide and water, stuff you wouldn't see coming out of the stack. Part of the engine working properly means it’s supplying the correct ratio of fuel to air needed to get the job done.

If something is wrong in the engine, this ratio could be off. Black smoke means there’s too much fuel or not enough air to make an ideal combustion reaction.

If you’re trying to trouble shoot fixing this problem, unfortunately there are lots of things that could be causing it.

Having excessive carbon buildup in the wrong places or a faulty turbocharger.
Dirty injectors are probably the most common culprit (but also most easily fixed by using a detergent fuel treatment).
A restricted induction system will affect the amount of air available.
Worn or sticking iston rings and cylinder components can affect the cylinder compression such that combustion is moved away from an ideal state (this requires a mechanical fix).
You could have incorrect valve clearance or incorrect timing.

Not to mention the possibility of poor quality or bad fuel, or even fuel that has an inadequate cetane rating.

It’s possible that there may be nothing mechanical off with your engine, but the fuel doesn’t have the quality necessary to burn properly in the engine without causing smoke. Of all the things mentioned, this is the one you want to be happening. Fuel problems are easily fixed, most of the time. Poor cetane rating is a common cause of black smoke, and easily fixed by adding a cetane improver. If it is the problem, you’ll see almost immediately results.

Black diesel smoke is such a broad problem that it’s impossible to troubleshoot with 100% what could be causing the problem, without getting into the engine. That’s where having a good relationship with a reliable mechanic can help.