Black Smoke From Diesel Engines Explained

With an ASE-certified master mechanic on staff, we get all sorts of questions about gas and diesel engine problems. Some of them are pretty general and hard to diagnose through a quick conversation (or a two-sentence email like "I hear a rattle. What is it?".

It is nice, though, when we're able to shed some light on a problem that makes another person feel more confident in being able to go out and make the right decisions in solving their problems. There's nothing worse than having to go to a mechanic feeling like you're at a big disadvantage because you REALLY have no idea what's going on with your engine.

"Black smoke" is one of those general symptoms that seems to indicate a problem, but which bears further investigation to figure out what the cause is. Any kind of smoke you can see would fall into this category. Because there's just got black smoke, there's also white diesel smoke and even blue diesel smoke. Each one might indicate a different type of problem So let's do a quick rundown of what to suspect when you've got diesel smoke where it shouldn't be.

Black Smoke from Diesel Engine

Black smoke is a common issue, caused by an imbalance in the air-to-fuel ratio. This can come from an excessive amount of fuel relative to the available air. In essence, seeing black smoke signifies that either too much fuel is being introduced into the fuel-air mixture, or there's an inadequate oxygen supply to properly burn that mixture. The black smoke you see consists of particulates, essentially large diesel fuel particles that normally would be burned with the fuel. Diesel engines emitting black smoke aren't going to achieve the optimal fuel mileage they normally should.

The leading culprits behind black smoke can include faulty injectors, a malfunctioning injector pump, a clogged air filter (leading to insufficient oxygen supply), a problematic EGR valve (contributing to valve blockage), or even a malfunctioning turbocharger. Some of these issues can be addressed relatively easily, while others may require more extensive attention.

Read our related article: How to Reduce Black Smoke in Diesel Engines.

White Smoke

White smoke means that the fuel that is being injected into the combustion chamber is not being burned properly. The common causes that produce white smoke range from something as simple as low engine compression or water in the fuel to the fuel pump timing being thrown off because something is preventing the fuel from getting to the pump in the manner necessary for the pump to time and work correctly.

Blue Smoke

Blue smoke results from burning engine oil. This is a mechanical problem because engine oil is lubricating oil - it isn't supposed to be getting into areas where it can be burned. This could be from a faulty injector pump or lift pump, both of which would allow oil to mix with fuel and be burned. The valves or valve stem seals could also be bad. Worn cylinders and piston rings (X-tra Lube can help with this problem) might also allow oil to seep where it shouldn't. Or you could have a problem as simple as having put too much oil in the engine. Any of this can cause blue smoke emissions.

Now that you've got an idea of the possible causes, if you're seeing any of these kinds of smoke, it's time to consult with a reputable mechanic to see how to eliminate them.

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