How to Read Diesel Smoke Signals

In early April, XDP published a useful tech talk article on how to diagnose what's going on in your engine by what your smoke is telling you. This article is a brief summary of the highlights from that story. We'll include a link at the end here so you can read further.

The article begins by stating flat out that there should be no smoke coming out of the tailpipe of a properly running diesel engine. The exception might be a small burst of black smoke upon acceleration in older diesels.

If you do have smoke, it will be either black, blue or white.  

Black Smoke is indicative of a problem with the air/fuel ratio. The culprit is often the fuel injectors, though not always. Fuel injector issues can include damaged nozzles, worn parts, over-fueling or incorrect injector timing. Sometimes there's a problem with injectors sticking open too long with common rail diesels.

Other areas to explore with a black smoke problem include clogged air cleaner, malfunctioning turbocharger, faulty injection pump, low compression, sludge buildup, excessive carbon in the intake manifold, and even low quality fuel.  

White Smoke will signal a different set of issues. Often it means that not enough fuel is being burned, or that unused fuel has slipped through to the exhaust. It could also mean that your engine is too cold on startup, or that coolant has somehow seeped into the combustion chamber.

With white smoke you may also have an injector issue, either damaged or incorrect timing. Other potential issues can include damaged rings or cylinder liners, water in the fuel due to a head gasket leak, low cylinder compression, malfunctioning glow plugs, or even a fuel pump issue.

Blue Smoke is the least common of the three.  It means you're burning oil in your engine. Not good. Underlying causes can include damaged, worn or sticking rings, damaged or worn cylinders, oil too thin for your application, worn seals, too much oil in the engine or a damaged lift pump. 

Whatever the cause, excess smoke is not something you really want to see. Here's the full article at XDP: Diesel Smoke Diagnosis

Related Link
Looking for Blue Flame Diesel Oil? Now Available at Xtreme Diesel Performance

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Over-Fueling Causes Black Smoke

Modifying a Cummins, Powerstroke or Duramax diesel engine with some sort of OBD-II plug-in or computer-controlled engine management system is one of the simplest ways to improve power for diesel racing and pulling. It’s really not that complicated. You don’t have to take the diesel engine apart, and the modifications cost far less than what’s required to get similar power gains as, for example, from a gasoline-powered motor. 

Because diesels “throttle-push” the amount of fuel injected into the engine rather than limiting the amount of air entering the engine, notable power gains can be achieved by just increasing the amount of fuel delivered. All that’s required is the aforementioned electronics box to alter the signals from the onboard computer that controls the timing and length of the fuel injector pulse and the fuel pressure. There are all sorts of diesel tuner boxes on the market that promise plenty of big power.  

While you can make more power with more fuel, if you don’t do it right, exhaust gas temperatures can go out of sight. This can spell eventual death for the diesel engine or turbocharger. Many diesel racers and pullers are unfortunately unaware of such potential engine damage. They love to increase power and are willing to assume some ownership of the risk, and that’s just what you do when you over-fuel a diesel. 

It is only after getting more airflow into, and out of, a diesel engine that max-power can be achieved safely.  These enhancements include a bigger or less restrictive intake path, inducting cooler air, bigger valves or different camshaft timing, a less restrictive exhaust system, increasing cylinder head flow through porting, turbocharging, or supercharging.

It’s no secret that max-power increases happen when you control both fuel and the airflow. If you don’t burn the extra fuel, then you’re increasing the exhaust gas temperature of a diesel engine  If the exhaust temperature isn’t too excessive, and if it occurs for only for a matter of a few seconds at a time, most diesel engines can withstand the abuse without incurring permanent damage, but there is always that risk.  

The telltale sign of over-fueling and lack of a balanced air flow strategy is excessive thick black smoke coming out of the exhaust pipe.

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