Seeing thick black smoke from your diesel vehicle’s exhaust can be alarming. The thick plumes are rarely a good sign. What does it mean when the smoke starts, and what can you do to reduce these emissions?
Black smoke is more common on older diesels than on the newer electronic diesels equipped with particulate filters. Even before particulate filters, light vehicle diesels were being designed to produce almost no smoke by careful injector pump tuning. This has been true since the early 1980s.
The black smoke from the exhaust of your diesel vehicle is likely a result of an imbalance in the air-fuel ratios in its system. When there’s too much fuel and not enough air in the mix entering the engine, the fuel doesn’t burn up completely. The diesel particles instead exit the engine and leave the exhaust as thick black smoke.
Diesel fuel fires on compression (2 degrees increase in temperature per pound of compression) rather than using a spark, and diesels don’t typically use a throttle plate to determine how much power the engine is producing. If more fuel than is needed is delivered to the cylinder during the power stroke, some of that fuel will exit the stack or tail pipe as hydrocarbon soot.
If more fuel than is needed is delivered to the cylinder during the power stroke, some of that fuel will exit the stack or tail pipe as hydrocarbon soot.
–Richard McCuistian, ASE Certified Master Automobile Technician
Pro Tip: To increase the power and speed of a diesel, you just add more fuel. The engine is always getting all the air it needs even if it isn’t turbocharged.
Typically, when a diesel emits large amounts of black smoke it’s due to aftermarket modifications (either electronic or mechanical) or some issue with the injectors or the injector pump. And although some diesel vehicles will naturally emit black smoke during hard acceleration, a consistent and excessive flow of black smoke isn’t normal. There are many things that could cause this.
It’s best to take your vehicle to a mechanic for a proper diagnosis, but here are a few common causes:
When the fuel injectors in a diesel vehicle are worn or blocked, it could lead to excessive injections of fuel into the engine. This results in the imbalance of air to fuel in the system, which leads to black smoke leaving your exhaust.
The air filter in your diesel vehicle maintains a specific level of differential pressure inside itself in order to keep the air flowing. There’s a pressure drop between the airflow entering the filter and the filter’s inner sections. When the filter fails, it tends to collapse in on itself, and the air that would otherwise come out clean will instead show signs of contamination.
The fuel injector pump is in charge of delivering fuel in precisely timed and measured amounts to the engine. If the injection pump isn’t as precise as it should be, the air-fuel ratio in your diesel engine can be thrown off, leading to black smoke from the exhaust.
Modern diesel engines are often equipped with a turbocharger. The turbocharger compresses the air flowing into your engine’s cylinder. This packs the air molecules closer together, which means more fuel can be added to the cylinder without throwing off the air-to-fuel ratios. When a turbocharger malfunctions, air compression won’t be as measured. The ratios will no longer be consistent, which can result in black smoke.
As part of the EGR system, the EGR valve helps limit nitrogen oxide (NOx) emissions from a vehicle’s tailpipe. If it fails, you might see an increase in emissions from your diesel vehicle’s tailpipe, including blackened smoke.
Pro Tip: Black smoke itself doesn’t indicate a presence of NOx. Black smoke is hydrocarbon soot and particulate matter from too much fuel and not enough air. NOx is an invisible gas and is created during high combustion heat where the nitrogen and oxygen in the air are bound together during combustion.
EGR recirculates cooled exhaust gas in a diesel to lower combustion chamber temperatures, reducing the amount of NOx created during the combustion event.
Your vehicle’s camshaft is the long bar in its engine with egg-shaped eccentric lobes. It manages the precise intervals that the engine’s valves and fuel injectors operate at in relation to the motions of the piston. If your camshaft lobes are worn out, these intervals are thrown off, and the air-fuel ratio can be affected, leading to black smoke.
In your vehicle’s engine, there are small gaps between the top of the valve stems and the mechanism that presses on them to open the valves. These are called valve clearances. Over time, wear and tear can affect your valve clearances, leading to poor performance, rough idle, and black smoke coming from your exhaust. The process of fixing the valve clearances is called adjusting the tappets.
When the exhaust system isn’t blocked, exhaust can flow freely. With mufflers and pipe bends, there is always just a bit of backpressure, but not much. But when there is abnormal exhaust system blockage of any kind, the engine will have to struggle to push the exhaust out. This is what it means to have excessive backpressure, which is not normal.
When there’s excessive backpressure, it throws off the air-fuel mixture because the engine can’t “breathe freely” and this can result in black smoke.
If your fuel filter is obstructed by contaminants, it can starve your engine of fuel during hard acceleration. This reduces the engine’s performance and can end up affecting the air-fuel ratio. Misfires and increased engine smog, which can appear as black smoke, are also a result of a dirty or clogged fuel filter.
Contaminated fuel can indirectly cause black smoke by damaging the other parts of the system. For instance, asphaltenes will clear engine injectors, but will tend to clog engine filters. Colloid carbon, which is a result of oxidized stored diesel, can also block the filters along with grit, sand, sludge, wax, and rust.
If you see black smoke coming out of your diesel vehicle’s exhaust, it’s important to have it repaired soon. The smoke can have an impact on both your health and the environment if it continues to be emitted by your vehicle.
Diesel exhaust is detrimental when inhaled in large quantities. The concentration of diesel particulate matter in black smoke only adds to its potency. Short-term exposure to these high concentrations can lead to dizziness, coughs, headaches, and irritation in your eyes, nose, and throat. Prolonged exposure can lead to lung cancer as well as cardiovascular, cardiopulmonary, and respiratory disease.
Diesel emissions can pose a grave threat to the environment. They can damage crops, trees, and other vegetation because they contribute to the production of ground-level ozone. These emissions can also affect the soil and natural water systems through acid rain.
The best way to keep your vehicle’s black smoke emissions to a minimum is to follow its regular maintenance schedule. That being said, aging engines will eventually decline in performance and efficiency, which can lead to increased black smoke emissions. Diesel fuel in particular can leave deposits in your engine that will result in these emissions.
You can add a detergent additive to your diesel fuel to help clean out these deposits. The additive can also reduce the amount of incompletely-burned diesel in your vehicle’s system, which will in turn reduce the black smoke from your emissions.
On older diesel vehicles with large turbochargers, you can also try adding a combustion catalyst to the fuel. Large turbochargers in older vehicles add too much fuel to engines that turn at low revolutions per minute (RPM).
Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.