4 Causes of Too Much Black Smoke From Exhaust in Diesel Engines

Modern diesels aren’t the sooty smoke machines of old. Though black exhaust plumes seem dirty, gasoline engines emit invisible emissions too. Properly running trucks with common-rail diesel engines should discharge minimal smoke.

But EXCESSIVE black smoke means something’s not right. It isn’t just bad for the environment, but bad for your wallet. Let’s take a look at the common causes of too much black smoke and how to fix the problem.

rolling coal

See Also: Causes of Blue Smoke From Your Exhaust

Top 4 Reasons for Too Much Black Smoke From Exhaust

#1 – Faulty Fuel Delivery

Over-fueling is a common cause of black smoke in diesel exhaust. This occurs when there is too much fuel being injected into the combustion chamber, leading to excess fuel not being burned off properly

Look at the fuel delivery system. Try the simpler stuff first. Go over the injector timing and the EGR system. If there’s an issue with it, an EGR valve may need to be replaced. If that doesn’t fix it, it’s most probably a mechanical issue.

Diesel fuel injectors play a significant role in over-fueling issues. Injector wear can cause the nozzle opening to enlarge or erode the injector needle, allowing more fuel to flow into the combustion chamber.

Check the injectors. It’s possible that they’re clogged up or that they simply need to be replaced.

Wrong valve clearances have been known to give the same problems, but that’s uncommon.

EGR Related DTCs: Code P0402, Code P0404

#2 – Dirty or Clogged Air Filter

dirty air filter

Black smoke exiting the exhaust means one thing: the fuel is only partially-burned fuel. A normal engine burns all of its fuel, giving out water and CO2. Black smoke indicates that something is causing the fuel to only burn partially and an incorrect air/fuel mixture is the result.

Because diesel fuel is only part of the mixture in the combustion process, we have to take a look at the other one: air. If there’s not enough air getting through to the engine, the mixture will be too rich, i.e. the ratio will favor the fuel.

Check the air filter for starters, and replace it if it’s dirty. If there’s not enough air getting in you’ve found your culprit.

See Also:

#3 – Bad MAF Sensor

MAF sensor

As noted above, a proper air/fuel ratio is important to make sure all fuel is burned. Not only do you need to make sure the air filter is letting in enough air, but you also need to make sure the vehicle’s computer is correctly measuring the amount of air coming in.

The mass airflow sensor (MAF) is responsible for making sure the correct amount of fuel is mixed with the air that’s coming into the engine. If the MAF sensor is bad, it may register more airflow coming in that is actually occurring. This results in an excess amount of fuel being injected into the engine. The end result is unburned fuel showing up as black smoke.

#4 – Excessive Carbon Deposits

If your car is older and has a lot of miles on it, it could very well be that it’s simply accumulated a lot of combustion product in critical areas resulting in less than ideal functioning. Because diesels run for longer and the fact that diesel is crude fuel, with no detergents from the refinery, diesel engines are prone to carbon buildup

The fix in most cases is easy. Add a detergent additive to your diesel fuel, and do it on a regular basis. My favorite diesel engine additives at this time are Liqui Moly Pro-Line Engine Flush and Hot Shot’s Secret Diesel Extreme. Both are known to do a fantastic job.

Effects on Your Engine and Exhaust System

Engine Cylinders

When there’s black smoke from your diesel engine, it could harm your cylinders. It means there’s partially burned fuel in the combustion chamber, which may lead to deposits on cylinder walls and pistons. These deposits can cause abnormal wear and tear, reducing your engine’s lifespan.

Catalytic Converter

Since black smoke indicates unburned fuel, it can also negatively impact your catalytic converter. Unburned diesel in the exhaust can clog and damage the cat, reducing its efficiency. As a result, your emissions will increase and you’ll likely fail an emissions inspection if your area requires them.

Exhaust Manifold

Excessive black smoke may also cause issues for your exhaust manifold. Over time, it can lead to corrosion and buildup, compromising its ability to funnel exhaust gases away from your engine.


The buildup of soot and unburned fuel in the exhaust system may result in your muffler clogging, which can ultimately harm your muffler’s performance.

Diagnosing and Fixing the Problem

using an OBD2 scanner

Check Engine Light

If your car is producing black smoke, the check engine light may come on. Use a diagnostic tool to check for codes related to mass airflow sensor, fuel injection, and EGR system. Fix the issues based on what the diagnostics tell you.

Fuel Additives

Adding fuel additives to your fuel tank can help clean engine deposits, improve fuel efficiency, and reduce black smoke. Look for additives that are specifically designed for diesel engines and follow the manufacturer’s instructions for best results.

As already mentioned, Liqui Moly Pro-Line Engine Flush and Hot Shot’s Secret Diesel Extreme are two well-known additives that actually do what they say they’ll do.

Timing and Pressure Adjustment

Improper injector timing and low pressure in the fuel injection system can cause excessive black smoke in diesels. You might need to adjust the timing and pressure to ensure complete fuel combustion. Unless you know exactly what you’re doing, you’ll want to get a mechanic involved on this one.

Air Filter Replacement

A dirty air filter restricts air flow to the engine and lead to incomplete combustion. Replacing your air filter is a common solution for reducing excessive black smoke in diesels.

Fuel Filter Replacement

A clogged fuel filter can restrict fuel flow and contribute to the black smoke issue. Regularly changing the fuel filter according to the manufacturer’s recommended interval will help maintain the fuel system and keep the engine running smoothly.

The Environmental Impact of Black Smoke

diesel truck black smoke

You might be wondering about the environmental impact of black smoke coming from diesel exhaust and whether environmental activists will start protesting outside your home so let’s dive into it.

To start, emissions from diesel engines do contribute to ground-level ozone production, which isn’t great for the environment, but is it really as bad as it appears?

You may have heard of the term “rolling coal,” where black smoke is intentionally produced by modifying a diesel truck’s engine. While it’s true that the black smoke emissions can harm the environment, the impact from rolling coal is unlikely to be significant unless lots of people are doing it simultaneously.

When you see black smoke coming from a diesel engine, it’s a sign that not all of the fuel is being burned properly. Normally, diesel engines are designed to completely burn fuel, resulting in the emission of water vapor and carbon dioxide.

Black smoke, on the other hand, represents partially burned fuel. This issue tends to be more apparent in older diesel engines since they don’t have computers that regulate emissions output like newer engines do.

So, while black smoke from diesel exhaust does have some negative environmental impact, it’s actually not that much different than the exhaust of gasoline-powered car that’s running rich. The biggest difference is that one is a lot more visible than the other.

Is Some Black Smoke During Acceleration Normal?

Yes, a small amount of black smoke during acceleration can be considered normal for a diesel engine vehicle. However, if the amount of smoke is excessive like a big plume or it changes at different RPMs and loads, it could indicate an issue with the air/fuel ratio.

Mark Stevens


  1. Particulate emissions are acutally a significant health hazard above and beyond normal CO2 emissions, with near term negative impacts on you, your kids, your dogs, your neighbors, etc. Rolling coal literally causes a feedback loop of stupidity—lol.

  2. Hi can You reommend a adishive to add. Must it be to. the oil or to the diesel. THANS

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