What Causes Black Smoke from a Diesel Engine During Hard Acceleration?

Diesel engines make louder noise and emit darker-colored smoke than petrol engines. While a puff of black smoke may seem normal with diesel engines, a cloud of black smoke should be a concern. Still, when accelerating hard or idling, little black smoke from diesel engines should never be a reason to panic because most causes are easy to fix.

I’ll explain the most common reasons this smoke occurs when accelerating hard or idling. You will also learn how to reduce black smoke from diesel engines when accelerating hard. Stay ahead of the curve by learning these guidelines.

black smoke when accelerating hard

Why Do I See Black Smoke From Diesel Engine When Accelerating Hard?

Little black smoke from diesel engines when accelerating hard or under load is a normal thing in older diesel engines. But with better fuel and additional filters, black smoke on newer diesel engines is a sign there’s something wrong in the engine. What are the causes of black smoke in diesel engines?

Black smoke when accelerating hard in diesel engines indicates there’s an imbalanced air-to-fuel ratio. This means there’s too much fuel or too little air in the engine, causing improperly burned fuel to escape through the exhaust tailpipe. The black smoke comprises carbons from unburned diesel in the combustion chamber.

The imbalanced air-to-fuel ratio can occur due to a clogged air filter, a defective turbocharger, a lousy MAF sensor, or fuel system issues. Let’s see how.

Clogged air filter

The air filter filters the air that goes into the engine, preventing dirt, debris, and other particles from entering the engine. If it gets clogged over time, it’ll prevent the right amount of air from entering the engine. When that occurs, the injectors will still inject the right quantity of fuel into the engine. Invariably making the mixture more of fuel.

The vehicle will emit black smoke even when idling and running at lower RPM, but it will intensify once you step on the gas pedal because more fuel is pumped in with little air.


Inspect the air filter if you suspect enough air is not entering the engine. It could be a clogged air filter, restricting airflow to the engine. Clean or replace the air filter as needed.

Bad MAF sensor

While not the most common, a dirty or bad MAF sensor can cause black smoke from the diesel exhaust tailpipe. Here’s why. The MAF sensor regulates the airflow in the engine. If it is dirty or malfunctioning, it can send the wrong signal to the engine control module, telling it too much air is entering the engine. To compensate for the too much air, the engine control module will send in more fuel into the engine.

In real sense, the air entering the engine is normal but the ECU think is too much and it is sending more than the required fuel to compensate for it. Because there is too much fuel in the combustion chamber, it will not burn properly before exiting the engine.

This scenario happens both when the engine is idling and running. But you notice it while accelerating hard because the ECM/ECU pumps excessive fuel when you step on the gas pedal.


Thankfully, this is an inexpensive and simple-to-fix issue. To address it, remove the MAF sensor and inspect it. Clean it with a mass air flow sensor cleaner if it is dirty. The cleaners cost 5 to 20 dollars, depending on your chosen brand. If that doesn’t fix the problem, then the sensor is faulty and needs to be replaced.

Issues with the fuel system

Issues with the fuel system are the most common causes of black smoke when accelerating hard. The fuel system comprises several components, such as the fuel pump, the injector pump, the injector nozzles, etc.

The fuel injector pump and nozzles can fail in two ways. Either they pump low fuel or excess fuel. If the pump or any of the nozzles are sending too much fuel into the engine, the fuel will not be burned properly before exiting the engine through the exhaust tailpipe. Making the engine produce black smoke, especially when accelerating hard. So, if you are wondering can diesel injectors cause black smoke, now you know.


Inspect the fuel system components, starting from the fuel pump to injector nozzles and injector pump. If the injector pump is sending too much fuel to the nozzles, have a mechanic service it. If the nozzles are bad, then you have to replace them.

Dirty EGR valve

Inspect the EGR valve if you are experiencing loss of power and black smoke from the exhaust on diesel engines. The EGR valve, also known as the exhaust gas recirculation valve, is responsible for resending a precise amount of exhaust gas back into the engine, which helps to minimize the creation of harmful oxides of nitrogen. This helps to lower fuel consumption, improve engine efficiency, and reduce nitrogen oxide emissions.

The carbon chucks in the valve could clog the EGR valve. When that happens, you will experience fuel inefficiency, loss of power, and emission of black smoke from the exhaust tailpipe.


If you suspect the EGR is bad, scan the vehicle with an OBD11 scan tool. Once it is confirmed the exhaust gas recirculation valve is the culprit, locate it in the engine compartment and loosen it. If it’s clogged, clean off the carbon deposits using EGR Valve cleaner. If that doesn’t fix the problem, change the valve.

Faulty turbocharger

The turbocharger is designed to increase your engine power and torque. The turbo is a device made of two halves. One half uses exhaust gas to spin a turbine that compresses the fresh air the other half draws in. The turbo improves the overall engine efficiency and increases performance by forcing more air into the engine so the vehicle can add more fuel while maintaining the ideal air-to-fuel ratio.

If the turbo is not functioning as it should because it has been damaged or failed, it will not provide the required boost pressure, causing inadequate air supply. This, however, causes issues because the car computer thinks the engine is receiving Y amount of air and sending an equal amount of fuel to balance the ratio. But because the engine receives X amount of air, the car computer has over-fueled the ratio. As a result, the excess fuel will burn off as thick black smoke coming out of the exhaust tailpipe.

Black smoke from the tailpipe due to a failed turbocharger is usually obvious when accelerating hard. Cracked turbo housing, failed turbo seal, failed bearings, and blocked oil return feeds are the usual causes of turbo failure.

Black smoke when accelerating hard and then stops shows improperly burned fuel is exiting the exhaust tailpipe, which could be caused by many things but the turbocharger is a common culprit.


The best way to fix a faulty turbo is to change the damaged parts or replace the turbo entirely. Take your truck to your mechanic and let them diagnose and see if you need to fix or change the turbo.

Incorrect engine timing

Engine timing involves the meticulous synchronization of fuel injection and combustion processes within the engine. The timing is kept accurate through the use of a cam chain or belt.

If the timing goes off by even a few teeth, it’ll alter how and when the nozzles inject fuel into the engine. This will lead to incomplete combustion—which means the fuels are not burned properly and are forced into the exhaust, where they will be burned if the exhaust is hot enough. This will generate black smoke as you accelerate hard.  So, if you were wondering why there is black smoke when I accelerate hard, now you know.


If you suspect your engine timing is off, contact your dealership or an independent shop that specializes in your car. Timing issues require a mechanic who knows the ins and outs of the engine. They will check the engine and correct it if it is the root cause of the black smoke under hard acceleration.

causes of black smoke in diesel engine

Final words

While a little puff of black smoke upon start-up on older diesel engines is normal, a cloud of black smoke when accelerating is never normal and should not be ignored.

Older diesel engines also give a little black smoke under normal driving, but excessive smoke shows there’s a problem that needs your attention. The first place to check is the turbo and the air filter as the black smoke means the engine is not receiving enough air and is not burning the fuel properly.

Osuagwu Solomon

Osuagwu Solomon is a certified mechanic with over a decade of experience in the mechanic garage, and he has over five years of experience in the writing industry. He started writing automotive articles to share his garage experience with car enthusiasts and armature mechanics. If he is not in the garage fixing challenging mechanical problems, he is writing automotive repair guides, buyer’s guides, and car and tools comparisons.

Leave a Reply

Your email address will not be published. Required fields are marked *

Recent Posts