Black Smoke from Diesel Engine Causes and Fixes

Ever noticed clouds of black smoke billowing from the diesel truck driving beside you? Despite black smoke's ability to make diesel engines look tough, it indicates something’s wrong under the hood! In this blog post, we’ll share common causes of diesel black smoke, how to fix it and, last but certainly not least, how to prevent it. 

Causes of Black Smoke from Diesel Engine When Accelerating

Wondering what causes black smoke from a diesel engine? Diesel engines have combustion chambers where diesel and air mix to create a spontaneous combustion. During combustion, fuel burns and is converted into mechanical energy that powers the vehicle. 

However, carbon buildup, a worn component or a faulty system can lead to too much fuel in the chamber. A rich fuel-to-air ratio inhibits combustion, leaving unburnt fuel to spew out of the exhaust as black smoke. 

Additionally, incomplete combustion means limited mechanical energy and loss of power. Keep reading to learn about more specific causes. 

Fuel-related issues 

A faulty fuel injector can over-fuel the combustion chamber and cause black smoke from a diesel engine. Fuel injectors release atomized fuel into the chamber, but if they don’t open and close properly, they may inject too much diesel, preventing proper combustion and allowing unburned particles to escape the exhaust as smoke. 

Air-related issues 

Dirty air filters can also cause a fuel-to-air mixture that is too rich. The air filter stops contaminants like dirt from entering the combustion chamber. However, a clogged filter inhibits airflow. Without the right amount of air, the fuel cannot burn properly and will flow out of the exhaust in the form of black smoke. 

A faulty mass airflow (MAF) sensor can also jeopardize the fuel-to-air ratio. Newer vehicles with electronic fuel injection systems have MAF sensors that measure the amount of air entering the chamber. If the sensor is not operating properly, the engine computer can't calculate how much fuel to inject, often resulting in too much fuel.  

Engine-related issues 

Excess carbon deposits in the combustion chamber cause black smoke from the exhaust. Older vehicles that haven’t been properly maintained are prone to this issue. 

A clogged exhaust gas recirculation (EGR) valve may also cause black smoke from a diesel engine. The EGR valve is part of the engine management system, which is responsible for returning exhaust fuel to the combustion chamber via the intake manifold to lower combustion temperatures and reduce emissions.   

As part of this system, the EGR valve connects the intake manifold to the exhaust manifold and manages the amount of exhaust sent back into the chamber. If the valve becomes clogged, the fuel cannot circulate back into the engine, resulting in soot buildup and black smoke. 

How to Reduce Black Smoke from Diesel Engines 

We know that black smoke happens when there’s too much fuel in the combustion chamber. Unfortunately, black smoke usually means reduced fuel efficiency and increased diesel particulate emissions as well.  

The first thing you should do at the sight of black smoke is visit a diesel repair shop. A trained diesel technician can narrow the black smoke down to a single cause and solution. 

Once a diesel technician addresses the issue, there are steps you can take to prevent it from happening again.  

Follow Manufacturer Maintenance Guidelines

Following the manufacturer's maintenance instructions can prevent the wear and tear that causes black smoke from a diesel engine and optimize engine performance. Preventative maintenance ensures regular oil changes, timely air filter replacement and prompt attention to worn-out parts that cause costly repairs and reduce fuel efficiency if left unattended. 

Use High-quality Fuel and Additives 

The American Automobile Association (AAA) conducted a study on fuel quality and its effect on engine performance. They compared the lowest additive concentration (LAC) premium fuel to TOP-TIER premium fuel. 

LAC fuel contains at least the number of detergent additives mandated by the Environmental Protection Agency (EPA). Unsettled by the EPA's standard, vehicle manufacturers banded together to create TOP-TIER fuel that includes quality detergents designed to minimize carbon buildup. 

AAA’s study found that vehicles operating off TOP-TIER fuel had 19 times less carbon deposits than those that did not. Less carbon buildup means less diesel black smoke!  

Use Fuel Additives

Diesel is a crude fuel that causes carbon buildup on engine elements if not refined. If you can't fill up with high-quality fuel, use an additive with polyether amine (PEA). PEA is a potent detergent that disperses to clear buildup from intake valves, fuel injectors and the combustion chamber. 

FAQs

Is black smoke from a diesel engine a sign of a serious problem? 

Determining if black smoke is a serious problem will require the eye of a qualified diesel mechanic. In some cases, it may just be carbon buildup that an additive can solve. In others, it may be a pricier issue, such as a faulty fuel injector. 

Are there any additives or treatments that can help reduce black smoke in diesel engines?

Yes, a refining detergent can prevent the crude fuel from forming carbon deposits inside the chamber. However, additives alone are not enough. Routine maintenance is the best way to reduce black smoke from diesel engines. 

Is it normal for diesel engines to smoke? 

Diesel engines should not smoke. Black or white smoke from your diesel exhaust may indicate an engine issue. A diesel mechanic can get to the root cause and help you find a solution. 

Black Smoke from Diesel Engine: Let’s Recap  

If there’s black smoke from your diesel exhaust, it’s likely due to an imbalanced air-to-fuel ratio. Clogged fuel injectors, dirty air filters and faulty engine components are usually the culprits. A trained diesel mechanic can help you promptly identify and solve the issue. 

If you're intrigued by the complexities of diesel engines and want to learn more, consider attending diesel mechanic school at Universal Technical Institute!¹ Through industry-aligned training, students graduate prepared to help diesel vehicle owners address and prevent issues like black smoke. Request information to speak with an Admissions Representative. 

Campuses That Offer Diesel Mechanic Training 

• Long Beach, California.
• Rancho Cucamonga, California.
• Sacramento, California.
• Avondale, Arizona.
• Austin, Texas.
• Dallas/Fort Worth, Texas.
• Houston, Texas.
• Miramar, Florida.
• Orlando, Florida.
• Lisle, Illinois.
• Exton, Pennsylvania.
• Bloomfield, New Jersey.