Diesel Engines And Black Smoke: What You Need To Know

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Black smoke is a common problem for diesel engine owners, but this symptom is a serious issue that you cannot afford to ignore. Black smoke could point to a series of different problems, some of which could ultimately result in catastrophic damage to your engine if you don't take any action. Learn why your diesel engine is emitting black smoke, and find out more about some of the mechanical issues that can cause the problem.

What black smoke means

Exhaust from your diesel engine typically contains carbon (or soot), semi-organic hydrocarbons, sulphates (primarily sulphuric acid) and water. If the exhaust fumes contain elemental carbon, you'll also start to see black smoke.

Elemental carbon starts to appear when the combustion process inside the engine is inefficient. Generally speaking, this inefficiency occurs because the engine is unable to properly combust the fuel supplied. Reasons for inefficient combustion can include:

  • Excess fuel in the engine
  • Fuel passing through the combustion zone too quickly
  • A lack of oxidants in the combustion zone to allow the fuel to combust efficiently

These problems can occur for several reasons. In some cases, the black smoke will worsen over time, while other mechanical issues can lead to a sudden increase in the problem, and you may see a lot of black smoke belching from the pipe.

Mechanical problems that can cause black smoke

Certain situations may temporarily cause black smoke from your exhaust. For example, if you drive up a steep hill, the throttle will open to inject more fuel, but the fuel-rich mixture in the engine may only partially burn. If this happens, you may temporarily see black smoke, which you probably don't need to worry about.

However, if you regularly see black smoke from the exhaust, you may have more serious, mechanical issues to content with. These include:

  • Over-fueling. Worn or damaged diesel fuel injectors commonly cause this problem. If the nozzle hole is too big, too much fuel can get into the combustion chamber. Similarly, dirty air filters can also cause this problem because they stop enough air getting into the combustion chamber.
  • Excessive oil consumption. Worn or damaged parts can increase the amount of oil that the engine needs to run efficiently. Common problems include worn valves, worn valve stem seals and worn cylinder liners. Excessive oil consumption can also occur if you use the wrong type of oil and/or fail to replace the oil often enough.
  • Other worn, damaged or faulty parts. Some of these problems may depend on the age and type of engine. For example, older vehicles with carburettors can suffer from black smoke if the automatic choke doesn't function as it should. On newer vehicles, faulty sensors can cause problems with black smoke. For example, a faulty oxygen sensor may result in an imbalance between the fuel and air intake, decreasing fuel efficiency.

Prevention and cure

Don't ignore a recurrent problem with black smoke. A mechanic can often replace or adjust a relatively minor part to fix the problem, but a larger repair or replacement bill can quickly follow if you allow the issue to continue. Serious problems could cause irreparable damage to your engine. For example, ongoing issues could eventually cause the engine block to crack, and if this happens, you may need to replace the entire engine.

As such, it's important to arrange regular inspections and maintenance checks to spot the early signs of problems that could cause black smoke. Check the owner's manual for details of the suggested service interval for your vehicle. If you are a high-mileage driver, you may need to book the vehicle in more often.

Make sure you always use the right engine oil, too. Again, you can check the details in your owner's manual, or you can talk to a diesel mechanic to confirm the type of oil suitable for the make and model of the vehicle you drive.

Black smoke is a common, unwelcome problem that affects many diesel engines. Talk to your local diesel engine repair shop for information and advice about any problems you have.