8 Causes of Carbon Buildup in Diesel Engines

Carbon buildup in diesel engines is a prime cause of the black smoke that diesel engines have a reputation for generating. Excessive idling by diesel engines is also a main cause of carbon buildup on pistons, rings, injectors, and valves.

The issue of carbon buildup is a bigger problem in diesel engines than it is with gasoline engines. Why is that? One reason could be the Federal requirement for certain detergents to be added to gasoline at the refinery. That keeps the injectors cleaner and the carbon load down in your car and is better than nothing (which is what diesel fuels have, with respect to added detergency), though that's not to say there's enough detergency in gasoline to keep everything completely clean for the entire life of the car. Diesel engines don't get any of that benefit.

So, what do other authorities have to say about it? One culprit being assigned blame for carbon buildup is direct injection systems. As vehicles have moved toward direct injection, mechanics are seeing more carbon buildup in the engine. The carbon builds up around intake valves and erodes whatever performance or mileage gains the engine gets from having direct injection in the first place.

How much of a loss? Studies have been done on diesel cars taking newer vehicles (starting at 15,000 miles) and comparing the horsepower of the vehicle at that point and then with another 5,000 miles added (along with the intake valve carbon deposits). At 15,000 miles, the cars had 324 "all-wheel horsepower". 5,000 miles later, the horsepower was measured at 317. Another 5,000 miles and the horsepower was down to just 305. That's a drop of almost ten percent in less than 10,000 miles.

Carbon buildup in diesel engines can be caused by several factors, often related to fuel quality, engine design, and operating conditions. Here are some of the primary culprits:

1. Poor Fuel Quality

Low-quality diesel fuel can contain higher levels of impurities and contaminants that contribute to carbon deposits. These impurities don't burn cleanly and can leave residue in the combustion chamber and on other engine components.

2. Short Trips and Low Operating Temperatures

Diesel engines that are used primarily for short trips and don't reach optimal operating temperatures are more prone to carbon buildup. When the engine doesn't reach and maintain the temperatures necessary for complete combustion, unburned fuel and soot can accumulate.

3. Excessive Idling

Extended idling can lead to incomplete combustion, increasing the likelihood of carbon deposits. This is because the engine operates at lower temperatures during idling, which isn't ideal for efficient combustion in diesel engines.

4. The EGR (Exhaust Gas Recirculation) System

Although the EGR system is crucial for reducing nitrogen oxide emissions, it can contribute to carbon buildup. The EGR system recirculates a portion of the exhaust gases back into the engine's combustion chamber, but these gases can contain carbon particles that may deposit on engine components.

5. Faulty Injectors

Dirty or malfunctioning fuel injectors can cause an improper spray pattern or fuel droplet size, leading to incomplete combustion and carbon deposits.

6. Engine Oil Quality and Change Intervals

Using low-quality engine oil or extending oil change intervals beyond what is recommended can lead to increased carbon buildup. High-quality engine oils contain detergents and additives that help minimize deposit formation.

7. Turbocharger Issues

For diesel engines equipped with turbochargers, any issue that causes the turbocharger to perform inefficiently can lead to carbon buildup. This includes problems like turbo lag or a malfunctioning turbocharger.

8. Overuse of Engine Braking

In some cases, excessive use of engine braking can lead to increased soot production, contributing to carbon buildup.

What to do about carbon deposits in diesel engines?

Go onto a diesel driver's message board and ask a question about preventing carbon buildup, and you'll get some interesting answers. One board we saw advocates flooring the engines 10-15 times a day if the diesel engine is a car or light truck. The logic there is that it closes the EGR valve at full throttle. We're not sure how feasible that is if you're driving in the city, and it certainly won't help your fuel mileage.

The best thing you can do to keep your diesel engine clean and free of deposits is to use a diesel fuel treatment with a high level of detergency. Diesel fuel has no requirement to come with detergency, unlike gasoline. You can use a "keep-clean" detergent, but you'll also get excellent results if you choose an everyday treatment with each fillup.

Apart from your driving behaviors, keeping the engine clean is the single best thing you can do to preserve the optimal functioning of your diesel engine.