What’s the Worst Thing for a Diesel Engine? 7 Threats To Avoid

Diesel engines are known for their power, efficiency and long lifespan—but only if they’re properly maintained. Like any complex machine, diesel engines have specific needs and neglecting them can lead to serious, costly damage.

If you're passionate about working with diesel systems and want to turn that interest into a career, the diesel technician training program at Universal Technical Institute (UTI) offers hands-on training in everything from engine diagnostics to fuel systems and preventative maintenance.^1,18 UTI prepares graduates with real-world skills that employers are looking for so they can start their career path with confidence.

In this guide, we’ll explore the worst things you can do to a diesel engine, why they’re so damaging and how to avoid them.

Why Diesel Engines Require Special Care

Diesel engines are built to handle heavy loads and long hours of operation, making them essential in industries like transportation, construction and agriculture. However, they’re also more sensitive to certain maintenance issues than their gasoline counterparts.

Diesel engines are particularly vulnerable to poor fuel quality, restricted airflow and cooling system inefficiencies. Because they rely on high compression and precise fuel injection, even minor contamination or injector clogging can cause significant performance issues. Dirty air filters or intake blockages reduce airflow, leading to poor combustion, black smoke and power loss. Likewise, cooling system problems like low coolant levels or a failing radiator can quickly lead to overheating and long-term engine damage, especially under heavy loads.

Diesel engines rely on precise timing, clean fuel delivery and strong compression factors that can be disrupted by neglect or improper care. Preventative maintenance and attention to detail go a long way in preserving engine performance, fuel efficiency and longevity. The best way to protect your investment? Know what to avoid.

9 Worst Things You Can Do to a Diesel Engine

Not keeping up with oil changes

Skipping oil changes is one of the fastest ways to wear down your diesel engine. Over time, engine oil loses its lubricity as it breaks down from heat, pressure and contamination. As the oil ages, it becomes less effective at reducing friction, absorbing heat and suspending harmful particles. In diesel engines, where internal pressures and operating temperatures are often higher than in gas engines, this breakdown happens even faster.

Contaminants like soot, fuel dilution and metal particles can accelerate wear on vital components such as bearings, pistons and turbochargers. Sticking to a regular oil change schedule based on your engine’s workload and manufacturer guidelines is essential to maintaining performance, preventing costly damage and extending the life of your diesel engine.

Letting the fuel tank run low

Running a diesel engine on a near-empty tank can cause serious problems. Sediment and contaminants naturally settle at the bottom of the tank, and when fuel levels drop, these particles can be pulled into the fuel system, clogging filters, damaging injectors and reducing fuel flow.

But the bigger concern is air entering the fuel system. Diesel systems rely on consistent pressure and flow, and introducing air can lead to hard starts, engine misfires and, in some cases, injector damage due to improper lubrication and cooling.

Low fuel levels can also increase the risk of moisture buildup, especially in environments with temperature swings. The more space in the tank, the more room for condensation to form, which can lead to microbial growth and fuel degradation. To prevent these issues, it’s best to keep your diesel tank at least a quarter full whenever possible.

Using the wrong type of fuel

Diesel engines require diesel fuel, plain and simple. Accidentally using gasoline in a diesel engine can cause severe damage. Gasoline lacks the lubricating properties that diesel fuel provides, which are essential for protecting high-pressure components like fuel injectors and the fuel pump. Without proper lubrication, these parts can quickly overheat, seize or wear prematurely.

In addition, gasoline combusts differently than diesel. It can ignite too early in the combustion cycle, causing engine knocking, misfires and potentially catastrophic damage to pistons and valves. Even a small amount of gasoline can disrupt the entire fuel system and may require a full flush to prevent long-term issues.

Using the wrong grade of diesel (e.g., #1 vs. #2, or off-road vs. on-road diesel) can also impact engine performance and emissions compliance. Always double-check the pump before refueling and make sure you understand which fuel type your vehicle or equipment requires based on climate, application and manufacturer recommendations.

Cold starts without proper warm-up

Diesel engines don’t respond well to cold starts, especially in freezing temperatures. Starting the engine without allowing it to warm up can cause incomplete combustion, rough idling and excessive engine wear. Cold temperatures thicken both diesel fuel and engine oil, making them harder to circulate through the engine. Thickened oil struggles to reach critical components quickly, increasing friction and wear during those first few moments after startup.

Diesel fuel can also gel in extremely low temperatures, restricting flow to the injectors and fuel pump, which may lead to hard starts or stalling. This is especially problematic without the right winter blend or fuel additives.

To protect your engine, use block heaters, glow plugs or intake heaters to warm the engine and fuel system before starting. Letting the engine idle for a few minutes gives oil time to circulate and ensures the fuel reaches the right temperature for proper combustion, reducing stress and wear on internal parts.

Overloading the engine

Pushing your diesel engine beyond its limits by hauling loads heavier than it’s rated for can cause excessive stress and long-term damage. Overloading increases engine workload, leading to higher operating temperatures, reduced fuel efficiency and accelerated wear on critical components.

When the engine is forced to work harder than intended, internal components like pistons, bearings and valves experience more friction and pressure, which shortens their lifespan. The added strain can also affect the transmission, drivetrain, suspension and braking systems, putting your entire vehicle at risk.

Prolonged overloading can even lead to thermal breakdown of engine oil, decreasing its effectiveness and further compounding wear. To ensure safe and efficient operation, always stick to your vehicle’s recommended weight limits, including the gross vehicle weight rating (GVWR) and tow rating, and evenly distribute loads to avoid unnecessary stress on the engine and chassis.

Skipping coolant changes

Coolant helps regulate engine temperature and prevent internal corrosion, making it critical for diesel engine health. Skipping coolant flushes allows the coolant to degrade over time, reducing its effectiveness and exposing the cooling system to serious risks.

When coolant breaks down, it loses its anti-corrosion properties and can become acidic, leading to rust, scale buildup and clogged coolant passages. This restricts flow and reduces heat transfer, causing the engine to overheat. Degraded coolant can also damage the water pump, radiator and head gasket, potentially resulting in costly repairs.

Signs of bad or failing coolant include:

• Rusty, sludgy or discolored coolant
• Persistent overheating
• Coolant leaks or low levels
• Sweet smell under the hood (indicative of a leak)

Regularly flushing and replacing coolant based on your engine’s maintenance schedule helps keep temperatures stable, protects vital components and extends the life of your diesel engine.

Neglecting filter maintenance

Diesel engines rely heavily on clean air and fuel, making filter maintenance critical. Dirty air filters reduce airflow, leading to poor combustion, increased fuel consumption and power loss. Clogged fuel filters can restrict fuel flow, starving the engine, damaging injectors and causing hard starts or stalling.

Over time, neglecting filter maintenance doesn't just impact performance; it can lead to long-term engine damage. Insufficient air or fuel flow can result in incomplete combustion, which produces more soot and unburnt fuel. This buildup can clog exhaust gas recirculation (EGR) valves and coolers, reducing their efficiency and contributing to higher emissions, rough idling and even EGR system failure.

In severe cases, dirty filters can lead to increased crankcase pressure and contamination of engine oil, accelerating internal wear.

Ignoring warning lights

Dashboard warning lights are there for a reason. Ignoring signs like the check engine light or glow plug warning can turn minor issues into major repairs. Address problems as soon as they appear to prevent further damage and avoid expensive breakdowns.

Failing to keep the engine clean

Dirt, grime and oil buildup can hide small leaks and lead to overheating or contamination. A clean engine compartment helps you spot issues early and ensures proper airflow for cooling. Regular cleaning also extends the life of hoses, belts and seals.

Read more: How Long Do Diesel Engines Last?

Train To Become a Diesel Technician at UTI!

Understanding how to care for a diesel engine is just the beginning. If you're serious about working with diesel technology, UTI’s Diesel Technology program can help you prepare to turn that passion into a career. You’ll get hands-on training from industry-experienced instructors, preparing you to pursue a future in diesel technology.

Interested in protecting and powering diesel engines for a living? Learn more about UTI’s Diesel program by requesting more info or filling out an application today!