Diesel Engine Troubleshooting: 7 Common Diesel Engine Problems

Diesel engines are known for their durability, fuel efficiency and high torque output, making them essential for commercial trucks, heavy equipment and industrial machinery. However, like any mechanical system, diesel engines can develop issues that affect performance, reliability and efficiency.

If you're interested in learning how to diagnose and repair diesel engine issues like these, the diesel technician training program at Universal Technical Institute (UTI) offers hands-on training in real-world shop settings.¹⁸ UTI’s program is designed to equip graduates with the skills and knowledge needed to tackle diesel diagnostics, maintenance and repair in a wide range of industries.¹

Let’s explore the seven most common diesel engine problems, their causes, symptoms and practical solutions.

1. Hard Starting or Failure To Start

One of the most frustrating problems with diesel engines is when they struggle to start or fail to start altogether.

Causes

Hard starting or failure to start can stem from several issues, including a weak or dead battery, malfunctioning glow plugs, air leaks in the fuel system, clogged fuel filters or low compression within the engine cylinders.

In cold weather, malfunctioning cold-start aids are a common culprit. While many diesel engines use glow plugs to preheat the combustion chamber, some newer engines rely on intake air heaters (also called grid heaters) instead. These devices warm the air entering the engine, helping to improve combustion during cold starts. If either system fails, it can make it difficult to start the engine in low temperatures.

Issues like gelled fuel, thickened oil or low cranking speed due to cold batteries can also contribute to poor starting performance. Diagnosing the root cause often requires checking both electrical and fuel systems, especially in cold climates where multiple factors may be at play.

Symptoms

When your diesel engine cranks but fails to start, takes an extended time to fire up or doesn't respond at all when turning the key, it's a strong indicator of a hard starting issue.

Solutions

To resolve this, begin by checking the battery voltage and ensuring all connections are clean, tight and corrosion-free—low cranking speed is a common issue in cold weather or with older batteries.

Next, test the glow plugs for proper resistance or power draw using a multimeter. Faulty glow plugs can prevent the combustion chamber from reaching ignition temperature. If your engine uses an intake air heater (grid heater) instead of glow plugs, verify that it's receiving power and heating properly, especially in cold weather.

Inspect the fuel system for clogged filters, cracked fuel lines or air leaks. Bleed the fuel system if necessary to remove trapped air, which can prevent proper fuel delivery and cause hard starting or stalling.

If basic checks don’t resolve the issue, perform a compression test. Low compression can make cold starts nearly impossible and may point to worn piston rings, damaged cylinder walls or faulty valves—all of which reduce the engine’s ability to generate the pressure needed for combustion.

2. Excessive Smoke from Exhaust

Different colors of exhaust smoke can indicate various issues within the engine.

Causes

Excessive exhaust smoke usually results from incomplete combustion, but several underlying issues can be to blame. Air restriction is a common cause. Beyond dirty air filters, restricted airflow can also come from clogged intercoolers or carbon buildup in the intake manifold, all of which reduce the oxygen needed for efficient combustion.

Faulty fuel injectors may lead to over-fueling, poor atomization or dribbling, which causes unburned fuel to enter the exhaust system and produce black smoke. Turbocharger problems are another frequent culprit. Low boost pressure can lead to black smoke due to insufficient air, while leaking turbo seals can allow oil into the intake or exhaust, resulting in blue smoke.

Low-quality fuel, whether due to poor grade, contamination or even using the wrong type, can also disrupt combustion and create excess smoke.

Symptoms

The color of the smoke offers valuable clues about what's going wrong. Black smoke typically means there’s too much fuel in the combustion chamber. This can be caused by faulty injectors, restricted airflow from clogged filters or intake buildup, or turbocharger issues that reduce boost pressure and oxygen availability.

White smoke often indicates unburned fuel, especially during cold starts, but it can also point to more serious issues like low compression or malfunctioning injectors. If the white smoke has a sweet smell or lingers, it may be coolant burning, potentially caused by a blown head gasket, cracked cylinder head or a leaking EGR cooler.

Blue smoke signals that the engine is burning oil, usually due to worn piston rings, damaged valve stem seals, leaking turbo seals or a failing PCV (positive crankcase ventilation) system. In all cases, excessive smoke is a sign that the engine isn’t operating efficiently and should be inspected before more severe damage occurs.

Solutions

Solving excessive exhaust smoke starts with ensuring the engine is getting clean air and proper fuel delivery. Begin by cleaning or replacing air filters, but also inspect for clogged intercoolers, carbon buildup in the intake manifold, or malfunctioning EGR systems, all of which can restrict airflow and contribute to incomplete combustion.

Servicing or replacing faulty fuel injectors is critical if there are signs of over-fueling or poor atomization. For the turbocharger, check boost pressure levels and inspect for oil leaks or worn seals that could allow oil into the intake or exhaust system, causing smoke.

Finally, always use high-quality diesel fuel and consider using additives to improve combustion and reduce soot. Addressing these areas will not only reduce smoke but also improve engine performance and longevity.

3. Loss of Power or Poor Performance

When a diesel engine loses power or struggles to perform, it can quickly impact productivity, fuel economy and drivability.

Causes

This problem is often linked to clogged air or fuel filters, a malfunctioning turbocharger, restricted fuel delivery or issues with engine sensors.

Symptoms

If the vehicle is slow to accelerate, struggles to maintain speed or loses power under load, it’s a clear sign of decreased engine performance. In many cases, a check engine light or warning indicator may also appear on the dashboard. This can provide valuable diagnostic information and help pinpoint issues related to sensors, turbocharger function, fuel delivery or emissions systems. Ignoring these early signs can lead to further engine strain and more costly repairs down the line.

Solutions

To address loss of power or poor performance in a diesel engine, start by replacing any clogged air or fuel filters, as these are common causes of restricted flow and reduced efficiency. It’s also important to inspect the turbocharger system and its related components, including the intercooler, wastegate and vacuum lines. Look for signs of low boost pressure, oil leaks or worn seals that could be affecting performance.

A diagnostic scan tool is especially useful in these situations. It allows you to check for stored fault codes that may point to issues with sensors or emissions components. Once the codes are identified, the underlying issues should be corrected before clearing the codes.

It’s also a good idea to perform a thorough visual inspection of the engine bay, checking for any obvious leaks, loose clamps or cracked hoses that might be contributing to the problem. Confirm that the fuel system is delivering proper pressure and that the injectors are functioning correctly.

This approach helps isolate the issue and restore performance without unnecessary part replacements or guesswork.

Read more: 5 Reasons You Should Consider Going to Diesel Mechanic School

4. Fuel Contamination and Injector Problems

Diesel engines are particularly sensitive to fuel quality, and contamination can cause major issues with performance and reliability.

Causes

Fuel contamination typically occurs when water, dirt or microbial growth enters the fuel system, often through condensation, poor tank sealing or dirty storage. These contaminants can clog fuel filters, damage injectors and reduce overall fuel system efficiency.

In addition, poor-quality diesel can be just as damaging. This may include fuel with low cetane levels, which leads to poor combustion and hard starts, or excessive sulfur, which can harm emissions systems and increase engine deposits. Fuel that contains the wrong or insufficient additives may also lack proper lubricity or stability, accelerating injector wear and clogging. Over time, these issues reduce performance, increase smoke and can lead to costly repairs if not addressed.

Symptoms

You may notice the engine idling roughly, hear excessive knocking or abnormal combustion noises, or experience a noticeable drop in fuel efficiency. While some knocking is normal in diesel engines, excessive or irregular knocking often points to improper combustion, which can result from contaminated fuel or faulty injectors.

It’s worth noting that pinging is more commonly associated with gasoline engines. Diesel knocking typically signals uneven fuel delivery, poor atomization or timing issues. These symptoms are strong indicators that something is affecting the combustion process and should be investigated promptly.

Solutions

The best course of action is to drain and replace the contaminated fuel, clean or replace affected fuel injectors, and use diesel fuel conditioners. Installing a water separator can also help prevent future contamination issues.

5. Overheating Issues

An overheating diesel engine can lead to serious internal damage if not addressed promptly.

Causes

Diesel engines overheat due to low coolant levels, a malfunctioning radiator or water pump, a thermostat stuck in the closed position, or blocked cooling passages that prevent proper heat dissipation. If overheating is not addressed promptly, it can lead to severe internal engine damage, including warped or cracked cylinder heads, blown head gaskets, cracked blocks or piston scoring caused by excessive thermal stress and loss of lubrication.

These issues often result in coolant and oil mixing, loss of compression and eventually engine failure, making it critical to identify and resolve the root cause of overheating as soon as possible.

Symptoms

A rising temperature gauge, visible steam from the engine bay or noticeable coolant leaks are all warning signs of an overheating diesel engine. Steam is a strong indicator that the coolant is boiling or escaping, but it’s important not to confuse it with smoke, which may suggest burning oil, fuel or electrical issues, entirely different problems with their own causes.

In addition to those signs, overheating can also cause reduced engine power, illuminated warning lights, or even a burning smell from hot fluids or components under stress. These symptoms signal that the engine is running beyond safe temperatures and should be shut down and inspected immediately to avoid permanent damage.

Solutions

Check coolant levels and top off if needed. Inspect the radiator and water pump for signs of failure. If the thermostat isn’t working properly, replace it. Flushing the cooling system can also help remove any internal blockages.

6. Oil Leaks and Lubrication Issues

Oil leaks and lubrication failures can quickly lead to engine wear and costly repairs if not detected early.

Causes

Oil leaks may be caused by worn or degraded seals and gaskets, which can fail over time due to prolonged exposure to heat, pressure or contamination. These components are critical for maintaining oil containment and can become brittle or lose integrity, especially in high-mileage or poorly maintained engines.

Loose or leaking drain plugs are another common source of oil loss. In some cases, the plug may be improperly installed, cross-threaded, or have a damaged washer or threads, allowing oil to seep past.

High crankcase pressure can also force oil out through weak points in the engine. This is often due to a clogged or malfunctioning crankcase ventilation system, which prevents pressure from being properly vented. When that pressure builds up, it looks for an escape path—often resulting in oil leaks at seals, gaskets or the dipstick tube.

Symptoms

You might notice oil spots beneath the vehicle, a drop in oil pressure or a burning oil smell while driving—all common signs of a lubrication issue. A drop in oil pressure can be sudden or gradual and may indicate an external leak or a more serious internal lubrication problem, such as worn bearings or an obstructed oil passage.

Additional symptoms can include blue smoke from the exhaust, which suggests oil is being burned in the combustion chamber due to worn piston rings, valve seals or turbo seals. Higher-than-normal engine temperatures can also occur when there isn’t enough oil to properly lubricate and cool internal components, increasing the risk of overheating and mechanical damage.

Solutions

To fix oil leaks and prevent further engine damage, start by replacing any leaking gaskets or seals. Common culprits include the valve cover gasket, oil pan gasket, rear main seal and timing cover gasket, all of which can degrade over time due to heat and pressure. If left unaddressed, these leaks can lead to low oil levels and internal wear.

Inspect the drain plug for signs of damage or improper installation. A worn or cross-threaded plug, or one with a damaged or missing washer, can result in slow oil leaks. In more severe cases, especially if the threads in the oil pan are stripped, the oil pan may need to be repaired with a thread insert kit or replaced entirely.

Performing a crankcase pressure test can help identify excessive internal pressure that may be forcing oil past seals or gaskets. Keeping up with regular oil changes also helps maintain proper lubrication and prevents the buildup of sludge and contaminants that accelerate seal failure.

7. Electrical and Sensor Failures

Modern diesel engines rely heavily on electronic control systems and sensors to maintain optimal performance, fuel efficiency and emissions compliance. Sensors play a critical role in managing fuel injection timing, air-fuel ratios, turbocharger boost levels and exhaust emissions. When these sensors malfunction—such as the MAF (mass airflow), MAP (manifold absolute pressure), fuel rail pressure or oxygen sensors—it can throw off the engine’s calibration, leading to erratic performance or engine warning lights.

Electrical issues are another common problem area and can stem from damaged wiring, corroded connectors, blown fuses, failed relays, or malfunctioning control modules like the ECM (engine control module). Even a minor wiring fault or poor ground can cause sensor readings to go haywire or communication errors between critical engine systems.

Causes

These problems often stem from corroded wiring, loose electrical connections, or worn-out sensors that no longer provide accurate data to the engine control system. Over time, exposure to heat, moisture and vibration can degrade electrical components and cause intermittent or complete signal loss. A faulty engine control unit (ECU) or failed control modules can also disrupt engine operation, sometimes requiring reprogramming or replacement.

In many cases, wiring faults such as frayed insulation, pinched wires or corroded connectors are to blame. These faults may not always trigger a warning light immediately, but they can cause inconsistent readings that lead to misfiring, poor fuel mapping or emissions system errors. Diagnosing the root cause often requires a scan tool and a detailed inspection of the vehicle’s wiring harness, grounds and sensor inputs.

Symptoms

Dashboard warning lights—such as the check engine light, glow plug warning or DPF/regeneration alerts—are often the first indication of an electrical or sensor-related issue. These lights can provide specific diagnostic codes that point directly to faulty components or system errors.

Engine misfires may also occur and are frequently linked to failed sensors, such as the crankshaft or camshaft position sensors, which affect ignition timing and fuel injection. Other symptoms include poor fuel economy, sluggish acceleration, rough idling or the engine entering limp mode. These issues often worsen over time if not diagnosed and repaired, as the engine control system struggles to compensate for incorrect or missing sensor data.

Solutions

To resolve electrical and sensor issues, start by inspecting all electrical connections for corrosion, looseness or broken wires. These are common culprits in diesel engine performance problems. Damaged connectors or weak grounds can interrupt signal flow and cause sensor misreadings or intermittent faults.

Use a diagnostic scan tool to check for error codes and monitor live data from sensors. This helps pinpoint failing components, such as MAP, MAF or crankshaft position sensors. Once identified, replace any faulty sensors or damaged wiring and ensure connectors are properly sealed and protected from moisture.

In some cases, replacing a sensor isn't enough—it may also require calibration or relearning using a scan tool to ensure it communicates correctly with the engine control module. Taking these steps helps restore proper engine function, improve fuel economy and prevent future electrical faults.

Get Hands-On Diesel Mechanic Education at UTI

Catching diesel engine problems early can make all the difference between a quick fix and a major repair. Whether you're maintaining your own vehicle or starting a career in diesel mechanics, understanding these common issues is essential.

UTI offers a diesel mechanic training program that provides students the hands-on training and technical skills needed to troubleshoot, repair and perform diesel engine maintenance.¹⁸ With industry-standard tools and real-world training, UTI can prepare you to pursue a career as a diesel tech!

Ready to jump-start your path toward a diesel career? Request more info or fill out an application today!