Diesel Engine Overheating: How to Diagnose the Problem

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Diesel engines are known for many things. Longevity and sturdiness are a couple of the traits that make diesel engines the right choice for long hauls and heavy loads. It’s not unusual to see a diesel engine with over 500,000 miles on it and still going strong. Pair the reliability of a diesel engine with the fuel mileage potential and you understand how these juggernauts are responsible for delivering the goods we use every day.

Diesel Engines are the Workhorses of the Road

Being a well-built heavy-duty engine isn’t enough to hit 500k miles and keep on truckin’. Regular maintenance and repairs are required on all engines to make sure they are meeting maximum efficiency. Diesel engines, like all engines, need regular oil changes, coolant testing, and flushes, the radiator kept clean of road debris and bug guts, and all other recommended manufacturer updates/maintenance. Check that owner’s manual, folks! There’s a lot of important upkeep info in there.

This article is focused on overheating diesel engines and how to identify the issue as early as possible. Spotting the issue before it causes major engine problems will save major money on repairs and prevent the loss of revenue from a truck parked for repairs, instead of working.

How do you know when a diesel engine is starting to overheat?

Fortunately, there are early ways to tell if your diesel engine is overheating:

1. Is your engine cooling fan running constantly? If so, this could be a sign that your engine is beginning to overheat. But why? Your diesel engine thermostat controls the fan clutch and coolant flow to the engine. It is designed to operate at a fairly small temperature swing. If your engine gets too hot it can cause internal damage, but if the temperature runs too low you burn more fuel and the engine runs less efficiently. If your fan is running constantly it could mean that your engine isn’t cooling to the level needed to shut the fan off.
2. Is the AC in your truck not cooling like it should? The temperature of your engine can have a direct effect on the temperature inside your cab. If your AC is working, but you’re sweating like a rookie driver at a weigh station, your diesel engine could be overheating.

There are, of course, other ways to tell if your engine is overheating such as warning lights, temperature gauges on the instrument cluster, and more. It’s best to pay attention to anything that your rig is doing out of the ordinary and get it looked at ASAP. The last thing you want to do is get stuck in the middle of nowhere with a load and the only way out is getting towed.

What causes a diesel engine to overheat?

Several things can cause a diesel engine to overheat. Here are common reasons:

1. Restricted diesel engine air filters will obstruct airflow through the engine which can cause a whole series of performance and efficiency issues including, but not limited to, overheating. It is important to check these filters on a regular basis. Make it a step in your pre-trip checklist or have air filters checked during routine maintenance along with  oil changes. If you are consistently in a dusty environment or one prone to bugs, checking your air filters more often is a good idea. Air filters clog faster with dirt and bug guts present.
2. Thermostat malfunction can be a symptom or a cause of your overheating engine. It is important to determine which came first, the chicken or the egg.  Is the thermostat reading the right temperature and the engine is running hot or is the engine running hot because the thermostat isn’t working?
3. A worn-out Fan Hub Clutch will make the fan operate improperly. A thermostat opens and closes circuits and hoses that help with the engine cooling. If the thermostat reads that the engine temp is climbing and closes the circuit to the fan clutch, the clutch should engage and start cooling the engine coolant by pulling air through the radiator and engine. If your fan clutch is worn or damaged, the fan will likely not engage and the engine will not experience cooling needed to prevent overheating.

A Clogged DPF (Diesel Particulate Filter) can be diagnosed in several ways but the easiest is the DPF light. Diesel engines are like self-cleaning stoves when it comes to keeping the DPF filtering properly. It will go through a process called Passive Regeneration where the engine pumps exhaust over the Diesel Oxidation Catalyst (DOC) into the DPF to convert stuck carbon to carbon dioxide so the gas will pass through. If passive regeneration fails because the engine isn’t creating enough heat, the engine will start the process of Active Regeneration. This kicks up the regeneration process a notch by injecting fuel directly into the exhaust that passes over the DOC heating the DPF which converts the soot or carbon into carbon dioxide so it can pass through the filter. Ash, on the other hand, has already gone through the burning process and cannot be cleared through regeneration and must be cleared by removing the filter and cleaning with compressed air. If the light comes on, you likely need to clean the filter of ash.

How soon should an overheating engine be fixed?

Immediately! An overheating diesel engine can be caused by any number of minor and relatively inexpensive circumstances. If you wait to resolve the issues that caused your diesel engine to overheat, the damage that can occur is exponentially more expensive to resolve. 

Seized engines, busted blocks, and any number of other major systems can occur by delaying a fix to an overheating diesel engine. Ignoring overheating symptoms can take you from a few thousand dollars for a repair, to tens of thousands for a rebuild… not to mention losing money during the engine downtime.

How do you diagnose what caused the diesel engine to overheat?

1. Verify lack of restriction in both air and exhaust. Check air filters and exhaust restrictions, such as making sure an emissions-controlled truck properly can pass a forced regeneration.
2. Ensure air can pass through the radiator and the radiator isn’t clogged from dust or bugs blocking airflow. Using a garden hose from front to back can show if there’s no flow through the radiator. Contamination of water during this step can indicate the need to deep clean the radiator.
3. Verify coolant quality by visually inspecting that no contaminants are present. Using a coolant tester, make sure the coolant is the right proportion.  Coolant that is too pure/strong could affect operation. 
4. Check for proper fan operation. Fans should operate with temperature/AC demand and a manual switch, if equipped.
5. Check the gauge and thermostat operation with an infrared thermometer to make sure the gauge is accurate and the thermostat is opening properly at around 190-200 degrees Fahrenheit.
6. Check for air bubbles in the coolant system while running (remove the cap cold first to prevent injury). Air bubbles in the coolant system indicate an engine diagnostic will be needed. Possible combustion in the coolant system could occur from  diesel head gasket or head failure, to a faulty air compressor. Seek professional assistance if air bubbles are found to prevent misdiagnosis, which can cause further damage and downtime.
7. If nothing is found, test using a factory-level diagnostic computer to help diagnose if turbo efficiency or fuel system issues could be a factor.

What tools and parts are needed to diagnose an overheating diesel engine?

Common preliminary checks could require the following:

• Common hand tools
• Coolant tester
• Infrared thermometer
• Garden hose
• Diagnostic computer for deeper diagnostics may be required

How do I fix an overheating diesel engine?

It could be as simple as cleaning the radiator, flushing the coolant, or replacing a thermostat or sensor, but it may require more extensive work like a DPF filter or fan hub rebuild.

Overheating issues can lead to engine work such as cylinder head/gasket replacement, or a complete inframe rebuild. Simple checks can help draw the line between a do-it-yourself parking lot repair to a garage repair requiring engine tools and repair abilities. HHP has available technical support to offer recommendations and suggestions during the diagnosis, and to help supply the parts to repair your diesel engine.