Cooling System Basics for Diesels

September 30th, 2011

By Diesel World staff

Photography by Matt Emery

So you think you have a hot diesel? Well, there’s hot and then there’s simply running hot, as in water, oil and EGT temps getting hot when you race or tow.

There are many reasons a diesel can run hot, and most of them can be easily remedied. The first thing is figuring out exactly what is causing the problem.


If you suspect your truck could have a heating problem, the way to test it, according to the experts we talked to, is to drive on a flat or slightly raised straight run at no more than 30 mph. Do this as far as you can. Is the truck getting hot? If so, you may have a cooling problem—as least as far as the radiator is concerned.

One of the possibilities is that there is not enough airflow through the radiator to cool the water to keep the engine at a reasonable temperature. To get to the source of the problem, you have to dig deep and eliminate possible causes—for instance, a stuck thermostat, bad water pump, even a build-up of bugs in the A/C condenser or intercooler.

We talked to six different diesel experts on the topic of diesel cooling systems, and they all offered some great advice and places for you to look if your truck is running hot.

For the most part, any late-model diesel running hot, without there being a mechanical problem, such as a faulty water pump or clogged radiator, is an anomaly nowadays.

Manufacturers put a lot of effort into design and testing of stock cooling systems. A good example is the cooling system upgrade in the 2006 Duramax, which included a larger radiator and larger cooling fan over previous years. The heating problems only surface when owners pull heavy loads up long grades.

You have to start somewhere, and you might assume that we would be starting with the radiator. That’s where the water is cooled, isn’t it? Yeah, it is, but let’s look at what makes heat in the diesel: fuel and combustion.

More Air Helps

Unlike gasoline engines, diesels get hotter (that is, higher EGTs) the more fuel you add. The bigger the bang, the bigger bucks it takes to keep that explosion under control. So, if you’ve added larger injectors and upgraded fuel system or a hot tuners, which adds more than 100 horsepower, things will get hotter than stock. There are two fixes: Either dial back the fuel or add air.

Usually, trucks with big tuners that still run the stock air box and exhaust are going to have EGTs that get close to the nuclear zone. And even worse, if the air filter element is dirty, less air means more fuel and more temps. So, at the very least, a clean air filter or the addition of an aftermarket air intake system is a must.

More Air: Part II

So, if a cold air intake system is a good way to keep an adequate flow of air into the engine, an even better way to get more air flow is with a larger turbo. Adding a larger turbo, especially when you’ve increased the fuel, increases the airflow into the engine, thus reducing the EGTs.

Put a heavy load behind a truck that’s overfueled, and there are going to be EGT issues. A larger turbo pushes more air into the engine and drops the EGTs. And lower EGTs mean less heat buildup in the cooling system, since the cylinder head will be cooler, and the coolant that passes through it will absorb less heat.

Ambient heat under the hood can also be an issue, so isolating the heat in the engine compartment is another tip. While the jury is out regarding whether a turbo heat blanket is a good idea on diesels, it will keep the heat produced by the turbo from affecting the rest of the engine compartment. The downside is that the excess heat buildup in the turbo will require you to extend you cool-down idling time so that the turbo bearing won’t gall with burnt oil.

Go With the Flow

Another way to make sure that your engine is running as cool as possible is with a free-flowing exhaust system. By allowing the spent fuel to escape more efficiently, the engine doesn’t have to work as hard to expel it, and there is less heat soak from exhaust back pressure. Again, cleanliness is important, and having a 5-inch system won’t do a lot of good if the muffler is not free-flowing. Choosing the least restrictive muffler is essential when choosing an exhaust system.

Another source of restrictions can be the catalytic converter and diesel particulate filter. While late-model trucks run hot enough to burn the soot off a cat, the DPFs can get clogged and push the heat back upstream, from whence it came.

Removal of emissions equipment is a federal no-no, but that’s not to say it doesn’t happen. Reduction in restrictions, both on the intake and exhaust side, is the name of the game, and removal of smog equipment is technically legal and is reserved ONLY for off-road and race vehicles, according to the letter of the law.

Water-Methanol Injection Systems

Keeping the internal temps under control during high heat situations, such as towing, can also be accomplished with a water-methanol injection system. The water-methanol combination acts to lower exhaust gas temperatures, since the water mist is a cooling agent. The methanol part of the equation is a secondary fuel that helps with combustion and adds more power.

Typically, a good water-methanol system will drop EGTs between 200 to 300 degrees. This is perfect for vehicles that tow, a situation during which EGTs can climb because of the load, according to the folks at Snow Performance. Since the systems can be tuned to run off specific engine perimeters, such as boost and EGTs, they can be set up to come on when the engine really needs a cooling spray.

Tuners, Chips and Downloaders

We are living in a “golden age” of diesel performance. Sure, some will argue that the golden age of performance was actually back in the 1960s and ’70s, with the heyday of the muscle car. Back then, if you wanted performance, you had to change hard parts such as cams, carbs, headers, etc.

Today’s computer-controlled diesels make things a lot easier. For example, adding 100 horsepower to a muscle car meant adding a camshaft, trick cylinder heads, bumping the compression and a bigger cam (or the easiest way: by adding a $4,000 supercharger).

Adding 100 horsepower to today’s diesel is as simple as plugging in a $500 tuner. But when it comes to towing and tuning, the key to keeping your rig cool is to not get too carried away when adding fuel.

We know that it’s fun to hit the loud pedal and makes lots of smoke, but remember that all that smoke is unburned fuel, and that adding more fuel means making more heat. Your EGT gauge, which is a must for rigs that tow, will tell you as much.

That leads us to another topic regarding engine rpms and heat buildup. While it may seem backwards, bogging a diesel engine makes it work way harder, and the result is heat buildup. Why? You’re adding fuel and not pushing enough air through the engine to compensate.

So under heavy load, try to keep the engine at or near its peak torque rpm and watch the EGTs. You’ll notice they will drop under load as rpm increases.

Fuel Air Separators

A finely tuned engine is a happy engine, and by providing a constant flow of fuel, without impurities or air, the engine will run at its optimum. By removing the air from the fuel, the engine utilizes every ounce of that fuel. Thus, by running better, the engine actually runs easier, needs less fuel to do the same job, and therefore, it will run cooler.


A larger-than-stock intercooler is a must everyone who does a lot of towing with their rigs. The intercooler is meant to take heat out of the air charge. Compressing air causes heat, so the intercooler is essential on turbo diesels. Denser air means more performance by providing cooler air into the engine.

Another point is that a larger intercooler won’t physically retain the same amount of heat that a smaller unit, because is has a larger surface area and more cooling fins per inch.

Oil & Trans Fluid

It should go without saying that fresh oil is better for your engine than well-worn oil full of contaminates. And for those who tow, the question becomes: Are synthetics better than good, old dinosaur oil?

Based on the data we have seen, we’re going with synthetics for heavy-duty use. Sure, they cost more, but in many cases, you can get an extended oil change interval with them and a big reduction in heat from a lack of thermal breakdown. The same goes for automatic transmission fluid. For tow rigs, opt for the best synthetics.

Deep Sump Trans Pans and Diff Covers

Another cooling principal is that the more fluid you can add to the mix, the longer it takes to raise the temperature of the oil. So adding capacity, via deep sump trans pans or even differential covers, helps keep the heat down. When towing, much of the load is spread across the rear axle and transmission. More fluid means less heat for both the trans and front and rear diffs.

By physically increasing the amount of oil in any given system, you will be increasing your truck’s ability to cool itself. Aluminum pans and covers have two pretty good things going for them, even without the increased capacity.

First aluminum is 70 percent more effective than stamped steel at transferring heat. These aluminum pans are also ribbed with cooling fins. It’s all about spreading the heat out over a larger surface. The fins may appear small, but you have to think of them as three-sided heat sinks hanging out in the breeze.

That’s a lot of surface area added to the pan’s/cover’s ability to shed heat. Because they are more rigid than a stock stamped steel pan, they help reduce trans flex that causes case-cracking.

Transmission Oil Coolers

Most automatic transmissions run their cooler lines through the radiators. Heat transfer from the hot transmission fluid into the coolant of the radiators is a major source of overheating; in fact, it’s one of the most common. Most late-model trucks use a combination of transmission coolers that includes an internal and external cooler.

Heavy tow rigs should consider upsizing that external cooler to a heavier GVWR rating to help dissipate more heat. And the key to adding an oil or transmission fluid cooler is to place it where it has good flow across it and is not blocking the intercooler or A/C condenser. Blocking off air flow up front will eventually reduce the effectiveness of the radiator.


Speaking of radiators, they are the heart of the entire cooling system. Radiators have gone through an evolution over the past 50 years. They used to be copper and brass—and for good reason: Copper and brass were very good at transferring heat. The also could be disassembled, rodded out and reassembled by a radiator shop. Unfortunately, they also had a tendency to corrode internally more quickly.

Today’s radiators have an aluminum center section (core) and plastic end tanks. More often than not, late-model radiator failures usually result from cracked end tanks.

In radiator speak, cooling is based on the numbers of cooling rows inside the core and the numbers of cooling fins per inch. There are some laws of physics that come into play here, but the basic thing to remember about a radiator is that air has to flow through it to dissipate heat via the cooling fins.

Most radiators are two- or even three-row affairs. By now, you’re thinking that a six-row radiator would be better, right? But technically, the thing would be so thick, it wouldn’t pass as much air as a thinner core. And keep in mind that with a diesel, you’ve got an A/C condenser and hot intercooler out front of the radiator, which also restrict air flow.

The same goes for the numbers of cooling fins per inch. Too many fins, and the air can’t get through. No air means no cooling. A stock aluminum radiator with the proper fins per inch should have the properties to contain the heat the engine is putting out.

Again, the OEM engineers have developed some of the best diesel radiators ever. You can idle a diesel in 100-degree heat with the A/C on and not worry. GM had a small problem on its earlier Duramax trucks with a radiator and fan setup that was too small for serious towing. That situation was fixed in 2006 with a larger radiator and larger-diameter fan.

The Thermostat Myth

According to one expert, overheating can also be cause by running the wrong thermostat. It’s his contention that the hotter the thermostat, the longer the coolant stays in the block to absorb heat before it travels to the radiator.

For the most part, diesels run a 195-degree thermostat, and some late-model engines even run two in the cooling system. If one of them sticks or is partially open, overheating may result. When the manufacturers say to service your truck’s cooling system regularly, now you know why.

The Simple Stuff: Radiator Caps

One of the most overlooked parts of the cooling system is the radiator cap. On a diesel, you’ll normally find that pressure cap on top of what is called the “degas” bottle, more commonly referred to as the overflow reservoir.

Here’s the science: It takes more heat to cause a closed system to boil than it does an open one. That’s the basis for cooling systems (and pressure cookers) since day one. And anything that leaks the required pressure will result in coolant getting hot faster, since the boiling point is essentially lower.

As a cap gets older, the seal gets harder, making it much more prone to cracking. With the cracks, you get pressure leaks. And since today’s caps are plastic, instead of the old-school steel, it’s just smart to replace them when you do a coolant system service.


If radiators have changed a lot recently, then what goes into them has, as well. Back in the day, filling up the radiator was as easy as pulling out the garden hose. Nowadays, there are specialty mixtures of things we can’t pronounce, but most experts agree that a 50/50 mix of filtered water and coolant (anti-freeze) is the best.

Pieces such as the water pump and various seals are lubricated by the coolant, so coolant is a must. And since the coolant is essentially the anti-boil element of the coolant equation, the water actually absorbs the heat from the engine and also dissipates it in the radiator.

As for the water, the experts I contacted when researching this article differed slightly on the water source. However, all of them said to use some form of filtered water and not the garden hose.

The problem with tap water is that it may have minerals that you really don’t want floating around your engine. They will cause deposits to build up in some places and eat the material out of other pieces. While the water doesn’t have to be Evian, it should be filtered or distilled.

Coolant Additives

We’ve all heard about additives that make water “wetter.” Now, that sounds tough to do, just on the face of it. There is even some controversy about these additives. There are folks who think that they work, and others who think otherwise.

So what do they really do for a diesel engine? A few of the manufacturers’ claims say that the additive infused water will bubble less and will form a barrier that separates the rest of the coolant from the various metal parts. It “softens” the water to keep hard water scaling from occurring and extends the life of the anti-freeze.

It’s also supposed to lower the operating temperature of the water. We have “tested” coolant additives in the past and have experienced a drop in water temperature. It did what it said it was going to do. All the other stuff about bubbles and hard water is great, but actually cooling the water is what it’s all about. Your results may vary, as they say.

Fans and Clutches

There seems to be a divide when it comes to electric vs. stock clutch-type fans. Both have their good and bad characteristics, according to the experts we talked to, so here’s what they had to say.

The clutch fan used to be “dumb,” but even bombs are smart nowadays. Somebody hooked up some wires to a clutch fan, and now, the computer controls some of them. That means they can be manipulated to come on at a predetermined temperature, instead of waiting for the clutch element to heat up and start the lock-up process.

Another benefit to the stock setup is that it’s basically bulletproof. One of the folks I talked to said that in his personal 1997 Power Stroke, he installed a “Snow Plow” fan clutch. We guess that for those whose trucks will be plow-equipped, Ford makes one that features a more aggressive action, which means that it locks up sooner than a standard unit does.

An electric fan, by contrast, is sleek and sexy. It’s usually made from shiny, black plastic, and some of the blades look almost as if Salvador Dali had designed them. They have long had the ability to be set to a desired temperature or even to have an on/off switch wired in.

With an electric fan, when you need air, you got air. And while they may have been a bit anemic in the old days, newer fans pass a lot of air, and they do it without taking any horsepower away from the engine.

Spinning a diesel clutch fan takes power, and diesel racers and sled pullers have made the switch to electric fans to gain horsepower and torque. In addition, electric fans can be set up to run after the engine is off for quicker cool-downs.


We have spent the last few pages telling you what you can do to help keep your rig as cool as possible. Now comes the hard truth that nobody is going to want to hear.

Look at the picture of this truck. Pretty neat, huh? Well, all those neat items, such as the grille guard and roof rack, not to mention the fact that it’s lifted on huge tires, are conspiring to make your truck run hot. Some should be obvious, such as the lights blocking out the radiator opening, thereby blocking out the air that is supposed to be passing unobstructed into the opening and said radiator.

Speaking of obstructions, any grille accessories such as stick-on overlays, and some aftermarket grilles, themselves, have openings that are way too small to allow a proper amount of air to pass by.

The OEs spend a ton of time on grille design and air inlet measures to make sure the engine gets enough air. If you’re going to opt for an aftermarket grille, make sure you get one with an “open” design so that air loss is at a minimum.

Big tires and lift kits put a big more aerodynamic drag on the truck. In addition, pushing a larger and heavier rolling mass takes fuel. There are also two schools of thought on lifted vs. lowered. Some say that a lowered truck is more aerodynamic that a lifted one; hence it pushes less air underneath.

The other side of the coin is that lifted trucks not only allow more air underneath to cool the drivetrain, they also offer a better pathway for the hot air under the hood to escape below the truck.

That’s the long and the short of it when it comes to cooling systems. As we said, the stock system is pretty good, and by making some strategic upgrades and simply keeping up with proper maintenance, there is no reason you can’t pull a heavy trailer without having to watch for the temp gauge to move into the red zone.

This article is in the October issue of Diesel World

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