Blowing off steam can be fun, unless it’s your engine that’s blowing the steam. Many people view a cooling system as a necessary evil, building the rest of your 4x4 with cool parts and then dealing with an overheating problem. But being nervous with an engine that tends to run hot takes the fun out of a trail ride, and overheating can leave you stranded and potentially damage engine parts.
If you approach the cooling system with the same enthusiasm as you do an axle upgrade, you can be rewarded with a dependable system that you will never have to worry about. You may also be able to gain some performance and fuel economy while you’re at it!
In order to diagnose an overheating problem or build a great system, it helps to understand how the cooling system is supposed to work. Your engine produces heat as it runs. The heat is transferred to liquid coolant which is circulated through the radiator with the water pump. The radiator transfers the heat from the coolant to the air that flows through the grille. The fan blows air through the radiator to increase the rate of heat transfer.
In addition to this basic system overview, there are a couple more things you should know about. The thermostat closes off the coolant flow to the radiator to let the engine come up to temperature and to keep the engine above the minimum operating temp. The system is pressurized to increase the boiling point of the coolant. As the temperature goes up inside the system, the pressure increases. The radiator cap has a pressure release valve that’s designed to hold a specific amount of pressure, usually about 16 psi. Once the system boils, air is separated from the liquid, and it’s all over, literally.
Thar She Blows!
If your 4x4 has an overheating problem, the first step is to figure out what is causing the issue. Here are a couple of rules of thumb. If your 4x4 keeps cool on the highway but overheats in traffic or on slow moving trails, you most likely have an airflow problem. If it usually runs cool at idle, but overheats at highway speeds, your radiator probably isn’t up to par. It could be too small, not efficient enough, or clogged. This is also the case if your truck runs cool until you get into situations where you’re making a lot of power; in a mud bog or at the sand dunes, for example. If you overheat while towing, it could be either radiator or airflow, or both. Towing heavy loads is one of the biggest challenges for a cooling system.
There’s also the situation where everything has been fine for years and then the engine starts running hotter or simply overheats all of a sudden. This is usually caused by parts going bad. If the problem starts all of a sudden, it’s often a failed thermostat. If the temperature gradually increases over weeks or months, a water pump or radiator could be the culprit.
Also consider when the overheating started. If you made changes that have added significant horsepower, then more engine heat can be expected. We’ve also run into overheating problems caused by the ignition timing slipping 5 to 10 degrees, or by switching to a carburetor that was much leaner. Think about what modifications you made just before the overheating started to see if it’s a cooling system issue at all.
If overheating only happens in traffic and at low speeds, you need more airflow. When you drive at highway speeds, you’re forcing more air through the radiator regardless of how much air the fan moves. In fact, most vehicles don’t need a fan at all on the highway unless the vehicle is towing a heavy load. But at slow speeds or a stop, the only airflow through the radiator is created by the fan.
If you have a belt-driven fan, do you have a fan shroud? If not, this could be your problem, but even a properly shrouded, belt-driven fan may not move enough air at low engine rpm to cool your radiator because the fan speed is related to engine rpm. When the engine is idling, a belt-driven fan is moving very little air through the radiator. This is when an electric fan is a nice cooling solution because it can spin at full speed, pulling maximum airflow, even when your engine is idling. Switching from a belt-driven fan to an electric one also gives you a performance benefit by completely removing the mechanical load of spinning the fan from the engine. This means more power to the wheels and even a fuel economy gain. Removing a belt-driven fan that mounts on the water pump also reduces the load on the pump. This can lengthen the life of the bearings in the water pump.
What if you already have an electric fan and you still have an overheating problem only in traffic? Then you probably don’t have the right electric fan. There are hundreds of different electric fans with various airflow specifications and mounting configurations. An electric fan that pulls 1,500 cfm doesn’t stand a chance of keeping 700 horsepower cool. You need to scale the cfm rating of the fan with the power output of the vehicle. But cfm isn’t the only factor. Pulling air through as much of the radiator as possible is important. That’s why electric fan systems with full shrouds work so well. Traditional “basket” electric fans are good as auxiliary fans on the front of the radiator when a belt-driven fan is still used, but are generally not recommended as the primary cooling fan.
In addition to airflow through the radiator, you should consider airflow in the engine compartment. If the hot air is trapped under the hood, you’ll be heating the radiator from the inside and outside when you’re idling or moving slow. The fan is also less effective if there isn’t a good path for the air to flow out of the engine bay. In some cases, cutting out part of the inner fender wells or putting louvers in the hood helps heat escape the engine bay.
The radiator world has completely changed in the past 30 years. Way back when, a four-core brass and copper radiator was the hot ticket to keep big-powered trucks cool. Having more cores was the only way to be better at that time. What we know now is that, while a four-core radiator might be good for holding a lot of coolant, it’s not that good at letting the air flow through the core to transfer the heat to the atmosphere. The thick core is very restrictive when you try to pull air through it with a fan. That’s one reason why the auto makers went to two-core construction in the ’90s, with tubes that measure 1-inch wide instead of the traditional 3⁄8-inch. Most performance aftermarket radiators now have the same two-core design with 1-inch tubes.
Aluminum radiators are now very popular, but guess what? Aluminum isn’t the most efficient material for heat transfer. When it comes to radiators, brass and copper is. Surprised? Aluminum is favored because the strength of the aluminum allows for a thinner core with much larger tubes; this promotes superior airflow and heat dissipation over its brass and copper counterpart. Also, because of their added strength, aluminum radiators can run a higher psi cap, which raises the boiling point of the coolant. For every point of psi, the boiling point is raised approximately 3 degrees. Aluminum radiators also look cool and are lightweight. The absolute worst construction for a radiator is composite: an aluminum core with plastic side tanks. Plastic is not very good at heat transfer, and these are prone to failures, especially in off-road vehicles where there is a lot of flex and vibration.
Most full-size trucks come with good-sized radiators (height and width). You may need to upgrade from a composite, single-core radiator to a performance two-core with either aluminum or brass and copper construction. Vehicles that are more likely to have cooling problems are compact trucks and Jeeps where underhood space for a radiator is at a premium, and where you’ve likely changed out the four- or six-banger for a V-8. This is when you’ll want to fit the largest width and height radiator possible, with a high-efficiency core for the best heat transfer.