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How Your 4x4’s Cooling System Works

Posted in How To: Engine on November 9, 2016
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Meh, your 4x4’s cooling system is simple…as simple as fluid dynamics, thermodynamics, heat transfer, material properties, specific heat, ideal gas laws, and a few more aspects of physics can be. Keeping your 4x4 cool in the heat of the desert while churning through sand or towing down the highway, scaling over rocks, or hitting the whoops is no simple task. With modern engines that burn fuel efficiently and make more power than ever this means more and more heat that has to get shed from under your hood to keep you headed down the road or trail. So getting back to that physics and fluid dynamics thing, we are not physicists, but we do have faith in science and a loose understanding of how vehicles stay cool. And while the physics of understanding how our 4x4s cool is certainly not simple we are happy to share what little part of it we understand. We also love learning and like to share our knowledge of how stuff works even if that means asking for the advice of experts like our friends in the industry. We’ll talk about the construction of radiators, (different types of metals…and plastics), their parts, fans, (mechanical vs. electric), fluids, water pumps, what they do, and what happens when any of these parts fail.

Radiators are designed to shed heat from the coolant of an engine to the air passing through them. They do this because of the physics of the materials that they are made of and the shape and size of these materials. The construction of radiators and the understanding of how they work has completely changed in the past 30 years according to our friends and experts at Flex-a-lite; When classic 4x4s were rolling off the assembly line 25 years ago, a four-core brass and copper radiator was required to keep V-8s cool. Bigger was the easiest way to make cooling systems work better at that time, and weight was less of a concern than it is now. What we know now is that, while a four-core radiator might be good for holding a lot of coolant, it's heavy and not that good at letting air flow through the core to transfer the heat to the atmosphere. A thick core is very restrictive when you try to pull air through it with a fan. That's why newer 4x4s went to two-core construction in the late '90s, with tubes that measured 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.

This is a new 4-core copper/brass radiator for a 67-72 Chevy/GMC Truck/Suburban/Blazer from LMC. Most newer radiators are going to be made of aluminum as it’s cheaper and lighter. Most copper/brass radiators are painted black while aluminum radiators are usually left bare metal. We like copper/brass because it can be repaired relatively easily even in the middle of nowhere. Factory and custom aluminum radiators that are properly designed do a great job of keeping just about any 4x4 cool.

Generally aluminum or brass/copper are used to transfer heat in radiators. Both of these metals are good at conducting heat from the fluids that fill them to the air that passes by them. Although copper/brass is slightly better at heat transfer, aluminum is lighter and less expensive. Aluminum radiators are also generally more uniform as all aluminum radiators are either welded together or aluminum brazed. This avoids issues with chemical reactions between different metals which can be an issue in brass/copper radiators where lead, tin, silver and other metals are used to solder or braze the components together. Aluminum will corrode when exposed to the incorrect coolant and electrical fields of a vehicle. Flex-a-lite recommends using a sacrificial zinc anode in conjunction with their aluminum radiators.
For any off-roader the ability to solder a brass/copper radiator while in the boonies makes them much more attractive than aluminum, but brass/copper radiators are becoming more-and-more rare. So while we’d prefer a brass/copper based radiator to an aluminum one, if available, a newer, properly functioning aluminum radiator beats an old clogged, poorly repaired brass/copper radiator any day. Automotive manufacturers struggle to shed weight from their vehicles now-a-days to fight for every drop of fuel economy. This is the reason almost all newer vehicles have aluminum or aluminum and plastic radiators.

Modern aluminum radiators like this one for a Chevy S-10 Blazer are thin and light weight, usually with one or two larger cores rather than the 3-4 smaller cores commonly found in brass/copper radiators. Aluminum radiators either have plastic tanks or aluminum tanks. All aluminum radiators need to be used with coolant that won’t attack the aluminum. Flex-a-lite recommends using a sacrificial zinc anode in conjunction with their aluminum radiators. We also recommend that you ensure that your engine block is properly grounded to prevent electrolysis damage.

The tanks of the radiator are where the coolant enter and exit the radiator. Tanks can be made out of aluminum, brass, or, as in many modern radiators plastic. Plastic is a poor conductor of heat and must be attached to the metal tubes of a radiator via resins and epoxies which shrink and expand differently than metals. Radiators can have tanks on the top and bottom (down-flow) or on the right and left side (cross-flow). Cross-flow radiators can also be dual-pass, meaning that there is a dam in the center of the radiator and fluid flows from one side to the other on the top of the radiator, and then back on the lower half of the radiator. The tanks also serve as a place for radiator mounting, inlet, outlet placement, and a home for the radiator cap.

Flex-a-lite radiators are made of aluminum and have extruded metal tanks with patented T-channels that increase surface area acting like a heat sink as well as offering modular mounting points these radiators are available as direct fit for bolt in applications or custom for that odd-ball project.

This is a brass/copper down flow radiator (left) from a Jeep Wrangler or CJ and a slightly mangled plastic and aluminum cross flow radiator (right) from a Jeep Cherokee. You can see tanks top and bottom on the radiator to the left and tanks on the right and left of the radiator to the right. Also you can see the tubes running down on the down flow radiator while the tubes run from right to left on the cross flow radiator. Cross flow radiators can be dual pass meaning that there is a dam at the center of the radiator that forces the coolant to go through the tubes in the top of the radiator in one direction and then back through the tubes on the bottom in the opposite direction. Dual pass radiators usually have the inlet and outlet on the same side of the radiator.

Tubes and cores:
You’ve probably heard about two, three, maybe even four-core radiators. What that means is the numbers of rows of tubes front to back that pass from tank to tank. This is where the coolant passes and heat exchange occurs. Core number should not be confused with the number of tubes stacked on top of each other from the top of the radiator to the bottom in cross-flow radiators or stacked from right to left in down-flow radiators.

Fins are just that, fins of metal designed to increase the surface area of the radiator so more heat can leave the system. Fins are attached; welded or soldered, to the tubes and act like a heat sink where the air moves through the radiator. Fins can become damaged by debris and pressurized water. They can also become plugged with mud, plant mater, bugs and more.

We talked the folks at AMC 4x4 in Glendale Arizona out of a mangled Jeep Cherokee Radiator so we could bring it home and cut it up so you could see its guts. We cut the radiator into several pieces to show you the tubes, fins, tanks, and more. This radiator is a one core cross flow radiator with only one tube (front to back) labeled as A running from tank to tank (labeled as B). We also cut away part of the tank so you can see the entrances of the tubes from the tanks labeled as C. Between the tubes you can see the fins, labeled as D, which are made of thin corrugated aluminum welded to the tubes to increase surface area and allow heat transfer. You can also see the fittings and part of the transmission cooler in the tank of the radiator. The transmission fluid flows through the tank of the radiator because you don’t want the transmission fluid in an auto tranny to be cooler than the engine.

Fins can become damaged and folded over or packed with dirt, leaves, bugs, and mud. If air can’t pass through the fins and past the tubes your 4x4 is going to get hot. It’s good to wash the fins out after hitting that local mud hole, but be careful pressurized water can damage fins, folding them over and reducing air flow. Small fin combs are also available to repair damaged fins.

Radiator Cap:
Your vehicles radiator cap is probably more than just a plug in a hole where you fill your cooling system. Many radiator caps are more like a thermostat than just a plug. They allow fluid to leave and exit the system as it heat cycles and allow for pressure relief when pressure exceeds what the system can withstand. They do this via a couple of gaskets and springs. Those gaskets can dry and crack and the spring(s) can lose their…springiness allowing fluids or pressure to be relieved earlier than designed, or retain pressure until another part fails.

A radiator caps is a little more complex than just a plug that keeps coolant in your radiator. It has springs that allow fluid in and out of the radiator as it heats up and cools down. Coolant leaves the system and collects in the radiator overflow tank when hot. When the cars radiator cools and pressure drops the cap also allows this coolant to be sucked back into the radiator. The radiator cap is also the cooling systems “blow off valve” if the pressure of the system spikes pressure is released to keep the radiator, engine seals and freeze plugs, and hoses from exploding.

Your 4x4s fans are designed to move air through the radiator when headed down the highway at speed or parked in rush hour traffic. Add in towing, winches, lights, bumpers, big tires, and their job becomes more difficult. Now add in heavy rock work crawling up a trail on a hot day or blasting through that mud hole or creek and know that your fan is fighting a loosing battle. Fans come in two flavors, electrical and mechanical (engine driven) and both have pros and cons but neither work well without proper fan shrouds to direct the air they are moving through the radiator.

Electric fans are ubiquitous on modern vehicles direct from the vehicle manufacturers. That means they work and work well. Electric fans are great for low speed use or sitting in traffic when air isn’t flowing around the vehicle. Many old school off-roaders wont rely solely on them as electrical and mechanical issues can cause your rig to overheat, but many electrical fan systems are reliable and efficient. Also an electric fan can be controlled by a switch meaning you can turn it off for that deep water fording. Often electric fans are the only type of fan that will fit with an engine swap.

This is a custom cross flow, dual pass, radiator with Flex-a-Lite’s thinnest dual electric fan set up. This radiator is being used to cool a big engine (a 505 cubic inch Dodge RB big block) in an old Jeep with a very small grille. Running a mechanical fan is not possible due to space constraints on this rig.

Mechanical fans are driven directly by the engine or via a fan belt, frequently attached to the water pump. They work well in situations where there is a lot of radiator (four-core), a transmission cooler, A/C condenser, and more that has to have air pulled through it. Mechanical fans also spin at the same speed as the engine so clearly they move more air as you rev the engine. That doesn’t help when you are stuck in traffic or are idling over obstacles on the trail…but mechanical fans are pretty simple and rarely fail. Some mechanical fans are mounted to thermally controlled fan clutches which can fail and allow your 4x4 to overheat.

Many old school off-roaders refuse to rely solely on electric fans to keep their 4x4’s cool. Mechanical fans are simpler and almost always work (unless the fan clutch has failed, in which case you can drive a sheetmetal screw through it). Mechanical fans don’t work quite as well to cool engines when the 4x4 is moving slowly and the engine is only idling. That’s where electric fans thrive…as long as they are working properly. All fans, both electric and mechanical do their jobs much better when used in conjunction with a tight fitting fan shroud.

Other parts:
Thermostats are made of different metals, springs and a wax that expands at a certain temperature. This design allows you to use your 4x4 in a wide range of ambient air temperatures from 130 degrees down to well below freezing. The spring allows thermostats to remain closed until the engine reaches a certain temperature, holding coolant in the engine until it reaches operating temp. Then the thermostat opens, coolant flows to the radiator, and cools until the thermostat closes. This cycle maintains your engines operating temp. Thermostats can fail open, or closed. A thermostat that is stuck closed will cause the engine to overheat. One that is stuck open may not allow the engine to reach operating temperature which can cause a drop in fuel economy. Thermostats fail when the spring(s) fails or when the cylinder containing the wax leaks.

You can test a thermostat by submerging it in water and then heating the water to the temperature where the thermostat should open. This temperature is usually printed on the thermostat and should range from about 160 degrees Fahrenheit to around 190.

Hoses and clamps:
We will assume that you know what a hose is. Their job in a cooling system is to direct coolant from the heat of your engine and in to your radiator or heater core via the water pump. In the cooling system hoses can fail because of excessive pressure, mechanical damage (like abrasion from a belt or pulley), age (cracking or pin holes), loose clamps, and more. Replace any hoses that have any kind of cracking, chafing, or blisters. There are two general types of clamps; one type, sometimes called a band clamp uses a screw and slots cut in a metal band, tighten the screw and the band gets tighter. The second type of hose clamp uses spring steel to clamp the hose down onto the fitting it is attached to. This type of hose is very common from the factory. Both types of band clamps have benefits. Spring clamps are fairly size specific and can be reused many times as long as they don’t crack or loose tension. Band clamps can fit a wide variety of hose sizes but the screw can get stripped or the housing holding the screw in place can pop loose.

Any hoses that look dry, cracked, worn, or cut should be replaced as soon as possible. There are two common types of hose clamps. Both have pros and cons to their usage. One benefit of factory style spring hose clamps is that they can change size and maintain tightness as parts of your 4x4 heat (and get larger) and cool (shrink). Screw tightened band clamps will remain basically the same size during heat cycle.

The waterpump on your 4x4s engine is fairly simple and as long as it is turning and maintained properly it should move coolant where it needs to go. Pumps can have mechanical failures to the bearings or seals and can also fail when a belt fails or if the impeller becomes damaged by rust or mineral build-up from improper coolant usage.

Your engines water pump moves coolant through the engine blocks water passages, past the radiator, through hoses to the radiator, through the tanks and tubes, and back to the engine. If the pump fails then things can easily get too hot. This is a pump for a Jeep 4.0L I-6, you can see the impeller that does all the work. These can become corroded, packed with junk, or a bearing or seal can fail.

Sometimes referred to as water your engines coolant is way more than just tap water, in fact you never want to use tap water in your radiator unless you have no other choice (we’ve used nasty creek water in a pinch, just filter out the tad poles and swap it out for coolant and distilled water when you get home). Most tap water sources have minerals in the water that can damage your cooling system. Instead use 50/50 coolant mix or straight coolant with distilled water.

A cooling system only works as well as the coolant it contains. Coolant, or anti-freeze helps to prevent damage to the engine when exposed to very cold temperatures. The proper coolant also protects the cooling system from corrosion and can help improve temperature transfer of the cooling system. Always use the proper coolant for your type of radiator and mix with distilled water to protect your cooling system. We like the 50/50 premix. Sure it costs more, but it saves us the extra trip to the store to buy distilled water.

Radiator Overflow tanks:
The first commonly available cars and trucks lacked radiator overflow tanks and as a result you had to re-fill their cooling systems every evening after driving them. Early explorers in the Sahara desert of North Africa driving modified model-T trucks figured out a solution to this. By running the radiator overflow hose from their cars and trucks radiators to a can mounted on the fender with some water in it. The water in the can cooled the steam blowing out of their radiators changing it back to water. This greatly lowered the amount of water these explorers had to carry with them and as a side effect they developed a part that has been on just about every liquid cooled vehicle since; the radiator overflow tank.

Radiator overflow tanks are often missing or damaged from old 4x4s, and not having one almost always ends in an overheated truck. Factory radiator overflow tanks are clean and simple, but you can make one out of just about any kind of bottle or tank. Connect a hose from the radiator to the bottom of the tank and allow a vent at the top of the bottle and voila you have a radiator overflow system. Our 1949 Willys Jeep CJ-3A has an old propane tank plumbed to act as a radiator overflow.

You can also make a rudimentary overflow bottle out of a sports drink bottle or an old coolant bottle (shown) and some hose. Just make a hole in the lid or near the top run the hose to the bottom of the bottle and fill it with some water or coolant (enough to cover the bottom of the hose). You want the hose to fit in your hole loosely so the bottle can vent and the coolant can overflow if the vehicle overheats.

Auxiliary coolers are very common on heavy-duty trucks and 4x4s. These small radiators can be used to cool engine oil, transmission fluid, power steering fluid and more. They are also easy to add to any of these systems on a vehicle that works hard. Coolers work for two reasons. One is fairly obvious, they work like a radiator to allow heat exchange between the fluids running through them and the ambient air. They also increase fluid volumes. More fluid takes more energy to get hot so that helps keep these systems inherently a little cooler.

Almost all modern 4x4s with automatic transmissions will have a transmission cooler built into the vehicles radiator. These coolers share space with the engine coolant so the transmission temp is kept at or near the engine operating temperature. Autos don’t like to get above 230 degrees, but they also work better when warmer than ambient air temperature. It’s also easy to add auxiliary coolers like this one to the auto transmission system (although you want to do it up-stream of the radiator based transmission cooler), engine oil system, and power steering system.

Throughout this article we’ve hinted at what causes different parts of the cooling system to fail. In most 4x4s cooling system failure is generally caused by lack of maintenance or physical damage while on the road or trail. That can be caused by a stick passing through the radiator and damaging tubes, a tank, or hose or years of neglect, introducing the improper coolant or mixing coolant with non-distilled water. There are also a few things that generally tax and overwork your cooling system and can lead to problems. Mud, sand, and towing are all very hard on your cooling system for various reasons.

Towing at or near your vehicles capacity means your truck is building power and power creates heat. If that toy-hauler has one too many coolers of beer in it, or your cooling system hasn’t been maintained properly the next long grade is going to let you know.

Mud and sand both afford a fair amount of resistance to your 4x4s attempts at moving forward or backwards when compared to rolling down the road. Mud also has the unfortunate side effect of clogging up small passages like those found on a radiator. Both sand and mud can be tons of fun to drive in, and we wouldn’t begin to try to keep you from doing something fun. Just keep this in mind as you are hitting that berm or are perplexed when your truck continuously runs hot after that weekend mud hole assault. Dedicated mud trucks


Fife, WA 98424
LMC Truck
Lenexa, KS 66219
AMC 4x4 Salvage

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