Jim Smart
August 28, 2005

Overheating problems are as Mustang-specific as leaking cowl vents, squeaky upper control-arm bushings, rattling doors, and crummy drum brakes. Some of us blindly accept overheating problems as typical old-car shortcomings, but it doesn't have to be that way. When these cars were rolling off three assembly lines four decades ago, they didn't overheat. It became a problem as our classic Mustangs got older, which should tell us something about cooling systems and how little we understand them.

This is a typical engine-cooling system. Coolant is in contact with virtually every nook and cranny of the water jackets and radiator. Heat is transferred to the coolant that's moved by the water pump through the block, heads, and radiator. It circulates in the engine's water jackets, taking on heat. When coolant temperature reaches the thermostat's rated temperature (160, 180, or 192/195 degrees F), the thermostat opens, releasing hot coolant to the radiator via the upper radiator hose where it's cooled and returned to the engine through the bottom hose and water pump.

Cool: How Does It Work?
Your engine's cooling system is designed to carry operational heat away from the engine to the slipstream roaring through the radiator. Operational heat comes from fuel/air combustion and internal friction. Heat is transferred from the engine's components into the coolant that flows through the water jackets to the radiator. As air flows through the radiator, heat is transferred from the coolant to the atmosphere via the tubes and fins that connect the radiator's top and bottom tanks. The engine's thermostat controls the flow of coolant out of the engine's water jackets into the radiator via the upper radiator hose. Coolant that's been cooled in the radiator flows into the engine's water jackets via the bottom hose into the water pump. It's a continuous cycle of removing heat from the engine via liquid, pumping it into the radiator, cooling it down, and returning it to the engine to continue the process.

Cooling systems fail to do their job when we don't do ours. Regular preventive maintenance is the first step toward reliability. Under ideal circumstances, you should drain, flush, and service your Mustang's cooling system every spring, and properly dispose of the old coolant via local recycling efforts. Always maintain the appropriate balance of antifreeze and distilled water (yes, distilled water to keep mineral content down). Keep an anticorrosion additive in the coolant along with additives that help transfer heat to the coolant. Most of us, with our busy schedules and lifestyles, understand this isn't realistic. Who has time?

Radiator Facts
Classic Mustangs didn't suffer from overheating problems when they were new because their radiators provided just enough cooling capacity to keep them out of trouble. As these marginal radiators took on scale and corrosion during use, their cooling capacity dipped to inadequate. This was especially true with those tin-can Falcon/Comet radiators common to pre-'71 Mustangs without air conditioning. Beginning in 1967, Ford used a higher-capacity radiator in air-conditioned and high-performance Mustangs, which helped cooling issues considerably. But, it didn't eliminate them entirely.

This is a line drawing of the radiator common to '65-'66 Mustangs, including non-air-conditioned '67-'70 models (six-cylinder shown). A weak link in these dinky radiators is the top tank, which tends to split and spray hot coolant all over the engine compartment.

Radiators, under ideal circumstances, have a big job. They have to transfer a tremendous amount of thermal energy to the atmosphere. Your Mustang's engine generates a lot of heat energy under normal operating conditions. When outside air temperature skyrockets in summertime, it becomes more challenging to transfer heat to the atmosphere.

If it's hard to understand the radiator's job, think of yourself as the radiator and the engine as a demanding boss who just laid a huge stack of paperwork on your desk. The faster you weed through it, the more your boss adds to the stack. Think of engine heat as the paperwork. A new radiator is like your mind first thing in the morning with a fresh cup of coffee. As the radiator ages, however, it becomes more like your mind at the end of the workday. For the radiator, it's scale and rust inside the tubes and tanks. For you, it's burnout from the workload. Eventually, the paperwork overwhelms you, just like excessive heat overwhelms an aging radiator.

New radiators do a good job transferring heat. The tubes, filled with hot coolant, transfer the heat to the fins surrounding the tubes. Air flows over the fins, designed to create as much surface area as possible, and carries heat away from the radiator. Think of the fins in your Mustang's radiator like the fins on your air-cooled lawnmower or motorcycle engine. Fins carry heat to the atmosphere through the increased surfaces and sharp edges.

We often expect too much of older radiators. The tubes become filled with scale, and they also get clogged with iron particles from the engine's water jackets. Proper cooling comes from good coolant flow through these tubes. When tubes become clogged, the radiator doesn't get rid of heat. Also, old radiators are often clogged with dead bugs and other debris in the fins and around the tubes, which further hinders cooling.

Whenever you're shopping for a new radiator, go for the most cooling capacity possible. If you're doing an original-type restoration, opt for the four-row-core, desert-cooler radiator, and specify automatic or manual transmission. They look dead stock and offer more cooling capacity than original equipment. If you're restoring a '67-'70 Mustang, opt for the largest radiator possible: the high-capacity version used on air-conditioned and big-block models. These radiators, especially with a four-row core, offer outstanding cooling capacity.

The thermostat is a small, temperature-activated poppet valve inside a small neck at the upper radiator hose. It controls engine coolant temperature by opening and closing as coolant temperature warrants.

Thermostat design has been virtually the same for nearly a century, and its function is simple. The spring keeps the poppet valve closed, and a wax-filled actuator opens it when things get hot. As the coolant inside the engine reaches operating temperature, the wax inside the actuator expands, pushing the poppet valve open against the spring pressure, which determines the opening temperature. The stiffer the spring, the higher the opening temperature.

Radiator caps are pressure rated for various applications. Classic Mustang radiator caps are typically 7-8 pounds. Caps for late-model Mustangs are in the 14-18-pound range because they run at higher temperatures.

Radiator Pressure Cap
The radiator cap keeps coolant inside the radiator and relieves pressure as necessary. Think of the radiator cap as a pressure-relief valve if coolant temperature becomes too high and pressure becomes unsafe. Radiator caps are also known as pressure caps because they relieve excess pressure at a given number of pounds per square inch (psi). This keeps us safe because it doesn't allow cooling-system pressure to become unsafe, possibly endangering anyone nearby. Older Mustangs call for radiator caps in the 7-pound range. Newer Mustangs use higher pressures in the 13-16-pound range because their engines run hotter at 192-195 degrees F.

Radiator-cap pound ratings tell us at what pressure the cap will unseat and release cooling-system pressure. A 7-pound cap will begin relieving pressure at 7 psi.

There are two types of radiator caps. Conventional caps have to be turned counterclockwise to the first stop to relieve pressure when hot. Safety caps, like Stant's Lev-R-Vent, relieve pressure when the lever is lifted. This keeps us safe from scalding-hot coolant. A hot engine generates a lot of cooling system pressure and heat--hot enough to cause severe burns. If you experience an overheated engine, allow it to cool before opening the radiator cap.

Cooling systems have to maintain a certain amount of pressure to keep the coolant from boiling. A good antifreeze/water mix helps raise the boiling point. But, when pressure is allowed to build inside the cooling system, it raises the coolant's boiling point even more. Unpressurized, the coolant will boil at a much lower temperature. This is why coolant explodes from a hot radiator when we remove the cap.

Boiling Points & Pressures

PressureBoiling Temp.
(Deg. F)
Note: Boiling point drops with altitude. At 2,500 feet, water boils at 207 degrees; at 5,000 feet, 202 degrees; at 7,500 feet, 198 degrees; at 10,000 feet, 193 degrees. Sea-level atmospheric pressure is 14.7 psi. By contrast, at 10,000 feet, atmospheric pressure is 10.0 psi.

Coolant Recovery
Beginning in the '70s, automakers started installing coolant-recovery systems to catch any coolant vented from the radiator. The coolant-recovery system is little more than a reservoir tied to the radiator overflow. The hot engine vents excess coolant to the reservoir. As it cools, it draws the excess coolant back into the radiator. This prevents coolant loss and contamination of the environment.

Water Pump
Like the radiator, the water pump loses its effectiveness with time and use. The impeller blades gather corrosion and scale, making them less effective. When you replace the radiator, replace the water pump too. Opt for one of the high-flow pumps, which looks stock depending on where you buy it. Edelbrock and Weiand offer good-looking pumps that will improve your cooling system's effectiveness. With a high-flow water pump, you increase the number of gallons per minute circulating through the cooling system. Keeping coolant on the move reduces an engine's operating temperature considerably.

Fan Club
The fan is one of your cooling system's most important assets. Yet it seems we choose cooling fans for all the wrong reasons--for the way it looks or because it's the correct original equipment. But, if you're going to drive your Mustang regularly, choose a fan because it works well. The most effective is the luxury-car thermostatic clutch fan, which works only when it's needed. It's quieter because it doesn't spin as fast as the water pump. Clutch fans, as their name implies, slip most of the time, engaging as often as a hot radiator and engine need them. Clutch fans go bad when they lose their hydraulic fluid, which is used to engage them. Without the fluid, they freewheel and don't cool anything. This causes overheating, especially when you're idling in traffic.

Also common is the flex fan. As its name implies, the flex fan moves big gulps of air at low engine speeds. As rpm increases--meaning the car is moving faster--the blades flex and flatten out to consume less power. Flex-A-Lite is best known for pioneering the aftermarket flex fan, hence the name. These lightweight fans haul down plenty of air at idle while consuming less power than a conventional steel fan as rpm increases.

Beginning in the '67 model year, Ford used flex-blade fans as original equipment. Unlike the Flex-A-Lite version, the Ford flex fan is noisy. The Ford fans often failed, which makes them risky to use. If you want a flex fan, opt for one from Flex-A-Lite.

The Flex-A-Lite flex fan is quiet, with a soft whine, and is very effective.

Outside of engine-driven clutch fans, electric fans are the most efficient. The downside is having to create a dedicated circuit for them via the ignition switch. The key element with electric fans is getting the airflow needed to keep the radiator cool. If you install an electric fan, wire it with a relay and thermostat via the ignition switch so it comes on as needed with the ignition on. This eliminates the risk of leaving the fan on and running the battery dead.

When you're choosing a fan, don't forget to use a shroud. Not all Mustang fans were shrouded. However, when the fan is properly spaced, shrouding improves airflow velocity through the radiator at slow or no speed. Watch out for proper fan depth, which is halfway into the shroud for maximum effectiveness.

This is another type of flex fan from Flex-A-Lite.

Oil Is A Coolant
Did you know engine oil isn't just for lubrication? Engine oil cools where it counts--at moving parts, carrying excessive heat away from critical parts like bearings, piston skirts, and cylinder walls. As oil travels over these high-temp surfaces, it's your engine's front-line defense against overheating. Main and rod-bearing temperatures can rise as high as 400 degrees F, which makes coolant temperature pale by comparison. Whatever your coolant temperature is, add at least 100 degrees for the oil temperature. This is why we suggest the use of synthetic engine oils like Mobil 1, Redline, Royal Purple, and Castrol Syntex. These oils stick around when conventional oils begin to break down.

Antifreeze And Coolant Additives
The marketplace overwhelms us with choices when it comes to antifreeze and coolant additives. For years, conventional ethylene glycol antifreeze and water was used to keep our engines happy. Today, there are all kinds of antifreeze types, including water and antifreeze already mixed in one-gallon bottles. There are also environmentally friendly antifreezes designed to keep things safe for ground water and pets. Prestone offers a long-term antifreeze that lasts five years. There are antifreezes designed for severe-duty applications. Cruise the Internet and you'll find all of them.

Likely the most revolutionary antifreeze is from Evans Cooling. Evans NPG antifreeze is expensive at nearly $30 a gallon, but worth every penny if you want foolproof corrosion protection and maximum cooling effectiveness. The Evans nonaqueous coolant requires no water--you run 100 percent of the stuff in your Mustang's cooling system. Expect to use at least two gallons for most Mustang applications. Your Mustang's cooling system must be completely void of water before servicing with Evans coolant, which offers a corrosion-free environment, a higher boiling point (369 degrees F), and a lower freezing point. It also conducts heat better than any coolant in the marketplace. Using it is foolproof if you follow the directions.

If Evans NPG seems like an unnecessary investment, consider this: It never has to be replaced, and cooling system flushing and servicing never has to be performed. A radiator should last the life of the vehicle with Evans inside. Evans NPG has been tested to last 500,000 miles in diesel trucks. After more than 500,000 miles, it was tested and proven to show no chemical deterioration. Consider it an investment you make once. Install it and forget it.

Water wetter additives improve the heat conductivity of your coolant. All it takes is one bottle to improve cooling-system function (when everything is working as it should).

Reasons Engines Overheat
Engines overheat for all kinds of reasons, both inside and outside the cooling system, including:

  • Cylinder-head gaskets installed backwards
  • Crumpled or pinched exhaust pipes
  • Improper ignition timing
  • Any airflow restriction in front of the radiator (such as a dirty air-conditioning condenser)
  • Lower radiator hose missing the anti-collapse spring (overheating happens when you hit the freeway, and goes away when you get off)
  • Belt slippage (yes, it happens)
  • Improper water-pump-pulley sizing
  • Blocked cooling-system passages (corrosion, gasket sealer, stop-leak, and so on)
  • Improper fan for application (such as a reverse-rotation fan installed in error or fan installed backwards)
  • Improper valve timing (cam sprocket installed improperly)
  • Improper engine building clearances (too tight)
  • Fuel mixture extremely lean (carburetor jetting)
  • Blown cylinder-head gasket (especially with unexplainable coolant loss)
  • Coolant in the engine oil