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.
This is the high-capacity Mustang radiator found on '67-'70 small-block Mustangs with A/C and all big-blocks. It's retained with saddle brackets at the bottom and a single top bracket. | 
A radiator is comprised of tubes that run vertically ('65-'70) or horizontally (cross-flow radiators from '71-'05 Mustangs). These tubes connect the top and bottom tanks (or left and right tanks with crossflow radiators). In conjunction with the cooling fins that surround them, these tubes carry heat away from the coolant inside. This is the Griffin aluminum radiator from Mustangs Plus. | 
Cooling systems are happiest with the 60/40 antifreeze/water mix most antifreeze manufacturers recommend. This ensures protection against freeze-ups down to -34 degrees F and protection against boilovers up to 230 degrees F. Don't forget anticorrosion additives, especially if you're running aluminum heads and intake. Coolant enhancers, like water wetters, help improve cooling efficiency. |
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.