Most boil-overs can be prevented with the right parts, coolant mixture, and knowledge
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 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
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.
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.
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.