October 20, 2023

It’s All The Same Stuff, Right?

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Radiator Caps And Their Effect On Cooling

By Jefferson Bryant

In today’s world of knock-off parts and overseas manufacturing, many of the parts you buy all look very similar. Often these parts perform nearly or as well as their counterparts, but that isn’t always the case. The Devil is in the details, as they say, and that is the way it goes inside your engine’s cooling system as well. Small details like what cap is on the radiator make a bigger difference than you might think on the efficiency of your cooling system. Did you realize that the radiator cap is not just a cap, but actually a critical component in your engine’s cooling system design?

While some newer vehicles have the radiator cap hidden and you access the coolant via an overflow tank, most vehicles have a standard radiator cap. When you remove or install a radiator cap, you have to push down. The resistance you feel is the cap spring that seals the cooling system. This spring is designed to maintain a specific pressure (PSI) inside the system. “How your cooling system pressure is managed is a critical component to the overall design” Phillip Cochran of U.S. Radiator told us, “Open or closed recovery systems change the pressures inside the main cooling system.”

Water boils at 212F at sea level, but 6200 feet the boiling point is just 200F, this is due to the lower atmospheric pressure at higher altitudes. 200 degrees is the operating range of most engines, so at operating temp, the coolant in the engine is boiling hot, but it isn’t boiling. This is due to the pressure that is maintained in the system. When you increase the pressure, the boiling point rises (about 3 degrees/lb). This does several things- the boiling point moves higher, and the coolant volume is maintained. Water expands when it is hot, but under pressure, it can’t. This is why when you take the cap off a hot engine it violently shoots out of the radiator. This isn’t caused by system pressure; it is the coolant rapidly expanding and boiling over. There is a maximum pressure for cooling systems and the radiator cap is what manages that pressure. Cap ratings range from 4 to 30 PSI. If a cap fails to vent, the pressure can build and cause the radiator or hoses to burst, leaving you with a catastrophic failure and likely a blown engine. Opening the radiator cap on a hot engine can cause severe injury, and you will lose a lot of coolant when you don’t have to. It is always best to let to the system cool down to a safe level before removing the cap.

The spring and seat in the cap mate up to the flange on the filler neck in the radiator, creating a valve. When the coolant gets hot and expands, and the pressure limit is reached at the radiator cap, the valve/cap opens to allow the system to bleed off that pressure. In most vehicles, this action allows some coolant to escape, which is usually directed into an overflow cannister. There are two types of cooling system, open and closed. A closed system uses a pressurized expansion tank that works with a closed-system radiator cap which draws recovered coolant back into the system once it has cooled. An open system is one that has a drain hose that sends hot coolant to a recovery tank (or to the ground), and is not able to draw coolant back into the radiator. In closed systems, the tank both receives and yields pressure when the cap vents, but it does not hold that pressure, it vents pressure to the outside. However, once the main cooling system cools, the expansion tank seals under vacuum, so that the coolant can be pulled back into the main system without drawing in air. Closed systems require a double-seal cap, if you use a single-seal design, it will not function correctly.

Open systems vent the pressure and coolant to a non-pressurized recovery tank. If you have ever seen an old hot rod with a beer or alcohol bottle next to the radiator, this is an example of an open system recovery tank. In the 1970s and earlier, many vehicles simply had a small hose that attached to the water neck and let the coolant drip on the road. It used to be very common to see a small puddle of green under cars in a parking lot. These systems use a single-seal cap. As a general rule of thumb, on an open system you fill coolant up withing 1” below the bottom of the filler neck when the thermostat is open and engine is at operating temperature. This allows the coolant room to expand within the cooling system and not purge to the overflow tank or ground. If you have an open system, remember that antifreeze is deadly to animals, so be cautious about leaving pools of antifreeze.

The first closed-loop cooling systems for automobiles came out in the late 1930s, but they were not standard until the late 1980s. Even GM has used both systems in the same model with different packages. For example, a 1968 Buick GS had a standard closed system, but that would be an optional accessory on a Skylark. There are different pressure rates for various engines and cooling system types. The most common are 15 PSI for closed systems, and 7-15 PSI for open systems.

This is important because the radiator caps for a closed system and open system are very different. Most closed systems use 15 PSI caps, whereas open systems typically use 7-15 PSI, these values change based on manufacturer, make, model, and engine. “A change in pressure of 1 psi results in a 3-degree change in temperature” say Cochran, “If your engine calls for a 15 PSI cap, but the one that is actually on the radiator is only holding to 7 PSI, you can see engine temperatures 27 degrees higher than you should.” Cheap quality radiator caps can have large variations in the actual pressure range, which can wreak havoc inside your cooling system. If you need a 16 PSI cap, but the cap that came with your overseas radiator is opening at 12 PSI, you could see 12 more degrees in engine temp. If your vehicle has a high-performance engine, especially in boosted or high-horsepower applications, 12 degrees can make a big difference in performance and engine life.

Let’s not forget about safety caps, these are the ones that have a little red lever on the top. This is not part of the standard operation of the radiator cap, but rather a safety feature that allows you to vent the pressure from the system through the recovery system should you need to open the radiator while the engine is hot. You simply lift the lever, the pressure vents and once it is fully vented, you can safely remove the cap. A warning!! Never trust a lever style cap to have released all the pressure. You can be severely burned. At U.S. radiator we recommend you take a hot cooling system to a certified repair shop that is experienced in cooling systems.

Details matter, even the smallest piece can make or break your entire cooling system. If your classic car has the same radiator cap that was on it when it rolled out of the factory, you can rest assured it is no longer maintaining the correct pressure. Over time, the spring weakens and the seal itself wears out. You should replace the radiator cap about every 5 years in general. If your cap is loose, physically damaged, the seal on the bottom is torn or badly scuffed, it should be replaced.  Just make sure you know what type of system your vehicle has, so that you get the right one. U.S. Radiator can help you solve your vehicle’s cooling issues, just give them a call at 800-421-5975 to discuss your vehicle.