The part of a vehicle known as the “radiator pressure cap” contains 2 valves. Its purpose is to contain coolant in the radiator and ensure that the cooling system remains pressurized.
The radiator cap is sometimes also called a coolant cap or an antifreeze cap. All three names refer to the same part.
Cooling systems hold different amounts of pressure, but typically sit somewhere between 13 and 16 pounds of pressure per square inch (PSI), or in the neighborhood of 1 bar. This pressure is controlled by the radiator cap and needs to remain constant.
Too much pressure will exceed the manufacturer’s specifications for the cooling system, and could cause cooling system component failure.
Too little pressure could cause the coolant to boil off. With little or no liquid around to cool the vehicle, the engine is likely to overheat.
Below are some symptoms you can look for to determine if you have a bad radiator cap.
Signs of a Faulty Radiator Cap
1) Leaking Coolant
If the radiator cap is stuck, pressure may build inside of the radiator, which could cause cooling system components to leak or burst.
If you notice coolant near the radiator or the radiator cap, then you clearly have leaky coolant. Check to see if the radiator has holes in it or if the cap looks worn or damaged. If so, then replace the cap.
2) White Streaks on Radiator
When coolant leaks out of the radiator filler neck and dries, it often leaves white streaks behind. While you may not notice coolant leaking from under the radiator cap, look out for these white streaks. They may tell you the cap is leaking under pressure or intermittently.
3) Overflowing Reservoir
Coolant goes into the reservoir tank as it expands. The radiator cap releases the extra pressure by sending some coolant into the overflow tank.
If you have a bad radiator cap, the coolant could get released too quickly and cause the reservoir to overflow. While you’re in there, check to make sure your coolant overflow tank is working properly.
4) Radiator Hose Collapses
You may have a bad radiator cap if the radiator hose collapses. The vacuum won’t be released by the radiator cap properly and it will cause the radiator hose to collapse during the cooling down period.
If this happens, inspect the cap to see if there is any damage. If there is, replace it immediately.
5) Radiator Hose Bursts
If pressure in the cooling system is too high, you are likely to see one or more hoses start to spray coolant all over the engine bay.
Most of the time the pressure isn’t high enough to rip the hose in half. You will often see a pinpoint leak that only sprays coolant when the car is warmed up.
A hose with a small hole may actually seal just fine when the car is cold. As you drive, pressure in the cooling system will build. The pressure will eventually be enough to force coolant through the tiny hole and your car will slowly lose coolant.
In this situation you’ll likely notice a leak on the ground or in the engine bay when you reach your destination, but not when you start driving. Check to see if your overflow reservoir is draining slowly over the course of a few miles. This may give you a hint that this is the problem.
6) Overheated Engine
Leaky coolant or air in the cooling system can lead to an overheated engine. If you notice your engine starting to steam from getting too hot, don’t look under the hood unless you’ve turned the engine off. Then, let the engine cool for some time before popping the hood.
That way, the engine can remain cool as you check it out. If there is coolant fluid near the radiator cap, there could be damage to the pressure cap. Check for that and replace as needed.
7) Air Inside the Cooling System
When your radiator cap does not seal properly, air could make its way inside of the cooling system. This will cause air pockets to get inside of the heater core, thermostat, and radiator hoses.
As a result, the engine will start to overheat because it cannot sustain a temperature that is consistent.
How to Check the Radiator Cap
Warning: Never open the radiator while it is hot! Allow the engine to cool completely before opening the radiator.
The radiator is under high heat and pressure. Attempting to open a hot radiator will cause hot steam and coolant to spray out and is very likely to burn you.
Inspecting the Cap
Before you get too deep into the diagnostics, double check that the pressure indicated on the radiator cap matches the cooling system pressure specified by the manufacturer. You can find this information in a repair manual, factory service manual, or online.
Visually inspect that cap to make sure the spring moves freely and there is no debris or corrosion under the cap.
Pressure Testing the Radiator
If you have a radiator pressure tester, then you can inspect the cooling system on your own. This may help you find leaks or determine if the radiator cap has gone bad. You may also be able to rent this tester from your local auto parts store.
To begin, pop the hood and find the radiator. It is usually located right in front of the vehicle’s engine. Now remove the radiator cap by pushing it down first and then twisting it counter-clockwise.
Take the cap adapter and screw it onto the pressure tester’s end. Keep screwing until you cannot screw it anymore. If you don’t know what the cap adapter looks like, check the label of the pressure tester’s packaging and see if it shows it on there. You can also look it up on the internet as well.
After you’ve secured the cap adapter onto the pressure tester, screw the other end of the cap adapter onto the radiator filler neck until it is tight.
The pressure tester should now be sealed against the radiator. If it is not, you may not be using the correct adapter for your specific radiator.
Using the pump handle, start pumping the tester until you reach the pressure indicated on your radiator cap. See if the gauge can store the pressure. If the pressure begins to fall and you’re sure you have a good seal against the radiator filler neck, you have a leak in the cooling system.
See if you notice any external coolant leaks while the system is pressurized, as they will be easier to find this way. Any leaking components will need to be replaced.
When you’re done with your pressure test, unscrew the radiator cap adapter slowly so coolant doesn’t spill everywhere. It may be a good idea to have a pan or bucket handy to catch any that overflows. Top up any coolant that was lost, and clean up any coolant that has spilled onto the ground.
What Does the Coolant Cap Look Like?
The coolant cap is typically made of metal and sits near the front of the engine bay. Sometimes the radiator cap is oblong in shape, but on some cars it is round. Since the radiator is pressurized, you will often see a warning label on top of the radiator cap that instructs you not to open the cap while the engine is hot.
A radiator cap is different from an overflow reservoir cap. An overflow reservoir cap is usually made of yellow plastic. A simple plastic cap is used on reservoirs that are not pressurized.
Please beware that some overflow reservoirs are pressurized on certain vehicle models. If you aren’t sure if your overflow reservoir is pressurized, check the owner’s manual before you try to open it with a hot engine. If you have a metal reservoir cap or your cap screws on, chances are you should wait for your engine to cool down.
Bad Radiator Cap or Head Gasket?
When your car overheats, sometimes it is hard to tell if you have a bad radiator cap or something more serious like a blown head gasket. To check to see if you have a blown head gasket, you can do a leak down test.
A leak down test shoots compressed air into each cylinder via the spark plug hole (after removing the spark plug). The head gasket is designed to keep the combustion chamber completely separated from the cooling system. If any air leaks into the cooling system, you will know you have a head gasket leak.
Radiator pressure tests can often tell you if you have a bad radiator cap. Once you’ve pressurized the system, you may notice coolant leaking at or near the radiator cap. This can happen if the cap itself has a bad seal.
Other Cooling System Problems (Bad Thermostat)
If you notice that your radiator cap is not getting hot even after the engine has been running for a while, you may have a problem with your thermostat. When a bad thermostat is stuck in the closed position, engine coolant is only able to circulate through the engine block and cannot make its way into the radiator. This could leave the vehicle with cold coolant in the radiator, even though the engine overheats.
Thermostats are typically straightforward to replace but require opening the cooling system. Any time you let air into the cooling system, this air needs to be bled out. Air pockets inside the engine can cause hot spots that lead to intermittent overheating.