5 Symptoms of a Faulty Radiator Cap and the Best Way to Test It

The part of a vehicle known as the “radiator pressure cap” contains 2 valves and its purpose is to store coolant and ensure that the radiator system remains pressurized. The interior of the radiator contains 16 pounds of pressure per square inch.

This pressure is controlled by the radiator cap and needs to remain constant. Otherwise, your car might overheat and its radiator might endure severe damage. Additional information about the radiator cap can be found below

Signs of a Faulty Radiator Cap

There are a few signs that will let you know if you have a faulty radiator cap. Below are the top 5 signs.

1) Leaking Coolant

If the radiator cap is stuck, fluid cannot get released. This will cause pressure to build inside of the radiator, causing the hose to leak or break open.

If you notice coolant fluid 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) Overflowing Reservoir

Coolant goes into the reservoir tank as it expands. The radiator cap is released by the pressure and the coolant is sent toward the overflow tank. If you have a bad radiator cap, the coolant will get released too quickly and cause the reservoir to boil over.

3) 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.

4) Air Inside the Cooling System

When your radiator cap does not seal properly, air will get 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.

5) Overheated Engine

Leaky coolant or air in the cooling system can lead to an overheated engine. If you notice your engine getting too hot, don’t look under the hood unless you’ve turned the engine off.

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.

How to Check the Radiator Cap

If you have a radiator pressure tester, then you can inspect the radiator cap on your own.

To begin, pop open the hood and find the radiator. It should be 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, take the radiator cap and screw it on the other end of the tester until it is tight.

Read also: Vacuum modulator functions, failure symptoms and replacement cost

Using the pump handle, start pumping the tester and see if the gauge can store the pressure. You should see the gauge read a minimum of 15 pounds.

If this amount begins to fall, then the pressure cap must be malfunctioning. If so, take off the pressure cap and clean off any debris that may be on it.

Now screw the cap back on and test it again. If it is still malfunctioning, then you will need to replace it.

7 thoughts on “5 Symptoms of a Faulty Radiator Cap and the Best Way to Test It”

  1. my car is a Subaru Impreza 2003 model.
    Radiator dries up every few kilometers.
    Radiator cap suspected.
    What is the right radiator cap?

    • Any auto parts store or dealership will be able to tell you which cap you need. Make sure you don’t have a leak in the radiator or overflow tank.

  2. My 1997 Buick Le Sabre consistently loses coolant. Car runs at 100 120 degrees. On Vent, you can get slight amount of warmth on passenger side. Driver side cold.

    • Need to figure out where the leak is coming from. Most common areas would be the radiator cap, a radiator hose, the radiator itself, or the thermostat. Worst case is a leaky head gasket.

  3. My radiator blew. Replace it with a new one plus new thermostat. Started 2.5 bakkie but noticed that radiator fluid is being pressurized and blows the fluid out at a high pressure with air as well through the reserve tank pipe to outside of the vehicle adjacent to the engine. Please advise

  4. Hi, my 94 honda accord just blew the cap off going down the interstate. So what caused this? Do I have a bad radiator cap or is my radiator going bad? Need to know if I put water in it can I still drive it home?

    • It’s likely a faulty radiator cap. It’s job is to open and purge excess coolant to the overflow tank when a set pressure is reached. If it’s not doing its job, that pressure has nowhere to release so the cap itself pops off. It’s possible (but highly unlikely) that it’s a blown head gasket but in my mind, replacing the radiator cap should fix your issue.


Leave a Comment