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.

21 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

    • Probably a blown head gasket. If you are getting P0301 codes for misfire thats likely why. Combustion gases are being forced through a head gasket leak into your cooling system which is why it over-pressurizes the system and the valve lets go at 16 PSI to save your hoses.

  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.

  5. I have a 2001 chevy cavalier. I replaced the thermostat my car started to run hot finally got all air from line and it idles and drives 195- 197. When I turn the key off antifreeze goes out the recovery tank over flow hose. Can any one please tell me why??

  6. I have Toyota Camry 2006 and my car’s radiator is going dry and every 2 days I need to fill coolant. I visited radiator mechanic but he didn’t find any leakage but auto mechanic suspected that it may be a gasket issue and the coolant is filling into the engine. We checked engine oil also we didn’t find water in it. Can you suggest me what should I do I’m just going to change the cap immediately after reading the above symptoms.

  7. Happy New Year. Appreciate your work in helping us with a good article and post. 2008 PT Cruiser Turbo. Radiator began leaking last summer. Had it replaced. Come winter, I found little heat coming into the cabin. The fluid lines into and out of the cabin show 195+ deg temp, so not the heater core. Have refilled the radiator with air removal via the air bleed valve, have replaced the thermostat and the radiator cap but the unusual problem remains that engine temp takes a long time to get to the mid (normal), then at highway speeds the temp actually begins to drop towards the quarter mark. Will get some heat into the cabin after like 30 mins of driving. In street driving the radiator fan cycles on and off as normal.

    Now, after a few weeks the CEL has come on with a code of P0128. Only thing left is the coolant temp sensor which is kind of hard to get to but could probably be done with removal of the left headlight, which’ll be a lot of work.
    Thoughts, please? Thanks in advance.

    • Found the information at Autozone’s Repair Guide:
      2008 PT Cruiser Coolant Temp Sensor Replacement
      Turbo Models
      Before servicing the vehicle, refer to the Precautions Section.
      Disconnect the negative battery cable.
      Partially drain cooling system below level of ECT Sensor.
      Support engine from below with a jack stand and piece of wood
      Remove upper torque strut. (One 18mm and two 15mm and wiggle the mount out)
      Disconnect ECT sensor electrical connector.
      Remove ECT sensor.
      To install:
      Install ECT sensor. Torque sensor to 168 inch lbs. (19 Nm).
      Reconnect ECT sensor electrical connector.
      Install upper torque strut.
      Fill cooling system.
      Connect negative battery cable.

      • Update:
        Turned out that it wasn’t my Coolant Sensor either.
        Then, while looking for the intake air temp sensor I discovered that the long black thick goose-neck hose that goes from the top of the air filter box to the turbo air intake was totally rotten at the bottom neck and was almost separated at that point.
        So, for any car that is over 5 years old I’d recommend checking the air feed hose first, for temperature issues.
        Heat around the engine seems to be enough to disintegrate the material of these hoses.

  8. Need advice. I was driving my honda accord 1999 down road and saw steam. It was a upper radiator hose with a tear in it. The heater went out.

    I replaced the upper hose. I filled radiator up and overflow to min. Heat came back.

    Now I see steam coming up. I’m not sure if it is from the antifreeze from the hose blowing or new issue.

    What would cause the hose to tear like this? I replaced the radiator, the cap and hose about 2 years old.

    How can I test to see if it’s the cap? I dont have any specific tools for this.

  9. Please I have a Toyota Matrix 2003. I have being experiencing overheating lately. The mechanic said it was a blown gasket, he changed it and washed the radiator tank. At a low of 1000 – 2000RPM the temperature is normal but from 2200-3000RPM the temperature goes high, on the high way the water heats up and boil, which evaporate through the expansion tank outside. Please what can I do because I’m out of ideas.

  10. Hi, I have a 2002 Toyota Hiace. The overflow tank overfills and liquid does not go back into radiator after cooling. Have had the pressure test, and my mechanic put in Chemi-Weld to rectify this problem. He talked Head Gasket issues and we are hoping not to have to go this route. I replaced the radiator cap and wonder if buying a cheap one was a bad idea. Should I get a genuine one from Toyota? The mechanic also said I had a tiny leak in my overflow tank. Would replacing this do the trick? Get both a new cap and overflow tank?

    • I would always go OEM on something like a radiator cap. If it’s not sealing, it will prevent flow back into the radiator. The leak in the reservoir can also be causing this since you need vacuum between tank and radiator to function properly. Check the coolant overflow hose for a leak as well.

  11. hi…i have a 2007 pt cruiser. top radiator hose is hot across engine and
    hot still at the thermostat housing but rad cap up top is cold or cool.
    I have intermittent warm heat at best.
    temp gauge is constantly running below first cool tick line….no overheating
    at all…


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