7 Symptoms of a Bad Radiator Cap (and How to Test)

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.

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

coolant leaking

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

white residue 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 leak

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

collapsed radiator hose

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

radiator hose ready to burst
radiator hose ready to burst

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

engine overheating symptoms

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 open 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.

35 thoughts on “7 Symptoms of a Bad Radiator Cap (and How to Test)”

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

    Reply
    • 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.

      Reply
  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.

    Reply
    • 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.

      Reply
  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

    Reply
  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?

    Reply
    • 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.

      Reply
  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??

    Reply
  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.

    Reply
  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.

    Reply
    • 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.

      Reply
      • 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.

    Reply
  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.

    Reply
  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?

    Reply
    • 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.

      Reply
  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…

    Reply
  12. It’s good to know that coolant fluid near the radiator cap could mean there is some damage to the cap. Last night I noticed that my engine was getting overheated in my old Ford truck and I’m worried that it could do some real damage. I’ll have to check it tonight and make an appointment to get it serviced at a car repair shop as soon as possible.

    Reply
  13. Hi my name is Scott and I have been working on these old Chevy motors since I was old enough to hold a wrench but this one has got me stumped it is running around 260 degrees I’ve changed thermostat I put a new clutch fan plus a small electric fan it is building up psi the hose never hose soft like it should when the thermostat opens I haven’t changed the cap but in my experience the cap will cause the hose to collapse other then checking the heads and gasket I am at a lose please if anyone has some kind of solution please feel free to let me know thank u so very much in advance

    What would make the engine build up to much psi other then a head problem

    Reply
    • It sounds to me like you have multiple cooling issues going on, which is making this harder to test. If I were you, I would start with the following:

      1. The higher the heat, the higher the pressure; figure out why it’s running at 260 degrees first. Most thermostats open around 160-180 degrees. If your thermostat is opening in the 160-180 degree ballpark, the next thing I’d do is a leak down test. If you get any air bubbles coming out of the cooling system, that’s a textbook blown head gasket.
      2. Figure out why the pressure is so high. I think this issue may be related to the root cause for point #1 above. If the radiator cap is old, I would try replacing it. Make sure you replace it with a cap that holds the correct pressure for your cooling system. You should be able to find this pressure rating online or in a repair manual.

      Reply
    • I would actually take a look at the thermostat. If the thermostat is stuck open, the engine would have a very hard time warming up to and maintaining operating temperature.

      Reply
  14. Hi my name is Dennis Matsha driving a Toyota Corolla 2002 model I just overhaul the engine February this year and suddenly the car is overheating, cause when i start the car the coolant cap pops up and I bought a new thermostat cause the heater was not blowing hot air. What seems to be the problem?

    Reply
    • Have you tried changing the radiator cap? Make sure you have the correct pressure cap. If that doesn’t help, more troubleshooting of the cooling system is necessary to pinpoint the problem.

      Reply
  15. the radiator draws from the tank but don’t return the coolant. The engine is hot and there’s pressure in the radiator, but coolant do not overflow into the tank. Does a bad cap do that? The pressure actually burst the plastic part at the top. It’s a 2001 Chevy tracker.

    Reply
    • Yes, a bad cap could absolutely do that. If the valve in the radiator cap doesn’t open at the right moment to bleed the excess pressure, you can damage the cooling system as it will end up holding more pressure than it was designed for.

      When you buy a new cap, make sure it has a pressure rating that is designed for your vehicle. For instance, don’t put a 16 psi cap on a vehicle designed for 13 psi.

      Reply
  16. Hi there, I have a 2007 Hyundai Tuscon I’ve taken great care of, she had a full tune up in March, with scones head gaskets replaced, and recently had some more work done. I was checking the oil 2 weeks ago and noticed the coolant reservoir looked a lil low. I took it to a local oil shop and they agreed, shot some water into it and charged me lol. They said to keep an eye on it, but honestly, I checked a day later and I think bc of the water, it was too transparent so I had difficulty figuring out where the level sat. I waited a bit and today I could see the coloring and its just at the low line. I spent $22 on the 50/50 mix and before I add it, I just want to check if I’m doing the right thing. I’ve seen no leaks, smell nothing, I’m not good at examining auto parts but I’m guessing at best, it could be the radiator cap. I’ll check that in a few mins. When a check engine light diagnostic is ran, only a gas cap error shows these days. It’s been on since I bought a new gas cap. Thanks so much!

    Reply
    • Some cars leak a little bit of coolant over time for a variety of reasons. This could be out the overflow reservoir, the reservoir hose, radiator cap, coolant hoses, or into the engine from a head gasket leak. Top it off to the full line and keep an eye on it for now.

      Check the coolant every day to see if the level drops. If it drops, try to figure out where it’s coming from. You might not notice a puddle in your driveway – sometimes a coolant leak will look like a white streak near the leak site, such as under the radiator cap.

      Reply

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