Replacing Radiator Cap - Bad Cap Signs & Symptoms & How Radiator Caps Really Work

When should I replace my radiator cap? What does the radiator cap really do anyway? These are common questions technicians will hear from customers. In this quick Tech Tip, Ross Hatton from the Autodata Training workshop talks about the importance of the radiator cap to a vehicle's cooling system and what the signs of a bad cap are!

[Cody_Powers]

I feel like the cap is the most overlooked overheating culprit

[Mike-mesp]

Thankyou! Yours is the first explanation I've come across that actually includes the inlet valve. They all talk about how the pressure is let out, but not about how the fluid gets back in upon cooling. Thanks to you I now know why my radiator is always getting low, a dodgy inlet valve which I never even knew existed!

[gomy]

My radiator cap failed in the way you said you’ve only seen 2 in your career. Went for short drive got home and heard bubbling from under bonnet. My radiator was making boiling sounds while my overflow tank was boiling and spluttering. A large puddle was forming around my feet. Unbelievable that all of this could come from a cap. I thought it did nothing but seal a hole!

[technicianumber]

Holy shit. I've replaced the radiator, the thermostat, the water pump, cleaned out my heater core and all I needed was this damn overlooked radcap. Thank you sir

[girohead]

Good quick overview, though I had to rewind a few times to understand what 'weakest link file' was..:P J/K, hence I'm from CA and had no suspicious of my cooling system, but I removed the radiator cap and see no spring or washer! I knew that wasn't correct but I am perplexed how long it's been like that or how it happened. One guy said he hoped the part isn't in my radiator or elsewhere - I think the hole is too small on my Toyota, but I'm going to check again as I just bought a replacement - I'm glad you mentioned how long they last, as I'm beyond that, so it's time. Thumbs up!

[angruoklok]

The rubber was definitely expended passed the metal so I replaced the cap and went for a nice drive w no problem. Just wondering if the 25 degree difference in air temps from yesterday when it overheated could make that big of a difference.

[Dave-qprq]

All radiator pressure caps should be bench tested prior to fitting. We never installed a new cap without pressure testing it first. If you don't have access to a system pressure tester, measure the cap and radiator neck carefully to ensure that the two rubber seals are slightly greater in distance than the two radiator neck seal faces. The pressure seal must be positive. .5mm minimum.

[markwinter]

Inlet valve (the small one) failure recently happened to me. Collapsed radiator hose due to the coolant contracting as it cools.

[michaelrs]

Greetings from Orange County, California. thanks for the tip on checking to see if the rubber plug is out of shape and hanging over the metal. Not sure how much over it has to be but on my 20-year-old Toyota it is a little bit all the way around so I'll be replacing that radiator cap, since I've been having some minor overheating issues and I can't figure out anything else that is wrong 👍

[kingsupreme]

Thanks a lot.i carefully trimmed the extra rubber from around the middle flange and now im all set.saved me a lot

[frogboyxyz]

Simple Question: the cap on expansion tank goes on by pushing down, then a quarter turn will secure it in place but it wiggles a bit on top, then push down and another quarter turn locks it down. It stays on both ways. It doesn't just screw on like a threaded part. So which one is correct? One quarter turn with a jiggle or two turns and no jiggle? Thanks

[natthawatphongs]

Thank for the tips. Its very useful. Great video Thank you

[carlodonnell]

Hi I replaced my radiator with an all aluminium radiator which came with a 19psi cap; but the max psi for the cooling system on my 2001 Dodge Ram v8 van is 18psi. The 18psi pressure cap can't fit the new radiator because the opening is deeper and the 19psi cap is bigger. Would it do any harm to use the 19psi cap that came with the radiator or would it cause any problems with other parts of the cooling system like the pump-seals or hoses?

[TheMultikitty]

So we had to replace my radiator in a 2008 Hyundai Santa Fe it was not a OEM radiator it was a brand called Auto shack that we ordered through Amazon how are the past year and a half we had issues with it looks like slowly leaking look like coming from cap so we changed cap it was one from the parts store it did okay for a little while then started seeing coolant same area at top so then we bought a Factory bran and it's doing the same thing so what kind of cap should I get it doesn't seem to be leaking from anywhere else what cap should I get

[Official_Golden]

Radiator caps do make a difference. On my 97 Trans am, it went faulty, it was overheating up to like 300 degrees, water bubbling and exploding from the resorviour, and even it was popping. Changed it, temps dropped to normal, cooling was great.

[jimhart]

Hi there, I have an issues with my cooling system, I have sent for repairs a couple of times.but unfortunately, It can't be found, I had replaced the water pump casing, hoses are been cleaned, but still having same problem over heating, but everytime I released hot air thru the cap, by slowly unfastened the cap, its turn normal again.. do you think the cap is the problem.. everytime I unfastened the cap the water is back on the max level again. Otherwise, it will stay at low.. and dry out my coolant. Please share your opinion with me. Thank you

[Yasserflehan]

Also you can press the intake hose by hand if the cup is bad it show the water passing the expand tank if is no water that mean this valve is good.

[debbiesmyth]

Hello. I have a problem I'm trying to get help with. I have a 2008 Chevy HHR LT and it has been overheating to the point where the Guage on the dash goes clear to the max as far as itbcan go and if you continue to drive (I live a long way from town) the Guage will drop real fast, like immediately to the lowest point on the Guage. And stay there. I have looked and looked for a replacement cap for this radiator
Cant seem to find one😅 anywhere. Meanwhile, my plastic radiator cap has gotten so hot that it has fused to my radiator threads. How do I get this plastic cap off? And more importantly, where can I find the correct cap for this 2008 Chevy HHR? I cant find one anywhere. Even the chevy dealer only has the plastic cheap caps. I don't want to go through this again. I am at my wits end trying to find a replacement cap. Please please pretty please help me. I can't afford to take this to a dealer for this.

[MajorCulturalDivide]

I've never heard the term "header tank". Is that the plastic reservoir?

[nikyabodigital]

There's a spring that popped off my Radiator cap when i opened it. Should i have it replaced.