# Radiator Cap



## zrsteve (Aug 28, 2014)

I have just started to notice that I am getting some antifreeze run over, it just started. The engine is not overheating (185*) but if I let the engine idle to temp or I drive it for 15-20 miles and shut er down I get liquid out of the overflow tube. No, I don't have an oveflow tank, which will be remedied shortly. I've got a 15# PSI cap on now, but was told to reduce it to a 9# PSI cap. Comments?


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## PontiacJim (Dec 29, 2012)

If you over fill the radiator, this can happen -providing no other cooling issues. Let it over flow. Once the engine has cooled off, remove the cap and see where your water level is. As long as you can "see" the level above the cooling tubes, you are good. It may over flow more on a really hot day. Again, let it cool and check. The coolant has to expand when it gets hot and the larger the radiator, the more it may expand. As long as you can see the level, you should be good.

My guess is you do not want a 9 lb cap. What that means is that when the system builds 9 pounds of internal pressure, the cap releases that pressure, and goes out the over flow, which also means this will happen at a lower coolant temperature and not what you want. Lowering the cap pressure will only aggravate the situation. The higher pressure cap actually allows your temp to run higher before the cap releases the pressure, so I would stick with a 12-15 lb cap UNLESS the manufacturer of your radiator says otherwise.


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## ALKYGTO (Mar 29, 2010)

How old is the cap that is on there now? May be time to replace if the spring is worn and decreasing the amount of pressure required to vent.


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## geeteeohguy (Feb 2, 2008)

What these guys said, only JIm isn't guessing. He's telling. A 9 pound cap will blow open at 9psi, so you will loose coolant much sooner and at a lower temperature. Whoever told you to go to a 9psi cap: avoid future advice from that person. They are misinformed. Stock PSI rated caps for these cars is 15psi. You need to A: verify your coolant level is not overfull (needs to be 2" down in the tank), and B: pressure test your cap. You'd be surprised at how many 15psi caps blow open at 5 to 7 psi. Good luck.


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## zrsteve (Aug 28, 2014)

Thanks to all for the advice. I noticed the fluid llevel in the radiator was low so I topped it off, that's when it began to overflow when at temperature. I'll just replace the cap with a new 15# cap.


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## kobold (Jan 5, 2019)

Have the same problem now with a new Stant radiator cap. (16 lbs)
( I thought, it looked nice with the red safety lever, and wasn't expensive...)
5 minutes after the ride, the cap opens, and the coolant flows out ( about 1/4 liter)
This happens now 2 times, and now I'll use my old cap again.(13 lbs)
I'm amazed about the quality from this ( assembled in mexico) caps....


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## Mine'sa66 (Oct 30, 2019)

A cooling system without a recovery tank does not work like newer vehicles. When coolant heats up, it expands. It has to go somewhere, that's why there's an overflow line. The correct fill for a non-recovery system is roughly a couple inches below the top of the radiator. Specifically, it's exactly the volume that your entire coolant load expands. That volume is how far below the top of the radiator the level should be. A coolant recovery system does just that, coolant expands, and gets pushed out into the tank. When the coolant cools off in contracts, causing a vacuum, drawing the coolant from the overflow, back into the radiator.
Folks having some coolant forced out the overflow hose are probably not seeing issues with the cap, most likely just too full. If you want to test, run the car, when it's fully hot, all the coolant that's going to be forced out is out. Next time you drive it, it should not leak anymore (or just a tiny bit). If you continue to vent a lot, then you probably have an issue.


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## histoy (May 22, 2019)

If the fluid level is high, the coolant will overflow when you shut off the engine, as previously noted. The coolant temp can increase by 20 degrees once you shut off the engine. This causes further coolant expansion in the radiator. If you have a direct reading water temp gauge, you can see this happening. I run the coolant level at 1/2" - 5/8" above the tubes in our "66, and that solved the problem you described.


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