Coolant overflowing from cap

[vertigo]

I flushed my coolant system and put a new cap on the radiator recently. Filled the radiator to the top and made sure there were no air bubbles. Started the truck up and brought it to operating temperature and after awhile the coolant started overflowing from the cap and not into the coolant reservoir. There was nothing clogging the line from the radiator to the reservoir and the cap was on there tight. What could be the problem?

[haysonj]

Sounds like a faulty cap or the wrong cap for your system

[Stocker]

^ What he said

[SS Tim]

It sounds like two problems. The cap is not sealing and the radiator is overfilled. The spec. is 3" below the bottom of the filler neck cold. Might want to take a look at section 6 of the factory service manual.

If you are running a recovery system you need a cap so designed that will allow the cooling engine vacuum to draw from the tank. The stock cap will only work correctly with a stock dump tube or a returnless catch can.

[mac808]

Fans electric or mechanical? Are they turning on when they are supposed to?

[Stocker]

Originally Posted by SS Tim

It sounds like two problems. The cap is not sealing and the radiator is overfilled. The spec. is 3" below the bottom of the filler neck cold. Might want to take a look at section 6 of the factory service manual.

If you are running a recovery system you need a cap so designed that will allow the cooling engine vacuum to draw from the tank. The stock cap will only work correctly with a stock dump tube or a returnless catch can.

I understand not filling to the top if you don't have a coolant recovery system.... but did any of these rigs come from the factory with one? The factory sealed systems I've seen have the radiator completely filled. My '72 has a recovery tank and the radiator is filled to the top.

Agree as to needing the correct cap, as posted above.

[Rufton]

I fill 3 60's vehicles to the top.
Made sure caps had valving for over flow tank.
Installed over flow tanks.
I like because conserves coolant and minimizes oxidation.
I have noticed radiator sometimes has small air cap when cool but didn't investigate.

[SS Tim]

Originally Posted by Stocker

I understand not filling to the top if you don't have a coolant recovery system.... but did any of these rigs come from the factory with one? The factory sealed systems I've seen have the radiator completely filled. My '72 has a recovery tank and the radiator is filled to the top.

Agree as to needing the correct cap, as posted above.

Everthing I have seen has been a loss system from the factory. The closed example was used for people who have modified their systems. Problem is not everyone understands the difference or has not been exposed to anything but a full recovery system.

A recovery system is plumbed in such a way that the fluid can vacuum return typically a hose extending to the bottom and maintains a minimum fluid level in the recovery tank as well. A catch can only catches it and does not have a return path and may easily be empty. If you have modified the cooling system in such a way as the fluid can return (tank and recovery cap) then yes fill it to the top and enjoy a little more heat rejection. But if there is not a path then use the factory level as intended to reduce the venting from the closed system to the ground or catch can.