Small block 400 with correct heads (steam holes), good radiator with a 16lb vented cap. Overflow is hooked up and it just keeps filling the overflow and never sucks any coolant back up. The system has pressure all the time! Even 8 or 10 hours after driving, it still has pressure when you pull the cap. Overflow is hooked to a nipple in the neck of the radiator. No oil in the water, and no water in the oil? Any ideas on what I can do here. I have replaced the cap, but it still does the same thing?

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Jeff Carpenter
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Sounds like you don't have the proper cap for a recovery system or the cap you have is malfunctioning.

What type of cap would I need?

go to your parts store and explain to them that you need a cap that will flow both ways and that has sufficient pressure to work on your application.

the cap that you need has two gaskets, one on the inside as any cap would have and one on the outside circumference where it seals against the outer lip of the radiator. Hope this helps.

EDIT: Stant part # 10231

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[This message has been edited by chev64 (edited 02-07-2002).]

Agree with the two-gaskets-on-the-rad-cap deal, but I wanna know how a cooling system has pressure ten hours after the engine is shut off! Pressure should primarily be the result of coolant expanding from heat. Once the engine cools off, there should be no pressure!

First guess: small leak of combustion pressure into the cooling system. Perhaps a tiny crack thay only leaks under fairly high pressure. The relatively low pressure in the cooling system doesn't leak back into the cylinder, so no obvious white smoke out the tailpipe.

Even the proper rad cap will not draw coolant from the overflow tank if there is no vacuum in the cooling system, and if you still have pressure after ten hours, there's no vacuum there!

I put a 396 in my 71 Chevelle and had a small
problem with water comming out the radiator drain hose. I put a new 16 lb. cap on it and I think it helped. A few months later the motor went down on me and so I overhauled it and had the heads redone and put a new water pump on it.Got it running and noticed antifreeze blowing on to the windsheild and fender. I ask the AutoZone guy and he said the radiator cap for that motor was rated at 16 lbs. and he would not advise using a different one.
So I got a $6.00 overflow tank from AutoZone and hooked it to my overflow hose. I filled the radiator and left the tank empty. When it gets warm it fills the tank about half full and when it cools it goes back in the radiator.
I never had this problem with my 454 or any small block that I had in the car before.

Schurkey,

It seems to be building a vacuum, not pressure. Sorry for the confusion. I take off the lid and the air goes rushing in.

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