Hey all, I am hooking up the rad to my 2nd atempt at a rebuild (1987 Base). I figured once I had all the coolant stuff hooked up I should use my pressure tester to see if I installed the coolant O-rings correctly this time. Well I hooked up the pressure tester and started pumping. The only thing taht happend was the coolant overflow started bubbling. So I dicided to cap the line that connects the radiator to the coolant overflow. Once I did this the system would hold 15psi just fine.

So I want to know if something is wrong with the over flow bottle or what?

if bubbles were coming up in your overflow bottle it means your pressure release cap was letting pressure leak out, check the cap, replace if necessary.



you should be seeing bubbles when you exceed the pressure rating of the cap btw, so if you had a 13psi cap and you went to 14 bubbles should be expected.

What pressure cap is everyone talking about, I only have one and its off because my pressure tester is mounted in its place. Gawd I really feel like and idiot today.

well if you're using the neck that the pressure cap goes onto for your checker then the checker must not be sealing well....



the location of the pressure cap varies between the s4/s5 cars... even within the series there are some variances.

Well I am thinking that the pressure tester seals fine cuz when i cap the top line on the radiator it will hold just fine.

I guess I should as if there is supposed to be some kind of one way valve in the overflow bottle cap?

he's sayin replace the radiator cap!! it has a diaphram in it set for 13 psi, the cap it'self will bleed any pressure off over that amount into the overflow tank

no what i mean is the pressure tester must have a rubber seal protruding from the bottom of it like the normal caps piston would have, this is what has to seal against the mating surface down within the neck.



The adapter for the checker is probably sealing fine with the top edge of the neck, but the part protruding that is simulating the piston in the pressure cap is not sealing well enough to keep the pressure from getting to the overflow tube causing the bubbles.



check out the illustration here for a better idea of what i'm talking about:

http://auto.howstuffworks.com/cooling-system7.htm



The red highlighted area in the flash simulation shows where the pressure caps piston seals (and where the checker must seal to prevent pressure from escaping into the overflow tube), that is where I suspect the problem is when checking pressure.