Hey all, I just got done pressure testing my coolant system, and I think something is wrong. If I pump the tester with everything set up normally, it just makes the coolant overflow bubble. But as soon as I capped the line on the top of the radiator it would hold 15 PSI with no problem.
Is my overflow bottle cap broken?

 

Sounds like the pressure cap is bad.

 

So the cap for the coolant overflow is a pressure cap?
Cuz it almost looks like the underside of it has a notch in it. I will try to post a picture after I get off work.

 

is it a Stant pressure cap or a Mazda cap? Use Mazda cap only cause it sounds like the pressure cap is bad.

 

Which pressure cap are you refering to? I don't have the radiator fill cap on there, because I have a pressure tester attached to the fill neck.

 

bump

 

Well lets see, if you have the pressure tester where one cap should be, and the other cap is not realeasing under 15+ PSI .. at least one problem has to be your existing cap. Switch the pressure tester to the other filler and see what happens to narrow it down.

 

Well, I don't have the adapter to put the tester on the overflow tank. I also don't think there is an adapter for it.
Do you guys think this is something that should keep me from starting the car?

 

Not the overflow tank, their should be a pressure cap on the radiator and another one where the upper radiator hose meets the block.

 

I don't think S5's have both.

 

I have an S4, and there is only one pressure cap, and its where the upper radiator hose meets the block. the only other things on the radiator are the air bleeder, thermo sensor on the bottom, and the radiator fluid level sensor that has a tube that connects to the overflow bottle.
If I plug the line that connects the radiator to the overflow bottle the system holds 15psi just fine, but if I leave that connected to the overflow bottle it just vents the pressure to the overflow bottle.