Radiator caps

[340Dart]

What is the difference between a vented and non vented cap. I bought a chrome cap and it wouldn't work. When the pressure got up it started leaking under the cap. I thought it was the stock radiator. But when I bought my aluminum rad it did the same thing. I put on the vented stock cap and it stops. It did this with other chrome caps so it wasn't a bad cap. They said they come non vented. I have a catch can but it leaks from under the cap. Thanks for any info to help me understand this.

Fred B

 

[340mopar]

All radiator caps are vented but the some caps have a gasket that seals to the outer ring of the radiator neck so the cap is sealed to the vent hose and catch tank. When the water gets hot it expanse so it goes into the catch tank, when the water cools it retracts so because of the gasket on the outer ring it can draw the water from the catch tank so the radiator always stays full. The chrome cap you have must not have he outer gasket so it leaks around the edge.

Chuck