Leaking Rad Caps

Hey all,

I was pressure testing my coolant system and found that my rad cap was not holding pressure. It bled out through the overflow almost immediately as I was adding pressure. I bought a new 1.4 bar cap and it’s doing the same thing. Anything above 5 PSI (.34 Bar) or so and it purges.

The pressure tester I made just tees into the coolant line above the thermostat and has a schrader valve on it so it’s easy to pressurize with compressed air. I used a tire filler with a gauge so I could avoid over-pressurizing it. I measured the residual pressure with a 0-15psi gauge so I don’t think it’s my measuring that’s out. As far as I can tell it never held pressure on the trails either, this bike is constantly bubbling into the overflow.

What’s going on??? The new cap leaks. The old cap leaks and it was only a year or so old. The filler neck looks OK, it feels smooth inside. Does the rad cap need to be hot to seal? Is the cap meant to be water-tight but not air-tight?

-Kent

I found my old, old cap and compared. They are not the same...

Old one on left:


The old cap holds pressure OK. Both new caps, PN 58035016000 from the parts fiche, do not. Depth and diameter measured the same. Not sure about spring pressure.

Is there a different PN I should be using to get the right part?

-Kent

Sure, thanks, Taffy. I always appreciate it when people respond to my questions in an effort to help me out.

But, I’m not sure what you’re getting so excited about, your diagnosis wasn’t exactly spot on. You made some suggestions, but if you read my post the problem appears to come from a difference in cap designs. The old cap works OK and the new caps don’t, despite being in good condition and the correct OEM part.

Cheers,

-Kent