Radiator cap too loose and leaking?!


Active Member
Aug 1, 2004
So, I just picked a built car with an unknown aluminum radiator.
We took two brief test rides before I bought it, everything looked good. It did burn outs and such. Cool.

Went out for a 30mi drive yesterday after getting it home, a "getting to know ya, easy drive". After a few easy ramp ups on boost, the temps shot up to 210+.
- It has a front mount and the oem fan, so I didn't worry too much.

After she cooled down, It was very low on coolant. Uh Oh.
It took a gallon of coolant.
Warming the car up, checking for leaks, it started flowing out of the cap, right at the neck. Like huge puddle in two minutes.

OK, I just put a new cap on the Beater ( which has an F-Body Radiator), lets swap it out.
- Same thing.
- No visible defects on the radiator.
So I pulled the cap off of the GN, same results.

I stuck my Snap-On pressure tester on the radiator- fan cycles, doesn't go above 170. No leaks.
- It seems like the inner seal, or plunger, on the Snap-On Tester is longer than all my caps which seals it up.

Is there a radiator cap with a longer "plunger", or am I missing something basic?


Well-Known Member
Feb 11, 2009
I wonder if some head gasket blow by is building pressure in the system. They make a cheap kit at auto stores
you poor into the cooling system and it will turn color if there's a problem.