Changed the stock rad cap to a J's Racing 1.3 one, and next day or so of driving I see coolant spills all over the parkinglot. At first I get scared seeing green puddle on the ground, wondering if i cracked the rad or something, and I notice the overflow tank and its FULL. Full as in waaaay above past the MAX line and overflowing through the lid of the reservoir.

Does this make sense at all??

 

only two things I can think of, first the cap is defective or not totally sealed when installed. Secondly did you fill the overflow tank recently when cold ?.

 

did you flush your coolent or anything and not bleed it?

maybe the radiator cap is a defect like jfusuion said then
air bubbles formed....

 

Swap back to OEM and see if the problem continues.

 

I never touched the coolant prior to adding the cap. Just went over to the car, switched caps. Next morning after a short drive to eat... I came out to see there was green fluid all over the ground and dripping.

Now my coolant is still over the MAX line but not that close to the lid anymore. STill didnt touch it. I havent had any problems after that initial spillover, and I didnt have to add any fluid in...

im so confused... its like coolant magically appeared...

 

You definitely want to change that cap back.

What I suspect is happening is that the cap is defective (or installed improperly) and the coolant boiled over, into the overflow. Pop the cap again and see if you can visibly see coolant in the radiator. If you can't go through the bleed process and fix the cap.

The cap holds pressure higher than 1 atmosphere so that the boiling point is increased. (in case you didnt know). When the cap leaks/is installed incorrectly the fluid might boil, which would push fluid out of the radiator into the overflow.