Possibly a head gasket problem. Overflowing at idle with the cap off points to the head gaskets. Are there any bubbles in the water? Also, bump the throttle with the cap off and see if the water jumps out of the tank. If so, then almost definitely a head gasket problem. The only other possibility would be a crack somewhere but the fact that it holds 25 psi indefinitely makes it unlikely that it is any thing else. It is also possible that the engine is overheating and blowing coolant out of the cap.

Rene

He should try swithing the cap with one on an engine without this problem. See if the problems moves. I had something similar last summer. Coolant would move to the overflow tank normally when it heated up but when it cooled the coolant didn't not get sucked back into the engine. Eventually my coolant was low in the engine.

Also if it wasn't mentioned, you can buy a automotive coolant system pressure tester to diagnose further.

BT

i have seen bad heat exchangers or coolers go bad and leak coolant into the sea water. no traces beacause of the amount leaking is diluted immediately. i think under a pressure test of each independantly you will find the leak.