Yanmar 2GM20 - Cooling-recirculating..?

[Rubikoop]

Sounds like you are describing a vented loop anti siphon valve.

[Sailmonkey]

Originally Posted by Rubikoop

Sounds like you are describing a vented loop anti siphon valve.

Or at least what should be a vented loop. Often the vent is nothing but a little pee hose plumbed into the cockpit drain. If its plugged you should really unplug it and fit either a proper anti siphon valve or send a small stream of water out where you can see it.