The 5/8" vented loop acts as an anti-siphon valve for engine cooling systems. It is installed in the raw water circuit between the exit from heat exchanger and the inlet to the exhaust riser.
Its purpose is to introduce air at the high point of the raw water cooling circuit when the engine is shut down. This breaks any siphon that might cause sea water to flow into an open engine exhaust port, into the cylinder, and ruin the engine.
The siphon valves are installed as high as possible on the boat to keep them above the waterline.
You should test your anti siphon valves yearly to ensure that they are working properly.
• Plug one end of the "U" and blow into the other side. No air should exit the top of the "U" at the duck bill fitting.
• With one end still plugged, suck on the other end and air should enter the "U" through the duck bill fitting at the top of the "U".
If either of the above tests fail, you should replace your anti siphon valve as it is not functioning properly and can result in a flooded engine or boat.