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.
Molded Marelon offers superior strength and corrosion resistance. Marelon is lightweight, corrosion and electrolysis free, ideal for thru-hull valves. Marelon is a modern material formulation of polymer composite compounds that uses resins and additives to produce an extremely strong marine-grade product that will not deteriorate like traditional bronze marine plumbing parts.
An anti siphon valve introduces a small amount of air into the hose whenever a small suction is detected. This breaks the siphon and prevents flooding your boat. There is a tiny rubber "duck bill" valve that is critical to its proper function. If the rubber gets old and stiff it may not function properly putting your boat at risk. 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.
Order the replacement part below under "May we also suggest".