T vent fitting in hot water line before mixing elbow.

[GMC]

Ericson 25 +, Volvo MD5B - which has run hotter than expected last few years. Two weekends ago, it got very hot and engine reacted by bogging down and I nursed it home. After working on it this last weekend I confirmed what I should have diagnosed before and that is that although some water was moving through system, it was not enough. One of the hoses had broken and was spraying sea water in engine compartment and that area where it broke was kinked. Anyway, I re-fitted and re-positioned hoses (checked impeller and such) and came across something that I don't understand. In the flexible hose that finally leaves the engine to go to mixing elbow is a metal t type fitting (midway down the hose) about 3-4 inches long that has a fitting that attaches a small diameter vent or relief tube that runs to a vent outlet outboard at the transom. The fitting doesn't look stock and since it fits into the hot water hose with a right angle and exits with a right angle, it seems as if it would restrict flow. I don't understand why the vent tube would be necessary in this location as there is relief coming 6 inches away at the mixing elbow. I took the fitting out and am running a unbroken hose from engine to mixing elbow. I am getting a much cooler engine and great water flow. Can anyone tell me what this fitting that I've taken out was for and more importantly, do I need it in there? Thank you very much.
Greg

[mitiempo]

It is there to act as an anti siphon which prevents water backing up into the engine. Sometimes it exits in the cockpit well as a visible telltale that cooling water is flowing. I would keep it. The diagram below is from Yanmar.

[GMC]

Thanks. Makes sense.