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T fitting in cooling line before mixing elbow.

GMC

Member II
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
 
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Loren Beach

O34 - Portland, OR
Senior Moderator
Blogs Author
Siphon Break?

If your boat had a Universal diesel, this would be the siphon break fitting, from the description. Those lead to a separate discharge fitting on the transom of sometimes even T-ed into the inside of the metal exhaust port on the transom.

The one on our boat is pictured elsewhere on the site.

I have never noticed one fitted on a Yanmar, and have seen so few Volvo diesel installations that I am unusually clueless...
:rolleyes:

Can you attach a photo of the part?

Loren
 

GMC

Member II
Think I've got it.

Loren,
I think you are right re: anti syphon. Found an article by Jerry Powlas and Dave Gerr from Good Old Boat. "In saltwater service, the siphon break valve may become clogged with salt crystals and either become inoperative (not break the siphon) or leak constantly. The constant leak failure mode can result in spraying seawater around . . . This seawater is needed to cool the exhaust. For these reasons, some authorities recommend dispensing with the siphon break valve and locating a tee in the line vented higher up, such as in the cockpit . . ."
That seems like what I've got (although not in cockpit). I will fashion a tee and put it back into the system. Loren, I had no idea what anti syphon was until you mentioned it, which led to this article, so you saved the day. Thanks,
Greg
 

mherrcat

Contributing Partner
Whatever the device, siphon break valve or "T" fitting, it should be located above the waterline.
 
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