A tale of two anti-siphon vents

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PDQfan
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A tale of two anti-siphon vents

Post by PDQfan »

So we know that our PDQs are equipped with an anti-siphon device because the engines sit below the water line. If we didn't have them, raw water could continue to siphon in after turning off the engine, backing up through the exhaust, and flooding the cylinder heads causing the dreaded hydrolock. These anti-siphon vents are made by Vetus and located at the top of the hose loop inside the access locker near the primary fuel filters.

How do we know if they are working correctly? Water should be flowing out the small weep holes when your engines are on.

My problem has been that the starboard one had a slow dribble, almost like a spit. The port one always flowed with a nice stream.

Making a long story short, I swapped both vent fittings today to try and diagnose the issue, and it was apparent that the starboard one was different and not like the new ones I saw online. Upon inspection, it contained a fitting with a much much smaller diameter outlet than the OEM, molded in, plastic version. I surmise that perhaps on the original install someone put the hose clamp on too tight and broke the plastic nipple so they tried to jury rig a different fitting. Not good. It severely restricted flow and easily got clogged with debris.

Anyhow, if you have restricted flow it's pretty easy to take the Vetus fitting off, unscrew the top, and inspect. Otherwise, you risk hydrolocking your engine over a $75 plastic part.

I would also suggest checking the weep action just prior to stopping your engine. If water is coming out then air will go back in and break the siphon when you turn the engine (and water pump) off. If they aren't flowing then close your seacock before stopping your engine (to prevent a siphon), and then take some time to diagnose.
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Bryan & Lori
"Island Time"
'05 MV 34 #74 100hp
duetto
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Re: A tale of two anti-siphon vents

Post by duetto »

hi,

itwas a long time ago but i believe we were warned to remove the spring in the top of the valve. not sure if they come with the spring and plastic plunger.
john & diane cummings
duetto mv34 #23
James Power
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Re: A tale of two anti-siphon vents

Post by James Power »

Hey PDQfan,

Yes very important to monitor the output on the vented loops, especially after lay up or not running the engines for a while, nice catch. I would guess that the modification was done after delivery and not at the factory.

James
PDQfan
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Re: A tale of two anti-siphon vents

Post by PDQfan »

John,

Mine don't have the valve.

My research tells me that the ones with the valves are more susceptible to clogging from salt crystals and debris and the ones without are more foolproof.

Units with the one way valve might get a drop or two of water occasionally and not the steady stream that we are used to seeing in the valveless units.

This is an excellent video on the concept: https://www.youtube.com/watch?v=Q_8QSdZdJPA

Bryan
Bryan & Lori
"Island Time"
'05 MV 34 #74 100hp
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