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Old 15-09-2013, 12:41   #1
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Sucking air at the antisiphon valve?

Hello, i have a v6 mtu diesel and a problem with an antisiphon valve which is sucking air. Maybe somebody has any experience with this so I will lay out some details:
There are two exhaust elbows at each side of the engine.
Water is injected at the cooling elbow after which there is a waterlift/muffler. One of these each side.
The exhaust hoses coming from the two waterlifts each make a 5 feet high loop. They are joined by a Y shaped manifold 14 feet aft and there is one exhaust through hull.
The cooling water exits the engine and then there is a y shaped manifold to feed water into the two injection elbows.
Before being injected into the elbow the cooling water makes the same loop as the exhaust, 5 feet up. And here there are two antisiphon valves installed. One for each unit.

The problem is that when i run the engine one of the antisiphon valves is sucking in air and the matching exhaust elbow starts to smoke a little. If i throttle down to 950rpm the problem goes away.

Has anybody ever experienced this problem?

I have checked:
Strainers
Water supplying Y manifold
Muffler on the side of the over heating elbow
First 15 yards of the exhaust hose coming of the muffler/waterlift
Exhaust elbow.

Everything looked clean and there was no carbon buildup anywhere.

Could it be anything to do with compression?
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Old 15-09-2013, 19:42   #2
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Re: Sucking air at the antisiphon valve?

"an antisiphon valve which is sucking air"
They only do that when there is a vacuum in the line.
If I understand you correctly that line is supposed to be under pressure.

"The cooling water exits the engine and then there is a y shaped manifold to feed water into the two injection elbows."
I think I'd check to see if that leg of the Y or the hose between the Y and the anti-syphon valve was blocked.
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Old 16-09-2013, 01:25   #3
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Re: Sucking air at the antisiphon valve?

Yes, the line should be under pressure. I checked if it was blocked. It is free.

I think I checked everything. That is why I thought maybe it has to do with a difference in compression betwern the two exhaust elbows?
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Old 16-09-2013, 03:42   #4
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Re: Sucking air at the antisiphon valve?

Did you try replacing the little rubber "duckbill" valve in the antisiphon?

You can test it by removing it from the antisiphon and blowing thru it in both directions. In one direction you should not be able to blow thru it; in the other, it should blow freely. I do this bimonthly.

There is a way to clean it also (from the Marelon site):

"To clean the duckbill, soak it in white vinegar for a few hours to dissolve salt and calcium deposits. When reassembling, remember not to over-tighten the cap. Replacement may be necessary if the vent cap/duckbill assembly is clogged, deformed or not functioning properly."


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Old 16-09-2013, 06:14   #5
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Re: Sucking air at the antisiphon valve?

Thanks. Yes i did. I actually taped of the valve during running and the tape would get sucked tight.

I am going to check the suction side of the system to see if air comes in anywhere
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Old 03-11-2013, 01:42   #6
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Re: Sucking air at the antisiphon valve?

I have the same problem with the valve.
It used to make a noise so I replaced it with a new one which is silent but still exhibits the same low pressure effect.
The pump pushes the water up to the top of the loop alright but I suspect the water is getting away quicker than it is being supplied.
Perhaps the exhaust injector is creating a venturi effect?

(I get steam in the exhaust above 2100 rpm but not sure that is the cause and am investigating flushing the water passages in the engine.)
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Old 03-11-2013, 07:48   #7
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From the description it's hard to see how there can be a vacuum in the exhaust when the engine is running. However, there could be a restriction in the injection elbow or further down stream in the exhaust system. This might have the effect of sending most of the water to the less restricted side and not properly cooling the restricted side.

Have you pulled the injection elbows to look for restrictions? They can get filled with hard deposits over time.

A mechanic can measure the exhaust back pressure to be sure it doesn't violate the engine maker's specifications. This is a fairly common problem too.
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Old 03-11-2013, 11:05   #8
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Re: Sucking air at the antisiphon valve?

A tight/cracked exhaust valve can pull a vacuum on an engine. It would be like a pulsing. So maybe check the exhaust valve clearance. Well intake too. If it still does it after setting valve clearance, you have a valve job in your future.

In olden days one would check for a burnt exhaust valve on a car by placing a tissue over the end of the tail pipe. You could watch it being sucked in for just a instant every two rotations.
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Old 03-11-2013, 14:40   #9
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Re: Sucking air at the antisiphon valve?

In my case (Yanmar 3GM30) exhaust and water injector new and motor like new (500 hours and top condition).
The loop is 4 ft above the engine however so perhaps there is inertia as it flows down hill?
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