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symptons of malfunctioning anti siphon valve?

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scott_duerring

Regular Contributor
Can a malfunctioning anti-siphon valve, cause a rough idle issue to occur? Just got done with a floscan install on twin 8.2 MPI Crusaders. My stb. engine has a rough idle, have thrown all but the kitchen sink at it to diagnose and repair. Except fiddling with anti siphon valve. In calibrating floscans the stb engine readout at idle on the gph was erratic, it went away upon increasing rpms under load. The trouble shooting from floscan of this condition suggested one of the causes to be a sticking anti siphon valve. So, that is what made me think that my rough idle may be caused by a screwed up anti siphon valve.
 
Depending upon how it malfunctions, yes...fastest way to see if it is the root of this evil is to remove (or replace) it. you can always use an outboard tank, with fresh gas, as a source to eliminate the onboard tank issues...
 
As you no doubt know, the anti-siphon valve is a check valve, so to speak, and more importantly it acts like a relief valve.
The ball seat tension should allow for fuel pump suction, but should also NOT allow for a gravity siphon should a fuel line become severed below the fuel tank level.

When these become sticky, they may "hammer" under suction (for lack of a better word).
If one was to "hammer", it may throw the flow meter sensor out of what due to the pulsing.

I'm with Mark..... replace them. Easy to do, and they do not cost much.


As for the rough idle issue..... I'm not quite certain how a sticky anti-siphon valve would affect your idle quality.... given that the engine operates OK at higher speeds.


.
 
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How about removing the Floscan sensor? If everything was OK before the floscan install, could be some kind of defect with the sensor.
 
rfdlou said:
How about removing the Floscan sensor? If everything was OK before the floscan install, could be some kind of defect with the sensor.
Click to expand...
If you remove the sensors, you won't be burdened with knowledge of the poor fuel consumption.
Think of how happy you were before you realized how poor it was. :D :D :D
 
RicardoMarine said:
If you remove the sensors, you won't be burdened with knowledge of the poor fuel consumption.
Think of how happy you were before you realized how poor it was. :D :D :D
Click to expand...
It just takes a little longer to find out. When you see a few of these on your credit card statement you will know
06-23-2013
BAHRS MARINA HIGHLANDS NJ $ 718.70
 
RicardoMarine said:
If you remove the sensors, you won't be burdened with knowledge of the poor fuel consumption.
Think of how happy you were before you realized how poor it was. :D :D :D
Click to expand...
HOW TRUE... my first trip with the floscans almost prompted me to put tape over the totalizer readouts... but all it all I think they will be a good thing. Really helps to find out where the "sweet spot" is.
Back to the original question, the idle issue has existed for some time so am sure it is not the floscan sensors. I figure the easiest way to check is to pull the stb fuel line and switch it with the port line hook up if it reaches. If it is the anti siphon valve then it should idle just like the port engine. Which is fine.
 
My first experience with a Flo Scan had a similar result--Holy cow, those suckers are thirsty! In time, and with some experimentation, I've found the sweet spot range: A bit slower but more economical, or a bit faster and less economical. Each has its place.

Jeff
 
do a search on this part number: EVM A/S L-170-1/2

they aren't commonly stocked, at least down here.

Although they could contribute to your idle issue, I'd suspect they aren't likely to....you can always replace them with plain hose barbs (much easier to find) and see what happens then...
 
Thanks for the information. It is worth a try, at least to get better readings on the floscan. I have literally tried everything else to deal with the idle issue apart from getting an entirely new engine, so if there is even a slight chance this may contribute to it, might as well, it will be about the cheapest attempt...
 
I may have lost the thread, but I hope no one is suggesting to replace the anti-siphon valve with a common hose barb. The ant-siphon valve serves an important safety function. And to answer the original question, yes, a sticky anti-siphon valve could possibly affect idle. If you have a mechanical fuel pump, the reduced amount of suction at low RPM may not fully open the check valve if it's gummed up, or it may pulse. If you are going faster, the higher flow rate may alleviate the issue. I would replace them as they cost about $7
 
gardnersf said:
I may have lost the thread, but I hope no one is suggesting to replace the anti-siphon valve with a common hose barb. The ant-siphon valve serves an important safety function. And to answer the original question, yes, a sticky anti-siphon valve could possibly affect idle. If you have a mechanical fuel pump, the reduced amount of suction at low RPM may not fully open the check valve if it's gummed up, or it may pulse. If you are going faster, the higher flow rate may alleviate the issue. I would replace them as they cost about $7
Click to expand...

I don't plan to replace it with a hose barb. I took Mark's suggestion as a test situation only. I do not have a mechanical fuel pump, but the Floscan gph meter at idle is behaving consistent with a sticking anti siphon valve according to their trouble shooting section of the install instructions. Will be going to the boat this weekend, so will try to replace and see what happens. Am going to switch it out with the port side to see if it affects anything.
 
scott_duerring said:
I don't plan to replace it with a hose barb. I took Mark's suggestion as a test situation only. I do not have a mechanical fuel pump, but the Floscan gph meter at idle is behaving consistent with a sticking anti siphon valve according to their trouble shooting section of the install instructions. Will be going to the boat this weekend, so will try to replace and see what happens. Am going to switch it out with the port side to see if it affects anything.
Click to expand...

If you are going to the trouble of removing both AS valves, why not just replace with new and be done with it?
 
Yep, I know I have 1/2 inch supply line, not sure of the the thread side, though. Just need to get to the boat and see I suppose every set up could be different depending on the boat manufacturer. I am thinking it cannot be any less than a 3/8 NPT... but I suppose it could be a 1/2 NPT as well.
 
In terms of the flo-scan issue, could also be a cracked pickup tube or a loose hose clamp...you can always plumb a vacuum gauge in the fuel line, right at the pump's inlet fitting...

I'd be surprised if the pipe fittings are other than 3/8" NPT
 
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