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Old 10-29-2019, 02:51 PM   #1
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Fuel sight gauge issue

I have two 62 gallon fuel tanks on my starboard side in engine room. They are plumbed together with valves to one sight tube. The top of sight tube is tied in at the top of one fuel tank in the same plumbing as my return line. I know it’s funky right?
There is a valve at the bottom of site tube. After shutting engine off it takes at least overnight for the fuel to reach what I think is the right level in sight tube. It does this even if I shut all the valves for the sight tube while running. As soon as I shut engine off and open valves to sight tube all fuel is sucked out of sight tube and takes what seems like forever to fill the tube back up.
Is there anything I can do to speed this up , short of adding a separate bulkhead in tank for the top of sight tube? I’m guessing it’s taking so long because air trapped in top of sight tube .
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Old 10-29-2019, 03:16 PM   #2
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Unless these are EPA gas tanks, they should be vented to atmosphere at the top. That is, with a overboard vent that is not plugged.
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Old 10-29-2019, 03:22 PM   #3
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Unless these are EPA gas tanks, they should be vented to atmosphere at the top. That is, with a overboard vent that is not plugged.
Diesel tanks ,vented at top and vents are open .
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Old 10-29-2019, 03:26 PM   #4
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I have two 62 gallon fuel tanks on my starboard side in engine room. They are plumbed together with valves to one sight tube. The top of sight tube is tied in at the top of one fuel tank in the same plumbing as my return line. I know itís funky right?
There is a valve at the bottom of site tube. After shutting engine off it takes at least overnight for the fuel to reach what I think is the right level in sight tube. It does this even if I shut all the valves for the sight tube while running. As soon as I shut engine off and open valves to sight tube all fuel is sucked out of sight tube and takes what seems like forever to fill the tube back up.
Is there anything I can do to speed this up , short of adding a separate bulkhead in tank for the top of sight tube? Iím guessing itís taking so long because air trapped in top of sight tube .
It appears you have a covered slip. Take the fuel caps off and see if it speeds up the process. If it does, suspect the overboard vents.
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Old 10-29-2019, 03:27 PM   #5
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Have you tried to disconnect only the top of the site tube? This seems to be indicating an unexpected plugging or valving in the system. Letting the top go free will divide the problem in half.
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Old 10-29-2019, 03:40 PM   #6
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It appears you have a covered slip. Take the fuel caps off and see if it speeds up the process. If it does, suspect the overboard vents.
Thanks ,I'll try that . A few weeks ago I had a tank go out ( not one of these). I was scrambling to get the fuel in that tank transferred to other tanks. In the process I slightly overfilled these tanks to the point of seeping out of the vent. I don't think the vents are clogged but maybe.
The tygon site tube has a 90 degree turn in it where it ties into the return. It has a slight crimp in tube that could be the culprit.
I guess I'm overthinking the fact that it being tied into my return has something to do with it.
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Old 10-29-2019, 03:41 PM   #7
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Have you tried to disconnect only the top of the site tube? This seems to be indicating an unexpected plugging or valving in the system. Letting the top go free will divide the problem in half.
Thanks I'll try that. See post #5
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Old 10-29-2019, 06:57 PM   #8
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I’m pretty sure now that this kink is the culprit . Sunday when we left the boat the level was 10” lower than where the level is now marked with yellow tape.

I’m going to hard plumb from the tank to the tubing including a hard ninety and replace tubing with straight piece. The tubing is no longer soft and that little kink has taken set .

Thanks guys . I think I was too focused on the tube being plumbed to my return that I over looked the blockage. I have never seen a sight tube plumbed that way .
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Old 10-29-2019, 10:59 PM   #9
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Originally Posted by Pack Mule View Post
Iím pretty sure now that this kink is the culprit . Sunday when we left the boat the level was 10Ē lower than where the level is now marked with yellow tape.

Iím going to hard plumb from the tank to the tubing including a hard ninety and replace tubing with straight piece. The tubing is no longer soft and that little kink has taken set .

Thanks guys . I think I was too focused on the tube being plumbed to my return that I over looked the blockage. I have never seen a sight tube plumbed that way .
I think you found your problem Marty
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Old 10-30-2019, 04:11 AM   #10
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I don't see the slight kink as a problem since the part at the top is air... it may have resistance but not sealed (at least from the photo). The problem is the top tube venting. If it were me, I would put a valve at the top of the sight tube and when you open the bottom valve, open the top one, and it will follow whatever tank level you have it switched to. Venting to one tank and sensing another is screwed up.

Another option would be to tee the two tank vents into the sight tube, so they share the vent and the tube.
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Old 10-30-2019, 10:52 AM   #11
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So nobody is mentioning a plastic hose for a sight tube ???

God forbid, a fire would melt that and feed the fire....

It looks like it's T'ed into the black hose and there appears to be a dip in that one.
Might have fuel collecting there blocking the air.
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Old 10-30-2019, 11:23 AM   #12
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So nobody is mentioning a plastic hose for a sight tube ???

God forbid, a fire would melt that and feed the fire....

It looks like it's T'ed into the black hose and there appears to be a dip in that one.
Might have fuel collecting there blocking the air.
The proper usage is to open the valves on the tank to the sight gauge tube and read the level, then close it back. The amount of fuel in the sight gauge tube is minimal.

Fire is the reason why you have a valve closing it off, at least on the bottom, preferably on the top too. It can't have spring closed dead-man valves to meet ABYC, from what I recall. There are some options for alternates to vinyl tubing but they have a risk of being broken since they are glass. for some reason, borate glass comes to mind here...
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Old 10-30-2019, 11:36 AM   #13
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Thanks for all the help guys. I just ordered some tygon tubing for sight gauge,3/8” id x 1/2” od. Thanks stubones I will add another valve to the top.
I turn all valves off after checking fuel level. The time it’s takes for checking level is my main reason for addressing this issue. I hope this will speed up the time it takes diesel to enter sight gauge.
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Old 10-30-2019, 12:47 PM   #14
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Years ago, I looked at a trawler that used plastic tubing for a sight gauge. To protect the tubing, the owner had snapped the tubing into the channel of a piece of shelf standard channel that was cut fit the tube snugly, twisted the channel around the back and leave the channel open so you could easily see the tube, but protecting the tube. The owner was worried about something sliding around in rough seas and hitting the bare tube, knocking it off and causing a fire hazard. It seemed to be a robust solution to his problem.

Something like this: https://www.amazon.com/Sterling-FAST.../dp/B0009SUWDI
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Old 10-31-2019, 11:36 AM   #15
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Just got this in today from McMaster Carr. Semi-clear soft Tygon tubing for fuels and lubricants. Hope to get it done this weekend.
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Old 11-01-2019, 07:39 AM   #16
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"I hope this will speed up the time it takes diesel to enter sight gauge."

If it is not the cure , I would get a pail and observe how fast different locations feed the system, it still could be a chunk of gunk stuck in a fitting, or a partially plugged valve.
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Old 11-01-2019, 08:16 AM   #17
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"I hope this will speed up the time it takes diesel to enter sight gauge."

If it is not the cure , I would get a pail and observe how fast different locations feed the system, it still could be a chunk of gunk stuck in a fitting, or a partially plugged valve.
I suspect it was plumbed fine until someone added the wooden frame on the tank top.
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Old 11-01-2019, 09:10 AM   #18
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Interesting use of the Tygon tube. I have 2 tanks down aft under the cockpit 500GPH. Sounds the perfect visual device. Many years ago I found some 3/8" red floating balls I dropped one in each tube in a previous boat too enable easier seeing of the depth of fuel. For the life of me I cannot seem to find any info on them. Has any one any idea where I could get them or similar?
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Old 11-01-2019, 10:58 AM   #19
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Interesting use of the Tygon tube. I have 2 tanks down aft under the cockpit 500GPH. Sounds the perfect visual device. Many years ago I found some 3/8" red floating balls I dropped one in each tube in a previous boat too enable easier seeing of the depth of fuel. For the life of me I cannot seem to find any info on them. Has any one any idea where I could get them or similar?
That sounds like a perfect set up but my vote goes to glass tubes.
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Old 11-01-2019, 11:03 AM   #20
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https://mpparts.com/part/10130187-re...-ball-10130187
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