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Old 01-03-2017, 02:12 PM   #1
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Fuel crossover valves and running dry?

My boat has twin engines, twin fuel tanks of 300 gallons each. It has two fuel manifolds, one for the fuel delivery, and one for the return to the tanks.

I ran both engines out of fuel this weekend. While running at cruising speed, one engine died, then the other. One tank was empty, the other tank contained more than 200 gallons.

The fuel crossover valves, both supply and return, were open so that both engines could draw from either tank and return to either tank.

My question: How did I suck one tank completely dry?

The valves have not been operated since I've owned the boat, and I've run at least 600 gallons though the engines in the past year.
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Old 01-03-2017, 02:19 PM   #2
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One tank may have a sticking anti syphon valve or a clogged pickup in the tank.


I always run with crossovers closed and each engine returns to the tank it draws from.
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Old 01-03-2017, 02:26 PM   #3
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Liquid takes the path of least resistance. You could have had a slight list, restriction, etc., that favored one tank for return and the other for supply. Boats/ships that use manifolds like you do, monitor their tanks while running. Multi tank vessels like mine use a day tank in or near the engine room for running and transfer from supply tanks as needed.
I don't know what engines you have, but some move much more fuel than they burn. Both to preheat the fuel and better filter. Some like mine also use the fuel passing thru the injectors to cool the tips. My Detroit mains together pump about 75 gallons an hour while using about 8 at cruising speed.
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Old 01-03-2017, 02:31 PM   #4
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All it takes is a little more pressure drop in the plumbing of one tank than the other then all of the fuel will come from the other tank. The pressure drop could be caused by a slightly plugged fuel pickup as noted or just more elbows on one side.

If the valves are left open when the engine is shut down, the two tanks will equalize but it may take a while. You probably cruised for quite a while between shutdowns and that was enough to run one tank dry.

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Old 01-03-2017, 02:44 PM   #5
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I have a similar setup. Two tanks, fuel manifold such that you can select supply and return to either tank, or with all valves open it will draw and return to both.

Well, tanks draw evenly. Most of the time. But on two occasions I starved my engine then checked the tanks- One bone dry, one plenty full.

I think what is going on is air. Some air in the plumbing means the draw or return will favor one tank or the other. My tank taps are on the top of the tanks, so it certainly is plausible for some air to be in there. Not so much an issue if your tanks are drawn from the bottom.

Now new rules for me. Valve engine to one tank or the other. When that tank gets low, switch to other tank. No more drawing from both at the same time.
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Old 01-03-2017, 02:52 PM   #6
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My rule is to return the fuel to the tank from which it is drawn.

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Old 01-03-2017, 03:04 PM   #7
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Well that's a novel rule!!
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Old 01-03-2017, 03:44 PM   #8
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My rule is to return the fuel to the tank from which it is drawn.
I do the same, unless I want to test that the overflow line on the the other tank works properly.
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Old 01-03-2017, 04:00 PM   #9
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Originally Posted by ABfish View Post
My boat has twin engines, twin fuel tanks of 300 gallons each. It has two fuel manifolds, one for the fuel delivery, and one for the return to the tanks.

I ran both engines out of fuel this weekend. While running at cruising speed, one engine died, then the other. One tank was empty, the other tank contained more than 200 gallons.

The fuel crossover valves, both supply and return, were open so that both engines could draw from either tank and return to either tank.

My question: How did I suck one tank completely dry?

The valves have not been operated since I've owned the boat, and I've run at least 600 gallons though the engines in the past year.
AB - I'm not 100% sure if I understand your fuel line setup.

Do your fuel supply and return lines come off the top of your fuel tank? If so then a separate line (and valve(s) is required for a crossover to equalize the level in the tanks.

Leaving both supply and return valves open would not be a dependable method to keep the tanks equalized, unless you had a crossover line connected to the bottom of both tanks.
I'd suggest running off one side at a time or there is a big risk of running dry on one side and/or overflowing on the other side.
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Old 01-03-2017, 04:23 PM   #10
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In heavy trucks with two tanks two pick up tubes and two return lines there is a splitter or flow divider in the return line that equally divides the return flow between the two tanks. It is a common failure point and when it does fail the truck will often run "out" of fuel with one tank nearly full and the other sucking air as the fuel takes the path of least resistance to return. Length of line and number of bends plays a role in this. As an example, if you put a 90 degree fitting in one line it will increase the resistance to flow in that line.
I would run my suction and return from the same tank unless I am moving fuel for trim or equalization reasons.
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Old 01-03-2017, 04:49 PM   #11
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I do the same, unless I want to test that the overflow line on the the other tank works properly.
That happened to me once, unintentionally!
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Old 01-03-2017, 05:00 PM   #12
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Quote:
Originally Posted by ABfish View Post
My boat has twin engines, twin fuel tanks of 300 gallons each. It has two fuel manifolds, one for the fuel delivery, and one for the return to the tanks.

I ran both engines out of fuel this weekend. While running at cruising speed, one engine died, then the other. One tank was empty, the other tank contained more than 200 gallons.

The fuel crossover valves, both supply and return, were open so that both engines could draw from either tank and return to either tank.

My question: How did I suck one tank completely dry?

The valves have not been operated since I've owned the boat, and I've run at least 600 gallons though the engines in the past year.
Can you post an in focus and well lit picture of your manifold?
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Old 01-03-2017, 05:59 PM   #13
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The puzzle for me was how I could have run the boat for one year, including an offshore passage of 80 miles, without encountering a major fuel imbalance. I'm the only person that's been in the engine compartment, and the valves have not been moved since I bought the boat.

Running out of fuel last Saturday 20 miles offshore, with 3 hours of daylight left, was not a pleasant experience. It was also embarrassing, though I felt better that I was able to diagnose the problem and re-crank the engines.

The good news is that I now have a thorough understanding of the fuel valving on my boat!

I don't have pics, but I do have a sketch-
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Old 01-04-2017, 12:07 AM   #14
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The puzzle for me was how I could have run the boat for one year, including an offshore passage of 80 miles, without encountering a major fuel imbalance. I'm the only person that's been in the engine compartment, and the valves have not been moved since I bought the boat.

Running out of fuel last Saturday 20 miles offshore, with 3 hours of daylight left, was not a pleasant experience. It was also embarrassing, though I felt better that I was able to diagnose the problem and re-crank the engines.

The good news is that I now have a thorough understanding of the fuel valving on my boat!

I don't have pics, but I do have a sketch-
Unless your piping comes from the bottom of the tanks, the tank levels will not self equalize.
Piping usually comes from the top of the tank with a dip tube inside. If yours is like that then the only surprise is it didn't happen sooner. As others have already said, better to use one tank at a time.
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Old 01-04-2017, 02:37 AM   #15
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I'd check the breather on the tank that had fuel left in it as well. If the breather was clogged the engines would pull fuel from the other tank. Mud wasps are a common cause of breather clogs.
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Old 01-04-2017, 05:31 AM   #16
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If your tank connections are in the top of your fuel tank, you either have a splitter/flow divider (as Bryan suggested) that has failed, or the system has relied on equal flow restrictions on the fuel lines to keep the tanks balanced. Something has upset that balance.

Perhaps a slightly blocked pickup line on the full tank outlet. It wouldn't take much blockage to give preference to the other tank.
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Old 01-04-2017, 06:34 AM   #17
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I only draw from the starboard tank and transfer fuel with an electric pump from the port tank through a fuel polisher, when there is a noticeable list (2 to 3 days of running). One less thing to worry about when you're underway.

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Old 01-04-2017, 07:26 AM   #18
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My boat has twin engines and twin tanks with a cross over valve/line at the bottom of the tanks. Stb engine draws from and returns to the stb tank. Port engine and gen set draw from and return to the port tank. Fuel gauge sender is on the stb tank, no gauge on port tank.

I've been running with the crossover closed, which is fine, but I'm always guesstimating the fuel level on the port side. I've been thinking of opening the crossover valve. The only reason I haven't is that my boat has a slight list to stb. Thoughts?
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Old 01-04-2017, 09:15 AM   #19
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All of my tank connections are at the top of the tanks. There is no "equalizer" at the bottom of the tanks.

From now on, I'm running with the crossovers closed. The only possible imbalance would come from the generator pulling from one tank.

Lesson learned.
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Old 01-04-2017, 09:17 AM   #20
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My boat has twin engines and twin tanks with a cross over valve/line at the bottom of the tanks. Stb engine draws from and returns to the stb tank. Port engine and gen set draw from and return to the port tank. Fuel gauge sender is on the stb tank, no gauge on port tank.

I've been running with the crossover closed, which is fine, but I'm always guesstimating the fuel level on the port side. I've been thinking of opening the crossover valve. The only reason I haven't is that my boat has a slight list to stb. Thoughts?
I would record the fuel sender level, open the crossover valve, and let it sit over night. Compare the before and after fuel levels to give you an idea of the imbalance in the tanks.

In hindsight, my boat developed a list on Saturday which I could not correct with trim tabs. Obviously it was from the unbalanced fuel levels.
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