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Old 06-29-2020, 08:38 AM   #1
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Fuel tank not balancing

We have a Mainship 390 with 2 Tanks port and starboard. I noticed a considerable list to Port yesterday (Fuel gauge was showing just off full) yesterday we fuelled up for the first time this year, and the starboard tank took $225.00 of diesel and the Port tank only took $50.00 of Diesel. both valves are open on the Equalization line at the bottom of the tanks and both Valves are open to the Racor, . I know the generator draws from the Starboard tank but we have only run it at Launch for about 5 minuets - any thought's
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Old 06-29-2020, 10:16 AM   #2
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I have this problem too though to a lesser extent. There are ganged valves that will switch the source between tanks, they have two circuits because you must switch the fuel return at the same time or you will be pumping spill fuel from one tank to the other until it overflows. As long as the genset returns to the tank it is drawing from, it isn't an issue.
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Old 06-29-2020, 11:22 AM   #3
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Do you have twins or a single?

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Old 06-29-2020, 11:37 AM   #4
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Had same issue last year, turns out the port side fuel filter was clogged. Once i replaced it boat was fine.
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Old 06-29-2020, 02:58 PM   #5
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Do you have twins or a single?

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Old 06-29-2020, 02:59 PM   #6
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Had same issue last year, turns out the port side fuel filter was clogged. Once i replaced it boat was fine.
Only has one set of filters for both tanks going to a single engine
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Old 06-29-2020, 03:06 PM   #7
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If both balancing valves are open and the tanks aren't balancing out that is the place to start. The supply/return valves shouldn't affect this.

Can you confirm the fuel level in the each tank?

What size is your crossover line?

Double check that both valves are open 100% and it's not an issue with the handle or stem.

If your crossover line is actually blocked I would be concerned that something is in the tank/line and will likely block the fuel feed to the engine at some point.

However it is more likely that there is a valve not set properly or there's another valve in the crossover link that has been closed. Maybe by a helpful mechanic in the offseason?

Can you post pictures of the valves?
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Old 06-29-2020, 03:19 PM   #8
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If both balancing valves are open and the tanks aren't balancing out that is the place to start. The supply/return valves shouldn't affect this.

Can you confirm the fuel level in the each tank?

What size is your crossover line?

Double check that both valves are open 100% and it's not an issue with the handle or stem.

If your crossover line is actually blocked I would be concerned that something is in the tank/line and will likely block the fuel feed to the engine at some point.

However it is more likely that there is a valve not set properly or there's another valve in the crossover link that has been closed. Maybe by a helpful mechanic in the offseason?

Can you post pictures of the valves?
Both are like this
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Old 06-29-2020, 04:59 PM   #9
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Possibly the fuel return is going to a different tank than the fuel supply is coming from. With P&S tanks feeding a single engine, the flow often favors one side. The balancing line is too small or maybe it is partially plugged to keep them even.

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Old 06-29-2020, 05:04 PM   #10
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What we are seeing is the shut off for the tank. There could very likely be a cross over valve some where else. I would follow the port fuel line all the way to the engine and then I would follow the return line all the way back to the tank. I suspect you will find more valves.
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Old 06-29-2020, 06:38 PM   #11
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What we are seeing is the shut off for the tank. There could very likely be a cross over valve some where else. I would follow the port fuel line all the way to the engine and then I would follow the return line all the way back to the tank. I suspect you will find more valves.
No this is the cross over - the shut off for the tanks is at the other end at the top of the tank close to the Racor, when I am changing the Racor I do not touch the valve shown just the one at the top on each tank that goes directly to the Racor
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Old 06-29-2020, 06:39 PM   #12
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What we are seeing is the shut off for the tank. There could very likely be a cross over valve some where else. I would follow the port fuel line all the way to the engine and then I would follow the return line all the way back to the tank. I suspect you will find more valves.
This line goes directly to the other tank with the same shut off at the other tank, Sorry the Picture for some reason is back to front
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Old 06-29-2020, 08:50 PM   #13
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How much is the boat listing? Maybe you need some ballast to even things out. If you list too much to one side the fuel will flow to that tank. Was your boat level at the time?
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Old 06-29-2020, 10:44 PM   #14
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Follow the fuel lines from the tanks to the engine and then the return back to the tank. There are more valves some where.
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Old 06-30-2020, 07:38 AM   #15
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How much is the boat listing? Maybe you need some ballast to even things out. If you list too much to one side the fuel will flow to that tank. Was your boat level at the time?
You know I think you have nailed it the boat lists to port slightly when both tanks are full, (we took the couch out which was a lot heavier), and all the Galley cupboards are on the port side, and probably as I run the engine it is drawing from both tanks and as they go down the starboard tank empties into the port tank when the boat is at rest, and increases the list, (Thus the Transfer) I am going to shut off the starboard shut off and see what fuel I put in each tank next fill up. I
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Old 06-30-2020, 09:03 AM   #16
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Might also look into the water tanks to see if they are affecting the list. Not sure how many or where they are. I know our 1985 34 trawler lists to starboard when the water tanks are empty. Batteries are on the starboard side.
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Old 06-30-2020, 09:13 AM   #17
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Responded to your other thread. Have you checked to see if the vent line to the tank that is not emptying is plugged?
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Old 06-30-2020, 09:16 AM   #18
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(we took the couch out which was a lot heavier),
New information...different solution! Makes sense. Too many suggestions without knowing the whole story!
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Old 06-30-2020, 11:42 AM   #19
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You know I think you have nailed it the boat lists to port slightly when both tanks are full, (we took the couch out which was a lot heavier), and all the Galley cupboards are on the port side, and probably as I run the engine it is drawing from both tanks and as they go down the starboard tank empties into the port tank when the boat is at rest, and increases the list, (Thus the Transfer) I am going to shut off the starboard shut off and see what fuel I put in each tank next fill up. I
Not sure if you are doing this but that crossover line is required to be closed when underway.

Also I would advise against running from both tanks at the same time with both returns open. Any flow issues and you could actually transfer all the fuel to one tank creating a very unbalanced boat. And maybe even a fuel spill. You should probably alternate every couple of hours.

BTW extra beer is always good portable ballast.
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Old 06-30-2020, 07:59 PM   #20
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Similar problem on my Willard 36 with single Perkins 4.236 75hp and twin saddle tanks of 225 gals each, though to a lessor degree than OP. For me, boat needs to be re-trimmed - no clogs or otherwise. Over the years, she got a bit heavy on port side, which caused more fuel to flow to port side, and trim gets worse until she has 3-4 degree of trim at rest. Not much but annoying. When she's done with her refit, I'll put 150 gals in each tank then use movable lead ballast to trim her to zero degrees trim. I consider this normal after 50 years and a major refit.

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