Fuel tanks not ballanced

We just returned from a long weekend and I have noticed that my starboard fuel tank is showing half empty while the port is showing full. The starboard talk has always drained a bit faster but this is more pronounced and has caused a slight list.

Valves on both fuel lines are open and the switch is set to both so the flow should be somewhat even. I was told it could be a clogged fuel vent or bad filter which I will check out. Any one else have this issue? Thoughts on how to fix.

Thanks

Assuming that you are using diesel, I would look at how your return lines are set up. I often balance our tanks by diverting the return to the low tank. Also assume that you don't have or aren't using a balancing line between the tanks.

Conrad said:

Assuming that you are using diesel, I would look at how your return lines are set up. I often balance our tanks by diverting the return to the low tank. Also assume that you don't have or aren't using a balancing line between the tanks.

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+1. Check that the returns aren’t both going to one tank. If they are you may spill diesel out through the vent.

Does the generator also pull from the lower tank ?

The generator does pull from the low tank, but is have noticed this issue after going out for the day. Will check the return lines when I get down there on Friday. I can always just fill the low tank more often but want to avoid stops to fuel dock if possible.

Lollygag1 said:

We just returned from a long weekend and I have noticed that my starboard fuel tank is showing half empty while the port is showing full. The starboard talk has always drained a bit faster but this is more pronounced and has caused a slight list.

Valves on both fuel lines are open and the switch is set to both so the flow should be somewhat even. I was told it could be a clogged fuel vent or bad filter which I will check out. Any one else have this issue? Thoughts on how to fix.

Thanks

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Disclaimer: I know less than nothing about all things mechanical.


I would see how the boat runs on just the Port tank. Can you make full power on just the port tank vs the starboard tank?

Another thought. If you are using your dash tank gauges, one or both may be faulty. I ran into this a while back and the cause and remedy was simple. When the gauges were calibrated by the builder, once the calibration was done they put a drop of glue on the adjustment screw to keep the calibration true. Over the years the glue came adrift and my starboard tank was reading quarter full when it was in fact empty.
Proper sight gauges rule if you can get them.

I am also planning to check the gauges but last time I topped the tanks the starboard one took 40% more fuel. In answer to the other question I will test running just the port tank this weekend.

Might be a plugged vent line as well.

I am planning to have the vent line checked too. Thanks

Check to see if the value between the 2 tanks is open.

Comodave said:

+1. Check that the returns aren’t both going to one tank. If they are you may spill diesel out through the vent.

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Yep check those returns. Before I completely understood all my five fuel tank systems , I completely emptied one tank and overfilled another with the return line.

This is a fairly recent issue so don't think it is a valve issue.

Lollygag1 said:

We just returned from a long weekend and I have noticed that my starboard fuel tank is showing half empty while the port is showing full. The starboard talk has always drained a bit faster but this is more pronounced and has caused a slight list.

Valves on both fuel lines are open and the switch is set to both so the flow should be somewhat even. I was told it could be a clogged fuel vent or bad filter which I will check out. Any one else have this issue? Thoughts on how to fix.

Thanks

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Had the same problem. It was a plugged vent line. Critters blocked the small exit holes. The problem was so acute that when I opened the deck fill there was a rush of air to relieve the vacuum in the tank on which the vent was plugged. Of course, much less fuel was being drawn from it.

catalinajack said:

Had the same problem. It was a plugged vent line. Critters blocked the small exit holes. The problem was so acute that when I opened the deck fill there was a rush of air to relieve the vacuum in the tank on which the vent was plugged. Of course, much less fuel was being drawn from it.

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Same thing happened to us. I was doing a lazarette check on a long run when I noticed the different levels in the tanks. Good thing I noticed, or we might have "run out of fuel" it the plugged line had not resolved itself by the time the other tanks emptied out.

I'd suggest checking the least complex theory and then work towards the most complex.

I had similar issue with my MS390. Since I have a single engine, my filters are installed closer to Starboard tank (with a long hose to port tank). this means that the pump will more likely pull from the starboard if there is small contaminant in the port hose. to solve my issue the easiest way, I closed the starboard valve (to force the pump to pull from port tank only ) then started the engine. it took few second then I have noticed some black crap in the bottom of my filter-1 that was in use. replaced the filter and run to boat on both tanks without any issue since. I do have my cross over line and both upper valves open all time.
Regards,

One more possibility. I once ran a tank dry while running on both. I looked at everything suggested above including changing my hoses. I even pulled the fuel pick up out of the tank to check if the screen at the bottom was plugged. No screen.

The problem took me about a year to find. I eventually pulled the pickup tube again and took a closer look. I discovered that the tube was threaded into the fitting, not soldered as I had assumed.

I unscrewed the tube and there was the screen, full of little black chunks.

I threw that damn screen as far as I could, pulled the one from the other tank and threw it too!

I want the dirt to be caught by my Racor, not somewhere in the bowels of the boat where it is a major job to get at it.

We had that problem as well...where the boat naturally listed to one side and the fuel tanks did not return to the correct locations....Make sure your boat is nice and level and then open all the lines so that the fuel naturally settles in their tanks and adjust any counter balance weights to handle the rest. We typically run with all of our tanks open, bow to stern, port to starboard and let. We used lead weights to redistribute the balance as over the years, past owners made additions and subtractions to the boat which changed everything around. Nice and level now.

Thanks all.

Vent lines- clear
Gauges- working fine
Valves - open
Next step is replacing the port fuel filter. Hopefully done tomorrow while i am still working so i can pay the mechanic.
If that does not work we will have to check the lines and other messy work.

FYI- gave the mechanic a bottle of olive oil hand pressed by a friend in Italy as an incentive to make this a priority over my dock mates pre-holiday requests.

dhays said:

Disclaimer: I know less than nothing about all things mechanical.


I would see how the boat runs on just the Port tank. Can you make full power on just the port tank vs the starboard tank?

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The main takaway should be that if your valves, for some reason, are set to draw from one tank and return to the other, you are in danger of spilling fuel without knowing it. Better to use one tank at a time (draw & return) then switching to the other tank at predetermined intervals to keep things even.

This was not an issue in the past it seems to have cropped up this season which makes me think there is something blocking the fuel flow. When we filled up yesterday the starboard tank had used 4x more fuel than the port. We are not going far so at this point it is an inconvenience but would like to get it resolved.

If you have a small compressor, you could try blowing back through the lines toward the tanks. You would need to remove the filters and make sure the valves are open on the lines you are blowing through. If one is plugged or partially obstructed, you'll know right away.

While it was never explicitly stated, lollygag's comments make me think this is a twin engine boat. That so? Anyway, this is intended to cover both cases.
If so, why are you running the boat with a common fuel system ("the switch was set on both.") supplying both engines? You run a lot less chance of being left powerless if the fuel sources are independent. In this configuration, return fuel should run from each engine back to the tank it is supplied from.
Also, the problem with one tank apparently not providing fuel would have been immediately identified when the engine it supplied died.
If you are a single engine boat, like my similar 30 Pilot, the fuel balancing hose runs between the two tank fuel supply valves. Shut off both valves and disconnect and remove the balancing hose and blow it down to assure it is clear. Now get a suitable container and place its open end at the valve of the high level tank and open the valve to find out whether or not it is clogged. If you truly have an obstruction it will reveal itself doing all this. No mechanic help necessary.
Also, if you have a single engine, it is entirely possible that fuel returns to only one tank - mine used to. It was about an hour long job for me to install a return hose to the other tank since the plugged fitting was already there. Another few minutes was required to install the two valves directing return fuel to either or both tanks.

A genset usually draws from one tank and returns to another.

My generator draws from and returns to the same tank.

Thanks for the advice Frolic, we have a single engine. The generator pulls and returns to the same tank. We did run this approx 10hrs over 4 days but have noticed this at times when we did not run the gen. We will be running many day trips over the next week so will see if the current fixes are helped if not will continue the troubleshooting that you have suggested.

Told my wife we could solve the problem by upgrading to a larger boat but she was not overly receptive.

rgano said:

My generator draws from and returns to the same tank.

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Mine too.

Lollygag1 said:

Thanks for the advice Frolic, we have a single engine. The generator pulls and returns to the same tank. We did run this approx 10hrs over 4 days but have noticed this at times when we did not run the gen. We will be running many day trips over the next week so will see if the current fixes are helped if not will continue the troubleshooting that you have suggested.

Told my wife we could solve the problem by upgrading to a larger boat but she was not overly receptive.

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So have you run the main engine with the low side tank valve shut off? If your main engine return is to the high side tank, you could end up dumping fuel out the air vent if you run this way too long, but if there is a bit of room left in the high side tank, you can get away with it for a few minutes. My 315 Yanmar returns about 11 gallons per hour at 2900 RPM.

My wife (she who does NOT do boat maintenance) wants our 42-foot Grand Banks back. I am liking simpler, although not much, and smaller and faster.

It would be helpful to understand how your fuel system is plumbed. There is lots of possibly good advice so far, but it all seems to be based on assumptions about how your fuel system is set up.


Is it a single or twin engine boat?


How is fuel drawn and returned?


Is there a balancing line between the tanks?


You mention a "switch". What's that and what does it do?


A sketch would be best.

Success, we changed the fuel filters and it appears the problem is resolved. Will know more once we make a significant run but we did 9hrs this weekend and the fuel draw was even. Will provide pics and a sketch if problem persists.

Thanks all