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Old 21-05-2018, 21:05   #1
er9
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Does diesel fuel return line have to be above fuel tank?

want to relocate fuel tank at some point in the future and have been wondering about this recently.

currently fuel tank is under starboard quarter berth and fuel return line (universal 5432) is mostly above the level of the fuel tank but its only 24" distance from engine to tank. If i were to relocate the tank 10' away under the cabin settee would i have to keep the return line above the level of the fuel tank? is 10' too far a distance for an engine to return un-used fuel back to the tank?

also while im at it...if i decided to add a second fule tank...do i have to have a return line to both tanks or just one? does it really matter?
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Old 21-05-2018, 21:22   #2
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Re: Does diesel fuel return line have to be above fuel tank?

Second question first. If you have to fuel tanks and a valve to switch with you to the two fuel tanks you must have a valve on the return line. If you omit the return line valve then fuel coming from tank A is being returned tag to tank B and tank B will overflow while tank A will empty rapidly.

The ratio of fuel pump into the engine to fuel returned to the tank varies by Engine quite a bit. But you can roughly guess that twice as much fuel enters the engine as the engine needs and the rest is returned to the tank. Fuel must be returned to the same Tank unless the two tanks are at the same level and no valve is used thus share common fuel level.

Exception is if one tag is used only to fill the other tank. In this case the fuel is drawn and return to one tank only.

On most engines fuel is returned to the tank using the excess pressure in the injectors. If the return line goes to a tank above the injectors there will always be some fuel pressure on the injectors and depending on the exact design they May bleed fuel into the cylinders causing severe engine damage. You might try a check valve to keep the fuel from bleeding back into the engine?
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Old 21-05-2018, 21:36   #3
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Re: Does diesel fuel return line have to be above fuel tank?

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Originally Posted by captstu View Post
Second question first. If you have to fuel tanks and a valve to switch with you to the two fuel tanks you must have a valve on the return line. If you omit the return line valve then fuel coming from tank A is being returned tag to tank B and tank B will overflow while tank A will empty rapidly.

The ratio of fuel pump into the engine to fuel returned to the tank varies by Engine quite a bit. But you can roughly guess that twice as much fuel enters the engine as the engine needs and the rest is returned to the tank. Fuel must be returned to the same Tank unless the two tanks are at the same level and no valve is used thus share common fuel level.

Exception is if one tag is used only to fill the other tank. In this case the fuel is drawn and return to one tank only.

On most engines fuel is returned to the tank using the excess pressure in the injectors. If the return line goes to a tank above the injectors there will always be some fuel pressure on the injectors and depending on the exact design they May bleed fuel into the cylinders causing severe engine damage. You might try a check valve to keep the fuel from bleeding back into the engine?
thank you for the great info. if i use two tanks...only one tank will feed the engine...the other will only be used to fill this tank when it gets low via some sort of electric pump or a simple valve setup.the new tank would be lower than the injectors so i'm assuming i wouldnt need a check valve?
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Old 22-05-2018, 05:59   #4
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Re: Does diesel fuel return line have to be above fuel tank?

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thank you for the great info. if i use two tanks...only one tank will feed the engine...the other will only be used to fill this tank when it gets low via some sort of electric pump or a simple valve setup.the new tank would be lower than the injectors so i'm assuming i wouldnt need a check valve?
What engine do you have? Maybe others with same can chime in on their configuration. My tanks are higher than my engines and its never been an issue. No check valve in line. Volvo MD2020s.
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Old 22-05-2018, 06:13   #5
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Re: Does diesel fuel return line have to be above fuel tank?

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What engine do you have? Maybe others with same can chime in on their configuration. My tanks are higher than my engines and its never been an issue. No check valve in line. Volvo MD2020s.
Thats good, they should be higher and feed by gravity, otherwise if the engine sucks fuel from below, you leave yourself open to a whole lot more possible senarios as to why your engine stops or ends up with air in the fuel system.
Thats why properly designed day tanks feed by gravity.
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Old 22-05-2018, 06:35   #6
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Re: Does diesel fuel return line have to be above fuel tank?

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Thats good, they should be higher and feed by gravity, otherwise if the engine sucks fuel from below, you leave yourself open to a whole lot more possible senarios as to why your engine stops or ends up with air in the fuel system.
Thats why properly designed day tanks feed by gravity.
Except....mine have pickup tubes on the draw side. Though in effect it works like a siphon, once fuel gets drawn up the tube and over to the downhill side it takes nothing to maintain the flow.
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Old 22-05-2018, 06:36   #7
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Re: Does diesel fuel return line have to be above fuel tank?

If the return goes into a rank at the top. No check valve is necessary. You should be ok as you describe.

What not use the existing tank as is and pump the second tank fuel into the existing as needed? Less new plumbing.
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Old 22-05-2018, 09:53   #8
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Re: Does diesel fuel return line have to be above fuel tank?

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Except....mine have pickup tubes on the draw side. Though in effect it works like a siphon, once fuel gets drawn up the tube and over to the downhill side it takes nothing to maintain the flow.
mine is currently the same setup except tank isnt above engine. the top of the tank is just about level with the injectors and 6" above the fuel pump but pickup and return line come from the top of the tank. my new preferred location under the settee would put the tank at almost exactly the same level, maybe a little lower but not by much and all plumbing would be the same just further away. its a universal 5432 engine.
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Old 22-05-2018, 10:07   #9
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Re: Does diesel fuel return line have to be above fuel tank?

Only regarding the second tank.

I have 2 tanks, only because I had to split the footprint of the existing tank. Long story, but the point is, the idea left my system vulnerable. You’re doing it for a different and better reason, but there are still vulnerabilities. Not a problem, as long as you know about it.

But really what I want to contribute is that if you’re going to involve an electric fuel pump, think about giving yourself the flexibility of incorporating a fuel polishing system in it, simply by putting in a dedicated Racor.

I never had any problem with fuel until I had a serious problem.
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Old 22-05-2018, 10:08   #10
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Re: Does diesel fuel return line have to be above fuel tank?

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Originally Posted by captstu View Post
If the return goes into a rank at the top. No check valve is necessary. You should be ok as you describe.

What not use the existing tank as is and pump the second tank fuel into the existing as needed? Less new plumbing.
i want to get the tank out of the quarter berth so i can open up the wall in the berth for engine access. it will also be where i house a new battery bank. there is no access to the engine compartment behind the engine except a couple of small openings just big enough to get batteries through or to do light maintenance. the engine, seacocks, wiring, plumbing etc in that area needs upgrading and attention. i need to be able to get unrestricted access. moving the tank will give me that access.

there are two huge 52 gal aluminum water tanks (actually fuel tanks used for water), one under each settee port and starboard, and the company that made them in 1978 is still in business and making them. got a quote for $700/ea to replace (they all leak). they wil also do custom tanks.

still trying to work out the weight distribution so i keep it the same as it is now but may just end up having one tank on starboard as fuel and one on port as water. addition of battery bank under quarter berth should keep weight distribution the same and give me all the access i need. just wasnt sure if it was feasible to run a return line that far. doesnt seem like it will be a problem which is good to know.
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Old 22-05-2018, 10:13   #11
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Re: Does diesel fuel return line have to be above fuel tank?

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Originally Posted by Minggat View Post
Only regarding the second tank.

I have 2 tanks, only because I had to split the footprint of the existing tank. Long story, but the point is, the idea left my system vulnerable. Youíre doing it for a different and better reason, but there are still vulnerabilities. Not a problem, as long as you know about it.

But really what I want to contribute is that if youíre going to involve an electric fuel pump, think about giving yourself the flexibility of incorporating a fuel polishing system in it, simply by putting in a dedicated Racor.

I never had any problem with fuel until I had a serious problem.
thanks for the info. im starting to lean towards a larger single tank instead of two. i really do like the idea of having a fuel polishing system in place though. i wonder can a polishing system just take fuel from the main tank, run it through a racor and return it to the same tank?
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Old 22-05-2018, 10:56   #12
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Re: Does diesel fuel return line have to be above fuel tank?

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thanks for the info. im starting to lean towards a larger single tank instead of two. i really do like the idea of having a fuel polishing system in place though. i wonder can a polishing system just take fuel from the main tank, run it through a racor and return it to the same tank?
Yep, that's what a fuel polishing system does. It takes dirty fuel from the bottom of the tank and returns filtered fuel to the top of the same tank.
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Old 22-05-2018, 11:38   #13
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Re: Does diesel fuel return line have to be above fuel tank?

Trying to handle weight distribution with tanks that aren’t centerline can be chasing your tail. I mean, they aren’t always full of the weight you’re trying to distribute.

If you go with any fuel polishing, run it while you’re at sea. Much better than polishing fuel while in a quiet slip.

My friend lost her boat because she paid someone to polish her tank while it wasn’t being sloshed around at sea. The polishing cleaned up the fuel just above all the sediment laying on the bottom of her tank. Result was her boat and a boat trying to rescue her, both wound up on the rocks.
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Old 22-05-2018, 12:12   #14
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Re: Does diesel fuel return line have to be above fuel tank?

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Trying to handle weight distribution with tanks that arenít centerline can be chasing your tail. I mean, they arenít always full of the weight youíre trying to distribute.

If you go with any fuel polishing, run it while youíre at sea. Much better than polishing fuel while in a quiet slip.

My friend lost her boat because she paid someone to polish her tank while it wasnít being sloshed around at sea. The polishing cleaned up the fuel just above all the sediment laying on the bottom of her tank. Result was her boat and a boat trying to rescue her, both wound up on the rocks.
yeah the distribution has been a bit of a challenge but have it figured out with changing volumes in tanks. spent a lot of time with this one and have been careful to keep it almost the same as what the boat was designed with.

sorry for your friends loss...hopefully one day i will have the $$$ to add a small backup motor inboard but until then im hoping a 55lb Mantus anchor will keep me off any rocks until help can arrive, if i cant sail out of trouble.
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Old 22-05-2018, 12:43   #15
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Re: Does diesel fuel return line have to be above fuel tank?

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sorry for your friends loss...hopefully one day i will have the $$$ to add a small backup motor inboard but until then im hoping a 55lb Mantus anchor will keep me off any rocks until help can arrive, if i cant sail out of trouble.
While an anchor is a safety device, ďuntil help arrivesĒ is not as handy as being self reliant. Hope this doesnít sound preachy.

Lost use of my Perkins 1,000 miles east of Hilo on the voyage from Mexico. No problem. My big outboard on my RIB had done plenty of towing of other peopleís boats. I could sail into flat water, deploy it, and all would be fine.

During the long passage, my outboard that had work so well before, didnít now.

Not a dire situation. Flat water and all. But help wasnít on itís way for quite a while.

My loss of use of my diesel had nothing to do with fuel quality. That came much later.

Like I said, I never had and fuel problems, until it was serious.

I had the spare engine you speak of. Still a good idea. But your own fuel polishing is a cheaper.

Wish I didnít have so many stories to tell.
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