School me on fuel return lines

[Chotu]

I have posted about this before. I keep going through a lot of fuel pumps. They don’t last as long as they should. I believe that it has more to do with them continuously trying to pump against a brick wall then it does with them pulling the fuel up from the bilge area.

In my other thread somewhere, we had talked about doing a day tank. I don’t like those. I just don’t want to do that.

Instead, what if I take a page from diesel fuel system set ups? What if I do a return line? At first I thought it was crazy. Something you should never do with gasoline. But the more I look at it, the more it seems like it could be the right way to go. And it would not have to bring any pressurized fuel inside the boat. Which is a hard rule of mine.

Here, in the first picture, we have the fuel system as it stands today. There are three fuel tanks. There is a complicated set of plumbing that acts as a selector between tanks 1, 2 and 3, there is a fuel pump, and plumbing to the four engines that it runs. Two propulsion engines and two generators.

And then now in the second picture, I have added the fuel return lines.



Main questions:

1. If I put a very small diameter return line at each of the engines, including the generator engines, will the fact that the return line is very small diameter keep some pressure at those points in the fuel system? Because it will not flow back as easily as it flows to the area?

2. Will the engines, including the generators, get enough fuel if I do this? It seems strange to me to have fuel leaking its way back to the origin point through a return line. Rather than keeping the full pressure of the pump at the carburetor or at the fuel injector pump. How does that work on a diesel engine?

[Bill O]

Looks too complicated. The day tank would be easier, but is always the problem of remembering to refill it or having enough space for an appropriately sized tank.

Our return goes into our bilge tanks (much deeper than yours) and is not a problem bringing it back up to the motor.

Edit: See previous post on return lines: https://www.cruisersforum.com/forums...on-186721.html

[Rumrace]

All pressurized fuel rail systems have return lines. Doesn’t matter the fuel

[rslifkin]

A regulator with a return line after the pump will definitely reduce the load on the pumps. I'd probably regulate down to 1 or 2 PSI, as you're just trying to lift fuel to the pumps on each engine and generator. Any fuel the lift pump moves beyond the demand will just be returned to the tank via the return line.

[Chotu]

OK. Thank you for the responses.

I realize it may look complicated, but there’s really no other way to do it. Because I have four engines that I’m supplying with fuel. It has to be complicated. It’s not just a single engine.

The one problem I run into is I cannot return the fuel to the tank. Because that makes things more complicated than one can even imagine. You would have to select the tank that you are drawing from, and then have an entirely new fuel manifold to set the tank that you are returning to. And it would be easy to mess that up and fill up a tank that you aren’t drawing from. Overflowing the fuel tank.

so, I think returning into the supply line just before the pump is probably the best bet for my situation.

i’m a little confused on the regulator part.

What is that doing? Is that restricting the flow of the pump? Because I want the pump to run free. But I still want a little bit of fuel up there at the carburetors for the generator and also at the injection pumps for the Outboards. and of course all four of these things should be able to run at one time technically.

So I was thinking a small diameter return line would keep some pressure up there while allowing the fuel to return right? I currently have 3/8 inch hose going to go to all of the engines.

[NPCampbell]

Originally Posted by Chotu

Main questions:

1. If I put a very small diameter return line at each of the engines, including the generator engines, will the fact that the return line is very small diameter keep some pressure at those points in the fuel system? Because it will not flow back as easily as it flows to the area?

It depends on the fuel pump.

I swapped out the stock 350 in a 74 Vette with a modern LS3 V8. The stock engine used a 3/8 supply fuel line and a 5/16 return. This was acceptable because the stock engine had a mechanical fuel pump whose output varied based on demand.

With the new engine, I converted to a constant pressure, electric, in-tank Walbro. Now the 5/16 return line was too small. At idle, the Walbro is putting out the same pressure and fuel as it does when the engine is at WOT. However, at idle, the engine only sips fuel and so the majority of the fuel returns back to the tank through the smaller return line. This creates head pressure which burns out fuel pumps. Additionally, if your fuel line run is long, fuel that returns to the tank has to travel twice as far as as the supply which creates more resistance and head pressure.

Therefore, with a constant flow fuel pump, your return line should typically be the same size or possibly larger than the supply fuel line.

Originally Posted by Chotu

2. Will the engines, including the generators, get enough fuel if I do this? It seems strange to me to have fuel leaking its way back to the origin point through a return line. Rather than keeping the full pressure of the pump at the carburetor or at the fuel injector pump. How does that work on a diesel engine?

I don't know about diesels but my LS3 is rated at 500HP and the fuel pump will handle around 700HP using a return style system. I did have to swap out the stock fuel rails because the stock LS3 didn't have return style rails on it.

[rslifkin]

Originally Posted by Chotu

The one problem I run into is I cannot return the fuel to the tank. Because that makes things more complicated than one can even imagine. You would have to select the tank that you are drawing from, and then have an entirely new fuel manifold to set the tank that you are returning to. And it would be easy to mess that up and fill up a tank that you aren’t drawing from. Overflowing the fuel tank.

You need a selector valve like this (but ideally for 3 tanks): https://www.groco.net/products/valve...-6-port-series

That selects both feed and return tank at the same time, so no concern for returning to the wrong tank.

[Chotu]

Originally Posted by rslifkin

You need a selector valve like this (but ideally for 3 tanks): https://www.groco.net/products/valve...-6-port-series

That selects both feed and return tank at the same time, so no concern for returning to the wrong tank.

that thing is pretty cool! I would have loved that from the start. I just made my own manifold out of a lot of ball valves.

But, I’m still a little leery of pumping gasoline down into the boat unattended. Gasoline return lines inside the hull give me a little pause.

I would prefer not to bring gasoline back down into the hull.

Would there be anything wrong with just creating a loop? Allowing the return line to feed into the supply line?

[Chotu]

Originally Posted by NPCampbell

It depends on the fuel pump.

I swapped out the stock 350 in a 74 Vette with a modern LS3 V8. The stock engine used a 3/8 supply fuel line and a 5/16 return. This was acceptable because the stock engine had a mechanical fuel pump whose output varied based on demand.

With the new engine, I converted to a constant pressure, electric, in-tank Walbro. Now the 5/16 return line was too small. At idle, the Walbro is putting out the same pressure and fuel as it does when the engine is at WOT. However, at idle, the engine only sips fuel and so the majority of the fuel returns back to the tank through the smaller return line. This creates head pressure which burns out fuel pumps. Additionally, if your fuel line run is long, fuel that returns to the tank has to travel twice as far as as the supply which creates more resistance and head pressure.

Therefore, with a constant flow fuel pump, your return line should typically be the same size or possibly larger than the supply fuel line.


I don't know about diesels but my LS3 is rated at 500HP and the fuel pump will handle around 700HP using a return style system. I did have to swap out the stock fuel rails because the stock LS3 didn't have return style rails on it.

Wow. That is one heck of a post. Thank you so much. That helps me understand the situation a lot better.

it’s exactly the opposite of what I was thinking with the return line. I would think it would need to be small so that there was a little bit of head pressure maintained.

currently, I have no return line. And my constant output pump is just banging its head against a brick wall basically. That’s why they are burning out I think.

and we are talking about returning from the rails. What about returning before the rails?

I basically have a pair of fuel injected outboards and a pair of carburetor generators. I was thinking of just putting a T in the line right before the carburetors on the generators, and a T in the line right before the internal lift pump of the Outboard.

Is there any reason that won’t work? I am seeing the return line, which is external to the fuel system of the engine in my case, as a way to just relieve some pressure and allow the pump to continually circulate and cool itself a bit.

[rslifkin]

A loop should work ok, but I'd be concerned about how hot the fuel may get going around in circles like that. Especially considering it's gas, not diesel.

[Chotu]

For complete information, this is the fuel pump I have started using. I broke one of these out. But it lasted quite a while.

https://www.amazon.com/SEACHOICE-50-...a-810457186453

[NPCampbell]

Originally Posted by Chotu

Wow. That is one heck of a post. Thank you so much. That helps me understand the situation a lot better.

it’s exactly the opposite of what I was thinking with the return line. I would think it would need to be small so that there was a little bit of head pressure maintained.

It's definitely not intuitive that the most stress is put on the fuel pump when the engine is idle.

Originally Posted by Chotu

and we are talking about returning from the rails. What about returning before the rails?

If you don't have vapor lock issues then you can put the return anywhere as long as you aren't generating turbulence with sharp bends, etc.

However, one of the benefits of a return style system is that it helps reduce vapor lock. It does this because fuel will no longer sit in one spot and be heated by the engine or exhaust manifold (your fuel remains a relatively constant temperature as well).

If your fuel pump is an in-tank version, I believe they have retrofit style pumps where the return is right on the pump, in the fuel tank.. This eliminates the need to have fuel switches while still being able to use a quiet, electric, constant flow pump.

[Searles]

Bite the bullet, fit a decent sized day tank,easy fuel returns ,low pressure on fuel lift pumps easy to access if done properly ,plus you will learn to love the simplicity .⛵️⚓️

[Manateeman]

Carter fuel pump. Lines the same size. Carter is a vane pump and used on aircraft and refrigerated sea containers. Military as well.
Don’t buy the Chinese copy.
Very quiet because it hangs in rubber mounts. Runs forever. There is a small screen inside to protect the vanes. This screen isn’t a Racor filter so the pump is not designed to pump dirt, just diesel or gas.
It’s metal not plastic. Put Tef-Gel on the electrical connections.
Happy trails to you.
Mark and the manatee crew who have used it to pump beer for which there is no FDA approval. Proof alcohol kills brain cells in mammals.

[Chotu]

Originally Posted by Searles

Bite the bullet, fit a decent sized day tank,easy fuel returns ,low pressure on fuel lift pumps easy to access if done properly ,plus you will learn to love the simplicity .⛵️⚓️

No day tank. It’s too complicated. I don’t want to have to manage my fuel especially in a bad situation.