Fuel return

I have a 1976 120 Lehman in my 1976 CHB. Can anyone tell me approximately how much fuel gets returned to the tanks at about 1750 rpms?
Thanks

At 1650, mine was returning so little, I got tired of waiting for the drops to show up.

My return lines feed right back into the supply lines in front of the primary filters.

Lehmans return almost no fuel. Bob Smith said it may return about a pint in a day's cruise.

Hmm, interesting. I wonder how much the Perkins 6.3544 returns?

Alot more. My '82 mainship had that motor, I could balance the boat in a days run due to return volume. My FL120 seems completely insignificant, although this boat is much heavier, wider, etc.

I had twin 6V53s in a previous boat. They returned way more than they burned. It did keep the fuel polished somewhat. In a way that is not good with the Lehmans.

Returning fuel is also a way to keep the injector pump cooler

Ok..but the question was answered and other engines are diffetent.....doesnt mean the return on a Lehman is detrimental.

Ok, thanks for all the input "BUT" I must have some other problem here goes!

I don't have fuel gauges just a really nice stick. I filled both tanks to with-in about 2" of full. My fuel system only returns to one tank (starboard), it also has an electric fuel pump. I was concerned that because it only returned to one tank that a should use some fuel from that tank first.

After running about 7 hours the boat was listing to the port side I measured the tanks with my custom stick and port tank was still about 2" down as is was turned off the starboard tank was down about 5".
So I switched to both tanks ran about 7 more hours when I stopped fuel was coming out of the over flow on the starboard tank!

Using the custom stick I re-measured the tanks, port down about 5" starboard full to the top!

Any ideas would be great!

Sorry for the long post

Is there a bypass on the fuel pump to the return?


Or just the single line from electric pump to injection pump?

Fuel pump to filter.

One more thing, there is a line out of the top of the filter housing that goes to the return line!

I don't think all Fords/Lehmans are plumbed the same. Last one I had in my shop for a rebuild I bench ran it and if I remember right it had a fair amount of return flow.

It's normal practice in the diesel world to have a drilled orifice or pressure control valve from injector pump to return. If you don't have that, it will not naturally purge air and can lead to stalling. On other engines like that (Yanmar GM) I have custom made a bleed rig to the return to get rid of the air problem.

So if I take the return line off the top of the tank where it currently returns and just "T" it into the supply line before the fuel pump will that cause any problems?

Thanks for the help!

Depends on the electric pump.

Might be better to tee it with valves to select the return to the same tank you are drawing from.

I took Bob Smith's class on the 120. We worked on it and then started it up. I don't recall that we even hooked up the return to the fuel tank. But we only ran it for about 15 minutes.

Island15

It sounds like you may be a candidate for a Groco 6 Port Fuel Valve.



This switches both the supply and return with a single lever and will permanently prevent the overflow you experienced. One valve, one handle, easy peasy

Island15 said:

One more thing, there is a line out of the top of the filter housing that goes to the return line!

Click to expand...

That bleeds off any air that might get in the system before the filters and helps prevent the injection pump from getting air locked.

Island15 said:

So if I take the return line off the top of the tank where it currently returns and just "T" it into the supply line before the fuel pump will that cause any problems?

Click to expand...

I wouldn't. You risk drawing from the return port on the tank, and typically they don't have a pickup tube. So if your supply side is restricted you'd draw air from the return side. Keep them separate.

I have a different theory.

The listing issue may not be only due to fuel tank levels. It may also be due to water tank or other storage issues.

When you equalize your tanks, the fuel flows from the high side to the low side, even if the low side has more fuel in it (and is overflowing).

LarryM said:

Island15

It sounds like you may be a candidate for a Groco 6 Port Fuel Valve.

This switches both the supply and return with a single lever and will permanently prevent the overflow you experienced. One valve, one handle, easy peasy

Click to expand...

That's exactly what I need . I have 5 tanks and valves scattered out everywhere. I just had a problem a few weeks back when I opened a valve by mistake and sucked one tank dry a almost overfilled another and wound up with air in the lines that shut me down .

The problem is I have no way to get the fuel back into the port tank.
There is only the valve for the fuel supply and fill hose! Is there something I'm missing?

Ski in NC said:

I don't think all Fords/Lehmans are plumbed the same. Last one I had in my shop for a rebuild I bench ran it and if I remember right it had a fair amount of return flow.

It's normal practice in the diesel world to have a drilled orifice or pressure control valve from injector pump to return. If you don't have that, it will not naturally purge air and can lead to stalling. On other engines like that (Yanmar GM) I have custom made a bleed rig to the return to get rid of the air problem.

Click to expand...

Can't speak for the Sp120, but for my SP135 it returns a significant amount of fuel. I've heard it's about 10x the amount it acutally uses and this seems correct to me.

My return lines go to a "T" and then another "T" at the top of each tank at the top of the sight tube.

Thus if I have the top of the sight tube open, fuel will fill the sight tube in less than 10 seconds. I keep that valve (top of sight tube) closed for that reason when underway as I want the fuel to return to the tank.

This aids significantly in keeping cool fuel going to the engine and aids filtering of the entire tank.

I keep the return of the tank I am feeding from open and the other tank closed. Otherwise, when i have had both open, it did empty one tank in the course of a day (maybe 100 gal) and then the engine died

Evidently, using these returns was the way the previous owners transferred fuel. One of my goals when i added the Fuel Polisher system was to be able to transfer fuel quickly.