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Old 09-09-2010, 07:57 AM   #1
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Return line question

Last weekend I was taking a better look at the fuel system because I want to install a polishing system next spring. My boat has two relatively new fuel tanks intalled aft of the original steel ones. The PO told me the old ones were empty and not used anymore. They are rusted and I am plannig to remove them.

But here is where things get weird. I noticed the return line from the engine was still going to one of the old tanks!! That started to make sense to me because that tank has a little fuel leak. I thought it might be a bit of old fuel left in the tank but now I'm thinking that tank might actually be full of fuel... So to get a better idea of the flow of fuel going in that tank I unplugged the return line and put it in a jar and started the engine. And this is where things get really weird. There was no fuel coming out of the return line... I ran the engine for about 5 minutes at different rpm and nothing would come out of the return line. Am I missing something here?

Here's picture of the return line.
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Old 09-09-2010, 08:44 AM   #2
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Return line question

Some engines return little fuel, it depends on the pump and design. Some return a lot. Contact your manufacturer or substitute and ask them what your "CORRECT" return rate*for your pump should be at various RPMs.*

I'm sure a few members here*have your same setup. If they have floscans they probably have your answer, but beware of the guesses that will flow your way. If you have the ability to return to either tank, draw from one and measure the rise in the other while at loaded (not at the dock) cruising RPM. I did this a few years ago and could see only a few* ( 2-5 at most) gph return rate.

My Perkins Sabres return less than my Westerbeke genset.

As far as de-commissioning the old tanks, be sure you don't have fuel in the tanks and a plugged discharge line on bottom. You'll figure this issue out real quick.

-- Edited by sunchaser on Thursday 9th of September 2010 08:46:36 AM
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Old 09-09-2010, 08:58 AM   #3
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RE: Return line question

It looks like you have a Ford Lehman.* If you do, try getting a hold of Brian or Bob Smith at American Diesel.** Phone: 804.435.3107

http://www.amerdsl.com/

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Old 09-09-2010, 09:35 AM   #4
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RE: Return line question

Yes I do have a Ford-Lehman 120. I wanted to have your opinion before calling them. I know they are very helpful but part of me hate to disturb them with a simple question when I don't have an order to place.
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Old 09-11-2010, 03:57 PM   #5
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RE: Return line question

Fotoman
Dont be afraid to call them. THey understand, and have built their business on helping Lehman and Perkins owners. They'll be glad to help educate you....then later on when you need parts you will do as I do - buy from them. (their prices are quite reasonable).

Here is the answer you are looking for:
The Lehman 120 returns 1/2 pint in 10 hours of running. Its almost zero return. I was with Bob when we ran one in the parking lot and no return line was even connected to any tank, Nothing came out. You could hook a clear gallon milk jug to it and run all day long and see very little accumulation. On my boat the return line only went to one tank and until I was in Bob's engine class, I didnt understand why.

You should, however, immediately disconnect that line from your old rusting tank.

R.
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Old 09-12-2010, 05:40 AM   #6
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RE: Return line question

Interesting this discussion, and probably explains why the return line on my L120 goes just a short distance back to the primary filter. Makes good sense now and quite a sufficient and elegant solution. Why take it all the way back to a tank, when there is so little anyway, and the primary filter is so close? Like Marine Trader, my boat still houses its old rusted main tanks, but they are sealed off, and I look on them as buoyancy tanks now, and will leave them there as not worth the hassle, danger and cost to remove. I don't need the space, so why not buoyancy....? As the only leaks in them are pinholes in the top, they would perform this function to quite an extent. Might help in an engine room flood until pumps caught up etc.....?
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Old 09-12-2010, 05:57 AM   #7
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RE: Return line question

By relocating the fuel tanks and leaving your old tanks in, you are changing the designed balance of the boat.

If it were me, I would remove the old tanks and replace them with water tanks. While traveling having lots of water capacity is really desireable to me.
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Old 09-12-2010, 06:12 AM   #8
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Return line question

Quote:
ralphyost wrote:

By relocating the fuel tanks and leaving your old tanks in, you are changing the designed balance of the boat.

If it were me, I would remove the old tanks and replace them with water tanks. While traveling having lots of water capacity is really desireable to me.
R,
Normally yes Ralph, but not in this case.* She was wildly overtanked originally, (1000L per side), so the replacement tanks (400L a side and plenty for our cruising), right behind the originals, with water tanks, (400L in between those, and 200L for'd - again plenty for our use), means she still sits nicely on her marks and handles accordingly.* Possibly even better than in original trim, as the original tanks are higher than I think is ideal, and I can actually surf her in a following sea, (I've seen 11.5 kn on the GPS on a good wave), with no feeling of losing steering at any point.

*


-- Edited by Peter B on Sunday 12th of September 2010 06:17:36 AM
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Old 09-12-2010, 09:29 AM   #9
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RE: Return line question

Fuel return obviously varies considerably with the engine make/model. Our old Volvo MD-47 90 HP returns a very large quantity of fuel which Volvo says is used to cool the injectors.
When we first got Penta I filled the tanks and started the engine to move forward at the dock and let things warm up. Imagine my shock when after about 10 mins. at idle the starboard tank was overflowing. I then found out that the return line was only run to the starboard tank, the equalizing line at 3/8 copper was way too small and that one should never run from the port tank when both tanks are full. The return volume is in the form of a steady stream and when running for a while is quite warn to the touch.
I cured all these problems by installing a Groco duel function transfer valve which in one valve assembly switches the draw and return so that now when drawing from the port tank, the port tank also gets the return. Another valuable feature is that when this valve is set between function the fuel is shut off creating a security feature. Although I left the equalizing line in place, in fact it does little in the way of balancing unless the tanks are below about 1/2 full.
Have a great day guys

John Tones MV Penta
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Old 09-13-2010, 09:12 AM   #10
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RE: Return line question

Thanks Ralphyost, that explains why I didn't see anything coming out. My plan is to remove the old fuel tanks and replace them with water tanks. I will then remove the original water tanks located under the cockpit. That should pretty much bring back the boat to its original balance.

Peter B: that sounds like a great solution. Food for thoughts.
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