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Old 25-04-2020, 02:41   #1
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Fuel Return Line Questions

The fuel return line is supposed to be low pressure and just allow fuel flow back to the tank.

*Can air in the fuel return line, assuming it can flow till empty of fuel, allow air to get into the injectors while the engine is at rest?

*Should the fuel return line slope down to the tank or can there be low points prior to entering the tank?

*A standpipe is sometimes recommended inside the tank. Is this installed to prevent air from entering the return line and going back up to the engine?

*If the return line of a generator is teed into the return line of a main engine, can the fuel return flow of the main engine create a suction on the return side of the generator such that air is pulled into the generator fuel supply system?
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Old 25-04-2020, 06:42   #2
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Re: Fuel Return Line Questions

1) no! The injectors are actuated from the pump or electronics, depends on which you have.

2) It doesn’t matter.

3) the return line doesn’t need a stand pipe, only the feed line, which should be about 1/2” above the bottom of the tank.

4) the return line should not be attached to the supply line. There is no pressure or volume in a return line. It should be free flowing. It can have air in it at any point, unlike the supply line.

Marine fuel tank supply and return should be from the top of the tank. Only a permanent drain plug should be at the bottom for maintenance only. Also, there should be an inspection cove on top as well.

Take a look here... https://newboatbuilders.com/docs/H-33_Diesel_2005.pdf
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Old 25-04-2020, 07:38   #3
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Re: Fuel Return Line Questions

Edit.... Last post.

2) But, vent lines need to be so the fuel drains completely back into the tank.

3) stand pipe, AKA pickup tube for a return line is to keep the fuel from foaming when it gets hot on returning. The fuel warms up as it returns thru the injectors.
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Old 25-04-2020, 08:31   #4
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Re: Fuel Return Line Questions

a diesel engine is not like a gas engine...

fuel is pumped from the tank to the fuel injector system, this is a very complicated piece of equipment that works under enormous pressure....think 36,000 psi....excess fuel that is not needed, simply bypasses this system and goes back to the tank.

The main injectors have tiny nozzles, usually smaller than the eye can see. In order to get the diesel to flow thru' these tiny holes, the enormous pressure is required.

The tank obviously must be vented to in order to allow fuel to be sucked out without creating a vacuum in the tank.

Earlier diesel engines had fuel systems which required one to " bleed" the system, so there was no air in the feed line, but the modern day diesel have self bleeding injector pumps.

Though I have seen many diesel / generator installments, most typically have their own independent delivery and return fuel lines. Keeping them separate eliminates the possibility of one system interfering with the other. The main feed lines should be obviously separated to ensure the one engine does not work against the other.

A thought to consider. Some boats have more than one fuel tank. if the tanks are separated, as they should be, there should be a switch that allows the return line flow between tanks to be switched as needed.The main feed lines from each tank are usually also controlled be a selector switch.

Nigel Calder wrote a good book about diesel engines entitled " Marine Diesel Engines". This will provide most any neophyte sailor an insight into the workings of a diesel.

Additionally, there are many excellent online references to diesel fuel management systems.

To go into detail on this forum would take an enormous amount of time.
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Old 25-04-2020, 09:24   #5
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Re: Fuel Return Line Questions

The return does not need to lead to the tank. On long trips the content of the tank warms and this stimulates bugs-development. Attach the return hose to the second inlet of the separate fuel filter!
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Old 25-04-2020, 09:54   #6
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Re: Fuel Return Line Questions

the return needs to go to the tank in order to avoid an air lock.
If you don't return the fuel to the active tank, it might overfill the other one.
Look up Groco 6 port selector valve, available for $150 to $350 depending on where you look.
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Old 25-04-2020, 10:28   #7
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Re: Fuel Return Line Questions

The fuel is also a coolant for the injector pump. Feeding the warm return fuel directly back to the feed could affect the cooling.
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Old 25-04-2020, 11:16   #8
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Re: Fuel Return Line Questions

...an integral part of any diesel engine installation is the installation of a correctly sized water/fuel separator...the Racor 500 is typically seen on sailboats....often in tandem.....it has a clear glass bowl allowing inspection of the contents that are fed to your diesel. A vacuum gauge attached to the top of this filter is another device giving you warning of a fuel problem about to happen.

...true...warm fuel/air/moisture does promote a bacterial slime which resides on the bottom of a diesel fuel tank. This is the reason the bottom of the pickup tube is located an inch or two above the bottom of the tank.

...I had the pleasure of removing a diesel fuel tank from a sailboat once. On a whim I removed the inspection covers and was surprised to see the bottom and lower sides covered in what looked like green slime. That tank was about 10 years old. I'm not sure how thick the green slime was, but I'm guessing at least 1/8"...

However, a good fuel polishing goes a long way in addressing this.

In 40 some years afloat, after a few learning hiccups, I eventually became fastidious in maintaining my various diesels in tip-top condition. Providing clean fuel and oil was high on the list. It's rare that I had a problem.

Picking up diesel in foreign ports often means decanting from a rusty 40 gallon container or other improvised means. Getting " dirty" fuel is a given.
There are a variety of ways to address this, but simply dumping it into your tank is asking for trouble.

At the end of the day, learning the ins and outs of all the various methods to ensure your diesel is fed clean fuel is a worthy exercise.
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Old 25-04-2020, 12:19   #9
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Re: Fuel Return Line Questions

Quote:
Originally Posted by delmarrey View Post
1)
2) It doesnít matter.
[/url]
Thanks for the answers to my questions.

I've been having trouble starting a Northern Lights M673. The return line had some bellies leading back to the tank. If I blow on the return line it takes a fair amount of pressure to push the fuel out of the bellies and into the tank. A lift pump might not be able to overcome the back pressure. It just seems to me that the return line should be self draining to avoid back pressure issues.

The return lines were re-routed with generally larger lines and slope back to the tank. However, there was no change in the hard starting problem.
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Old 25-04-2020, 12:30   #10
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Re: Fuel Return Line Questions

The starting issue persists with the M673 generator.

If the bleed screw on the fuel filter is opened when the lift pump is operating, fuel and air leaks out till I tighten the screw.

Then if the lift pump is turned off, and if I valve off the fuel supply line, and I open the bleed screw, all the fuel under the bleed screw flows away. There are no leaks so where does the fuel go? If it goes down the return line, doesn't that mean it has to flow through the fuel pump, and does that mean I have a faulty fuel pump?

Anybody have a fuel flow schematic for the M673?
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Old 25-04-2020, 12:38   #11
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Re: Fuel Return Line Questions

*Can air in the fuel return line, assuming it can flow till empty of fuel, allow air to get into the injectors while the engine is at rest?
Not that I've ever heard.
*Should the fuel return line slope down to the tank or can there be low points prior to entering the tank?
There can be low points, it's gravity, the low points will always have fuel in them but fuel will rise and go to the tank.
*A standpipe is sometimes recommended inside the tank. Is this installed to prevent air from entering the return line and going back up to the engine?
No pipes in any I've had. Just a fitting on the top
*If the return line of a generator is teed into the return line of a main engine, can the fuel return flow of the main engine create a suction on the return side of the generator such that air is pulled into the generator fuel supply system? I wouldn't think so.

I had one small diesel engine that had a fuel inlet barb fitting on the engine. If you turned that fitting any direction other than as supplied the engine wouldn't run! Weird. I think it would get an air bubble in it.
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Old 25-04-2020, 13:10   #12
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Re: Fuel Return Line Questions

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Originally Posted by MicHughV View Post
A thought to consider. Some boats have more than one fuel tank. if the tanks are separated, as they should be, there should be a switch that allows the return line flow between tanks to be switched as needed.The main feed lines from each tank are usually also controlled be a selector switch.
If one has any form of built-in polishing system then it is advisable to enable return fuel to be sent to a different tank than that which is supplying. I have two valves on my system, one that directs the supply from either tank to the engine and a different one directing the return to either tank.

Seems to me that it is pointless polishing dirty fuel and then sending the cleaned fuel back to the dirty tank. I donít know how polishing is done on boats with just one tank. I suppose jerrycans must be the solution. I have 450l of fuel capacity - thatís a lot of jerrycans.
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Old 25-04-2020, 13:33   #13
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Re: Fuel Return Line Questions

...sometimes your best bet is to have a qualified diesel mechanic come and look at your problem. A hard to start diesel engine can have it's problem originate in a dozen different areas, which would be impossible to diagnose on an internet chat forum. More often than not the mechanic will point to a problem outside your general knowledge or experience with diesel engines. Additionally he will come equipped with all the diagnostic tools...compression tester and whatnot. A simple issue such as a starting battery not up to snuff could be at fault. A simple vacuum gauge can also provide a wealth of information, etc, etc.

Yes, I know, being a sailor it's hard to pry open your wallet for "advice"...but in my experience, when you really need that engine to start..and it doesn't...that is not time to have recriminations about things you should...or should not have done.
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Old 25-04-2020, 13:39   #14
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Re: Fuel Return Line Questions

fuel is pumped from the tank to the fuel injector system, this is a very complicated piece of equipment that works under enormous pressure....think 36,000 psi....excess fuel that is not needed, simply bypasses this system and goes back to the tank.

Not sure what the pressure on common rail systems is but the injector cracking pressure on my Yanmar is about 2,500 psi.

To provide a long working life there are no elastomer seals in either the injector pump or the injectors and consequently there is leakage past the plungers and needle valve lifters in the injectors. It is this leakage which is being returned to the tank in the return line.
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Old 25-04-2020, 13:40   #15
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Re: Fuel Return Line Questions

Quote:
Originally Posted by MicHughV View Post
...

Earlier diesel engines had fuel systems which required one to " bleed" the system, so there was no air in the feed line, but the modern day diesel have self bleeding injector pumps.

...
Did you mean fuel lift pumps?
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