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  Topic Review (Newest First)
04-19-2017 01:54 PM
Re: Return diesel fuel to the tank?

Originally Posted by roverhi View Post
...For the Bounty engineer, not knowing where the fuel return line was plumbed may be excusable. Not knowing that there was a fuel return line means he was incompetent.
IIRC, the engineer essentially admitted he was put in a role that was over his head. That was SOP for the Bounty, whose crew was dominantly some version of newbies and mis-fits.

I wonder what ever came of the lawsuits. They would be public, unless they were settled, but I don't recall where they were filed.
04-19-2017 12:51 PM
Re: Return diesel fuel to the tank?

The fuel return line to the tank on my Yanmar 3GM30F pumps a lot more fuel than actually goes to the injectors and is burned in the cylinders. If that fuel isn't piped directly to the tank, it will probably back fill into the tank from wherever it 'Tees' into the fuel delivery line. On the Yanmar, there is enough pressure that the hose clamps need to be seated and tightened properly or they will leak a significant amount of fuel.

For the Bounty engineer, not knowing where the fuel return line was plumbed may be excusable. Not knowing that there was a fuel return line means he was incompetent.
04-19-2017 11:40 AM
Re: Return diesel fuel to the tank?

Good idea on the day tank. While we ware covering basics don't be afraid to replace ALL fuel lines, with the exception of flared copper which could be too old to. Until doing that on a 35 year old Boat, don't post "What do I do" about my fuel supply problems. Sounds hard line but just do it.
04-19-2017 10:33 AM
Capt Len
Re: Return diesel fuel to the tank?

Resurrected old thread but always new readers and facts dond't usually change. Seen many return lines cut back into fuel supply line so wonder where the venting of air and cooling effect is. I vented return to a day tank which overflowed back to tank .This ran stove and was emergency engine supply and handy for bleeding at filter change. Return lines under the valve cover can fail and flood the crankcase and even if external drip on hot engine. Just some thoughts on past misadventures
04-19-2017 02:08 AM
Fish head Hi guys
I'm a deisel mechanic i have had some experience and might be able to shed some light.

In modern deisel engines,
The fuel system has a lot of work to do.
Pressurising deisel to extremely high injection pressure takes a lot of energy and generates a lot of heat.

In many comman rail deisel fuel systems, the fuel coming out of the return line is very hot.
So the return line is often sent back to the tank.

A fuel tank is there to store fuel, but another fuction of the fuel tank is to allow the fuel to cool down through a combination of mixing with the fuel still there. and the tank looseing heat to the atmosphere.

Some of you my have noticed the extremely high rate at witch the deisel is pumped through the fuel system and out the return, deisel isn't just fuel for the engine, it cools and lubricates the fuel system and keeps it primed.

I worked at a Hino dealership a few years ago, I over flowed a 20L Gerry can in what felt like a minute from a return line from a Hino 500 FM just idleing away, I had no idea it cycled deisel that fast lol

Any way I Hope this helps
03-22-2013 11:30 AM
Re: Return diesel fuel to the tank?

See FIG 1 for a good pic on how an old fashioned injector pump / injector works.

Fuel from the pump is delivered to the injector at 1500psi + .

The injector piston [or spool if you prefer the part beneath the spring anyway] is a good fit in its cylinder but still leaks a small part of the fuel into the the very top area connected to the return .

This leakage MUST occur to keep things lubricated. How much they leak is not that important as long as they all leak approx. the same amount. Hence the manufacturers rebuildes supplying matched sets of injectors.
03-21-2013 02:12 PM
Re: Return diesel fuel to the tank?

Many of the small Hp Yanmars have the fuel return line connected at the top of each fuel injector. The lines are run together then connect to a hose that takes fuel back to fuel tank. I guess you could connect the line to the fuel intak, but because this fuel return line may have air in it, that could cause a problem by air locking your fuel system. I do not seem to get any fuel returned to the tank through this line at idle (have not checked at full throttle), but the engine runs fine.

I do not understand how the injector works. Is it the fuel pressure that causes the injector nozzel to spray fuel? Why do you need a fuel return. Does not the fuel injector pump meter correct fuel to the injector? I can see where the fuel mechanical pump (or electric if that is what you have) could push too much fuel, but with a diesel, I would think the fuel would be restricted by the positive displacement fuel injection pump.
03-21-2013 10:07 AM
Re: Return diesel fuel to the tank?

All injector pumps HAVE to have a 'leak off' pipe.

It may do but does not HAVE to return to the tank.

It may return to the fuel feed pipe anywhere between the tank and the inlet side of the low pressure 'lift' pump.

So what the Bounty guy said might be correct.

Now as the qty of fuel that leaves the high pressure pump through the leak off line, is small, much less than the qty actually consumed by the engine using it as a way of polishing the fuel is a non starter. [ There is an exception, the old CAV rotary pumps did return significant amouts. I am not sure if it would be enough to 'polish' fuel though.

Ex diesel mechanic and instructor.
03-20-2013 04:05 PM
Letrappes When that line plugs up you have a problem. Guess how I know this...
03-20-2013 02:29 PM
Re: Return diesel fuel to the tank? Yanmar has a low pressure(about 3 psi +/-) return line to the diesel tank. Easy to see from pictures from following the high and low rail lines(metal piping) to and from the top of the injectors.
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