Fuel Return Line Questions

[Locquatious]

Originally Posted by Locquatious

*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?

Actually, the M673 generator has a check valve between the bleed off return line passing by the injectors and the injector pump. I disassembled that portion and found some dirty fuel in that check valve. There is a possibility that the check valve did not seat which would allow air from the return line to enter the injection pump.

For those who might say I'm not changing filters enough, be kind. The boat was 50° over on it's side after Irma. The check valve ball may not have been seated properly for several months till the boat was righted.

[Locquatious]

Originally Posted by MicHughV

...sometimes your best bet is to have a qualified diesel mechanic come and look at your problem.

During the pandemic it is harder to get qualified help. Some mechanics don't like compromising social distance on boats. I have been in touch with an excellent mechanic by e-mail and I have found one qualified mechanic who came out. He did check the start battery- great shape. He concluded compression is not the problem. He advised re-plumbing all the fuel feed system. Two tanks, two Racors, two engines, two lift pumps. That's a major project that requires going to supply stores multiple times to get the right fittings, valves, and hoses. I am in the Caribbean and one parts store won't let anyone except employees in the door. I had the parts to do the return line upgrades but not the supply rebuild.

Sometimes we're on our own. If the gen doesn't start, we lose refrigeration. The boat yards are closed so we can't get out of the water and go home. We can't move to other countries. We have to do our best with our own mechanical skills. There is YouTube, websites, manuals, and the Cruiser's Forums. DIY is best.

[AndyEss]

Small diesel engine fuel injectors don’t run at 36,000 psi - you got a decimal place wrong.

For my VW Pathfinder (50hp 4 cyl) the fuel injectors open at about 2,800 psi. The compression ratio of the cylinders is 23:1, so the backpressure on the injector is at most about 325 psi.

Diesel engines are not ultra-high pressure bombs just waiting to go off. On the other hand 2800 psi is nothing to sneer at - a leak in a pressurized fuel line will easily penetrate skin and inject diesel directly into your body.

Be careful with diesel fuel systems.

[Wotname]

Originally Posted by Locquatious

.......
Sometimes we're on our own. If the gen doesn't start, we lose refrigeration. The boat yards are closed so we can't get out of the water and go home. We can't move to other countries. We have to do our best with our own mechanical skills. There is YouTube, websites, manuals, and the Cruiser's Forums. DIY is best.

I get it .

From your descriptions so far, I don't think there is anything wrong with your return line although that check valve you mentioned upthread might be an issue.

Although there are differences between engines, as a generalisation many small marine diesels are similar in operation.

The lift pump will provide way way more fuel than the injector pump needs with the excess being simply diverted into the return line. If your injector pump has a return line connected, you can almost be sure it operates this way. If the injector pump does not have a return line connected, then it doesn't - obviously.

As a test, you could (most likely) be able to set up a gravity feed directly into the injector pump. Even just a funnel and some tube is all that is needed for testing purposes. If the engine keeps it's prime between starts, then you have a small air leak somewhere between the main tank and the injector pump. Keep moving the temporary gravity feed back towards the tank until you discover where the problem starts again.

FWIW, gravity feed fuel leaks out in the same spot where air leaks in when using a lift pump but leaking fuel is easier to see than leaking air.

[Wotname]

Re injector pump pressures. A mechanically injected small diesel will usually be in the order of 2,500 psi while an electronic common rail injection will be ten times that.

As some others have noted .

[skipperpete]

Originally Posted by Locquatious

During the pandemic it is harder to get qualified help. Some mechanics don't like compromising social distance on boats. I have been in touch with an excellent mechanic by e-mail and I have found one qualified mechanic who came out. He did check the start battery- great shape. He concluded compression is not the problem. He advised re-plumbing all the fuel feed system. Two tanks, two Racors, two engines, two lift pumps. That's a major project that requires going to supply stores multiple times to get the right fittings, valves, and hoses. I am in the Caribbean and one parts store won't let anyone except employees in the door. I had the parts to do the return line upgrades but not the supply rebuild.



Sometimes we're on our own. If the gen doesn't start, we lose refrigeration. The boat yards are closed so we can't get out of the water and go home. We can't move to other countries. We have to do our best with our own mechanical skills. There is YouTube, websites, manuals, and the Cruiser's Forums. DIY is best.

Duplicating the fuel system is probably not the best first step, and the little non return valve is an important part of the system because it does connect and disconnect the return circuit to the supply from the filter head.......but have you tested the glow plug circuit and individual glowplugs. NL’s are a bugger to start from cold if they’ve been standing for a while. Is the fuel tank lower than the engine or higher?
An optional fuel supply solution might be to install a day tank for the genset, this was once a fairly common arrangement but requires quite a lot of real estate for a 4 gallon tank but provides a positive head for the generator and if you filter the fuel into that tank it will always be a known clean tank.
Pete.

[RaymondR]

Originally Posted by Wotname

Re injector pump pressures. A mechanically injected small diesel will usually be in the order of 2,500 psi while an electronic common rail injection will be ten times that.

As some others have noted .

I knew they were higher but thought about 6,000-7,000 psi. We are not mislabeling kPa are we?

And it sounds like another good reason not to have common rail in a boat. A small leak with a bunch of atomized diesel in an engine box with electrics sound a bit bombish to me.

[Wotname]

Originally Posted by RaymondR

I knew they were higher but thought about 6,000-7,000 psi. We are not mislabeling kPa are we?

And it sounds like another good reason not to have common rail in a boat. A small leak with a bunch of atomized diesel in an engine box with electrics sound a bit bombish to me.

Nope https://en.wikipedia.org/wiki/Common_rail

At those pressures, they don't like dirty fuel either

[Locquatious]

As a follow-up, the generator is running much better now. The biggest change was replacing the glow plugs. It is surprising that glow plugs are needed in the tropics. But the earlier combustion during start aids the starter motor turning over the engine.

Other changes were made. All fuel connections were tightened. A weeping shut-off valve was replaced. A second boost pump was installed since the bottom of the fuel tank is 45" below the inlet of the generator. Northern LIghts specifies 39" as the max, but that is probably for shorter fuel lines.

Thanks for all the advice and help.

[FPNC]

*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?

YES. IF the fuel return line is running down hill and empties out it can allow air backways into the injectors via the return rack. Run the hose with a lower loop before entering the tank so that it does not empty out from gravity. Does it happen all the time on all engines? NO. But it can create a pesky starting issue that is intermittent and very gremlin like.

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

Add a loop per above.


*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?

Maybe/maybe not but I like to keep systems separate to avoid this kind of thing. It can really complicate diagnosing problems in the future

[GordMay]

Northern Lights Models M673l , M673ld2, M673l3, and M673ld3

OPERATOR'S MANUAL ➥ https://www.northern-lights.com/medi...fs/OM673L2.pdf

PARTS MANUAL ➥ https://www.northern-lights.com/medi...dfs/P673-2.pdf

INSTALLATION MANUAL ➥ http://www.ironbarque.net/Boat%20Doc...Manual1000.pdf

I encourage anyone, who downlaods useful documents, from links (links often go ‘dead’ with time), to then upload the document to our
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