How has my engine been running with the fuel hoses swapped?

[Ryban]

A long story short:

After a little over a year of owning Tarka, with her Beta Marine 14HP diesel, I think I discovered today that her fuel lines have been backwards this entire time. What I mean is, the pick-up port and the return port on the diesel tank were connected to the return line and secondary filter, respectively.

How has my engine been running for over a year this way?

I believe the return port does not go to the bottom of the tank, as when I pressurized the tank through the vent port with a dinghy pump, only air ever came out of this port. The other port, the one that was connected to the "return line", however, spat out diesel. This is also how I discovered that the lines were connected backwards.

My other reason for thinking the lines were connected incorrectly is because the return port is attached to what I think is an electric fuel tank sending unit (not connected to anything electronically). It looks like this: https://www.amazon.com/Moeller-Marin.../dp/B000MTI15U

I swapped the lines today, and the engine ran fine after bleeding. But it ran fine (I think?) the other way around, too.

[RaymondR]

Maybe the engine is economical enough to just run on the fumes and does not need liquid fuel?? Want to sell it??

[Ryban]

Haha, there are many days where I want to sell it...

It is very economical, but I think that's just because it's a small engine.

[Gafferfan]

Hi!

Love your channel and watch you regularly!

You are doing great!

[sharutika30]

Hi I had a similar happening with Sharutika. She has a Perkins 4-108. I had shut off the fuel to the engine for some maintenance and went away for lunch. I came back days later and forgot about the shutoff and went for a 3 day journey. No problem running on the return line only. It seems as long as there was no air in the system it would feed the injection pump from the backdoor drawing from the return line that is quite far down in the tank. I now have a shutoff in the return line.
Cheers
Dan Timms
Van Anda British Columbia

[Cheechako]

Boat motion allowed the return to pick up a taste of fuel often enough to run?

[transmitterdan]

Often the return line has the same dip tube as the supply line. It makes the plumbing idiot proof.

[S/V Alchemy]

Originally Posted by transmitterdan

Often the return line has the same dip tube as the supply line. It makes the plumbing idiot proof.

And detection-proof, too, perhaps.

[Ryban]

The return line being far down into the tank was a possibility, but when I removed the sender unit (with float attached), where the return fitting is attached, there's nothing sticking down into the fuel. It's flush with the top of the tank. It's possible that maybe something was improvised and it fell down into the tank, but I didn't see anything at first glance, so that doesn't seem to be the answer

It's possible for a diesel engine to back draw from the return line? Does this also mean that the fuel that has been entering my injectors has been unfiltered for almost a year?

[DeepFrz]

Give us a drawing of your fuel system.

[Ryban]

This is an image of how it was connected BEFORE I swapped the fuel lines, and how it has been operating for at leas the last year and 4 months.

(image modified from the MX manual)

[transmitterdan]

I don’t think it’s possible for the lift pump to work if the suction side is exposed to air. But, if the lift pump is operating “backwards” then it’s possible that the engine will run.

As Deepfrz said, you need to trace out all the goes-intos and goes-outofs to figure out what is happening.

[transmitterdan]

Originally Posted by Ryban

This is an image of how it was connected BEFORE I swapped the fuel lines, and how it has been operating for at leas the last year and 4 months.



(image modified from the MX manual)

There’s an easy way to tell. Pull the supply hose off the tank and put it into an empty jar. Start the engine and tell us what happens.

[parkstone bay]

without seeing the full system - it is, I suggest, UNLIKELY that the system can have been running with the two tank fittings transposed. The "normal" return line (from the injectors) should not have a dip tube, or feed from the bottom. Why not? - it's just returning fuel into the tank, usually just dropping the fuel into the top; there is no reason to return it to an "underwater" location. The "normal" feed via the filtration system, on the other hand, will have a dip tube, or, on some installations, take from the bottom. I'd have thought that if the actual fuel feed were taking from the return tank end, for some reason, it would rapidly run out of fuel, because the end of the return tube is nowhere near the level of the fuel.

[transmitterdan]

One reason to return the fuel “underwater” is to stop foaming. Another reason is to make the installation idiot proof.

I agree, it cannot run if the lift pump is sucking air.