Fuel return back to engine

[ridgy]

I have a Yanmar 2GM and the fuel return pipe goes back to the fuel filter rather than the tank. I hadn't seen this before but it's obviously been like that for a long time.however it sort of negates the whole point of a return.

This year I replaced a leaking fuel pump and first time out the pressure was causing leaks around several fuel connections.

Is this a vaguely normal arrangement? Seems daft to me.

 

[john_morris_uk]

It’s normal on some engines. I suppose it means the fuel in the tank isn’t getting slowly scrubbed through the various filters.

 

[Mister E]

Used to be quite common on smaller commercial vehicles.

 

[ridgy]

How is the pressure from the lift pump regulated then?

 

[RunAgroundHard]

Perkins 4236 I own is fitted like this, return back to the fuel filter on the engine. I have a water separator and fuel filter, mounted off the engine, before the filter on the engine. This engine has been the most reliable, easiest to bleed engine I have ever worked with. The pressure in the lift pump, may be regulated by a spring operated relief path, where a ball gets lifted at a certain pressure to by pass flow, but I am not sure and it will depend on the pump make and model, no doubt.

 

[lustyd]

I can't see how it would make a difference either way since the fuel filter is open to the tank so the pressure would be identical whether it drops into a tank or drops into a filter which is open to the tank. Bleeding air our might be slightly harder as it wouldn't bubble out into the tank, but other than that I don't see much difference.

 

[geem]

I can't see how it would make a difference either way since the fuel filter is open to the tank so the pressure would be identical whether it drops into a tank or drops into a filter which is open to the tank. Bleeding air our might be slightly harder as it wouldn't bubble out into the tank, but other than that I don't see much difference.

Not quite. You have the small suction pressure between the tank pick up pipe and the fuel filter that might be a couple of feet of head pressure depending in where the tank is located. On my own boat, the difference is about 5 ft of head pressure plus about 8 ft of pipework. Mine fuel return goes to the tank. If the tank is higher than the engine it's irrelevant

 

[lustyd]

I'm not sure that's right. Additional fuel would still need to be sucked from the tank so the head pressure would be the same, although the quantity of fuel being sucked would be slightly different. In both systems the only opening is in the fuel tank.

Edit: also, with the return at the fuel tank you also reduce the pressure after the pump, equalising the pressure gradient between the two systems due to gravity.

 

[Metalicmike]

Assuming we are talking about the pre filter before the lift pump, any pressure difference is irrelevant. I quite like the idea as long as it isn't going to cause air ingress.

 

[B27]

On an engine like a Yanmar GM, which is capable of self bleeding, it seems a poor arrangement., because any air will be retained in the closed circuit instead of going to the tank.
It makes a lot more sense on a Tractor, where bleeding is easy because the tank is higher than the engine.

 

[Boater Sam]

All the BMC engines that were used in boats for many years have fuel returns and bleed offs connected to the fuel filter.

 

[rogerthebodger]

The CAV filer has 2 inputs and 2 outputs for those connections as on my Perkins 4236

 

[Bilgediver]

How is the pressure from the lift pump regulated then?

The lift pump is self regulating. It draws in on the upstroke and a spring applies pressure to the diaphragm ensuring fuel delivered as required and no more.
On engines with a return to tank the lift pump delivers on every stroke with the surplus going to the tank via a restrictor.

 

[geem]

The lift pump is self regulating. It draws in on the upstroke and a spring applies pressure to the diaphragm ensuring fuel delivered as required and no more.
On engines with a return to tank the lift pump delivers on every stroke with the surplus going to the tank via a restrictor.

And the you get some fuel filtering as a result.
My engine uses an electric fuel pump. It moves 500% of the fuel required as normal revs. A nice bit of bonus fuel filtering

 

[vyv_cox]

I have measured the return volume on my Yanmar 3GM30F. The volume was tiny, simply the fuel that is spilled inside the injector as the needle lifts and seats. I forget the exact amount but it was < 10 ml/minute. There is no weir at the pump on this engine, which on engines that have it deliver the spill back to the tank.
When I had a Bukh the fuel tank became quite warm with returned fuel from the pump but never from the Yanmar, which unlike the OP's returns to the tank.

 

[ridgy]

Interesting stuff. The return goes to the secondary filter on the engine. It seems that the pump is not regulating properly and putting too much pressure in. I may have been understating when I said that some joints leaked. What happened is that the smoke alarm in the engine box went off and we found one joint between the two injectors spraying significant diesel over the hot engine which set the alarm off. Unfortunately we were half way up the mersey channel at night in thick fog at the time so we had to anchor outside the channel and remake the joint while a cruise ship trundled past. It still peaked and then half an hour later the return pipe blew off completely and there was more like 10ml per second coming out of it. For the last two hours my mate had to kneel in front of the engine holding it on while getting sprayed with fuel. There were no hose clips on the joints suggesting there should not be this sort of pressure on the lines. The following morning we remade all the joints with cable ties and it got us 3 hours up to Warrington with just one joint dripping. Must be 5 litres of diesel in the bilges now.

 

[geem]

Interesting stuff. The return goes to the secondary filter on the engine. It seems that the pump is not regulating properly and putting too much pressure in. I may have been understating when I said that some joints leaked. What happened is that the smoke alarm in the engine box went off and we found one joint between the two injectors spraying significant diesel over the hot engine which set the alarm off. Unfortunately we were half way up the mersey channel at night in thick fog at the time so we had to anchor outside the channel and remake the joint while a cruise ship trundled past. It still peaked and then half an hour later the return pipe blew off completely and there was more like 10ml per second coming out of it. For the last two hours my mate had to kneel in front of the engine holding it on while getting sprayed with fuel. There were no hose clips on the joints suggesting there should not be this sort of pressure on the lines. The following morning we remade all the joints with cable ties and it got us 3 hours up to Warrington with just one joint dripping. Must be 5 litres of diesel in the bilges now.

I have done that trip to Fiddlers Ferry a few times. It not a place to lose the engine.