Yanmar Air in Return Fuel Line

[robbieg]

We are trying to track down an irritating air leak in a 3GM30. By using clear pipe and a jury rigged tank we have been able to ensure no air bubbles are in the system by the time the fuel gets to the high pressure pump. However, in the return line from the injectors to fuel tank we are still getting air bubbles-the more bubbles the faster the engine runs

I have been told this is normal since the fuel aerates under high pressure at the injectors and this goes back via the return line and then "falls out" of the fuel as is settles in the tank. Anyone any experience of whether this is normal or no-it sound plausible and since we can see the fuel is air free at the high pressure pump the air must be something to do with the high pressure side.

TIA

 

[Simondjuk]

Does the engine run ok? If so, put an opaque fuel line on so you can't see the bubbles and forget about it.

More seriously though, I trial ran a 1GM10 recently with no leak off pipe in place, putting a cloth round the banjo bolt to collect the return fuel, only there was no fuel. Well, barely a trace. I mentioned this to someone else and he told me he was aware of a boat that had the leak off line going to a jar rather than back to the tank (by way of a semi-permanent jury-rig by the sound of it). Apparently, it took many hours of engine use to even half fill the jar. I'm not sure if the quantity of leak off fuel on the 3GM30 should be similarly small, but if so, it would certainly explain the presence of bubbles in the return.

 

[Seadawg33]

I'm not a mechanic but I can't see that bubbles in the return lines would cause any problems. If the engine runs well . . . but the bubbles must have some source. have you checked all of the hose ends to see if the hose clamps have chewed a hole in the hose. Had that happen on the intake line and it would kill my engine after a few minutes. As long as you have extra fuel line it is just a matter of cutting off the end of the hose and reattaching it then bleeding the system. Also use the hose clamps that are not perforated b/c the are more prone to cause small leaks.

 

[rotrax]

We are trying to track down an irritating air leak in a 3GM30. By using clear pipe and a jury rigged tank we have been able to ensure no air bubbles are in the system by the time the fuel gets to the high pressure pump. However, in the return line from the injectors to fuel tank we are still getting air bubbles-the more bubbles the faster the engine runs

I have been told this is normal since the fuel aerates under high pressure at the injectors and this goes back via the return line and then "falls out" of the fuel as is settles in the tank. Anyone any experience of whether this is normal or no-it sound plausible and since we can see the fuel is air free at the high pressure pump the air must be something to do with the high pressure side.

TIA

Hi, It is probably because the return line is not under pressure.Air bubbles on the return side are not detrimental in any way and are quite normal. I have seen many injector pumps on test rigs and bubbles in the return line were always visible on the rigs with clear return pipes.

 

[vyv_cox]

The explanation you have been given is correct. I had a transparent fuel tank on a previous boat and the return fuel always appeared aerated. On my current car the return fuel is both aerated and hot, presumably for the same reason. There is a heat exchanger in the return line, which is vulnerable to puncturing by road stones. Recently cost me nearly £100 to replace it.