3GM30f loses prime

[jpstevens]

An appeal to the accomplished members of this forum that know far more than I do about the 3GM30 fuel system:
If I prime my 3GM (bleed screw on secondary filter and bleed screw on injector pump, sometimes injectors) it starts instantly, but after two days idle, it will not start until I bleed again. I have replaced lift pump, Racor filter, secondary filter (including CU washers at banjo fittings) and hose from Racor to the Stewart Warner electric prime pump. I checked the pickup tube at the fuel tank for leaks, all connections for leaks, and am now totally without ideas, and would like to tap into the collective wisdom of the forum.

[tkeithlu]

Have you checked the bleed screws? You've been opening and closing them frequently.

The leak may be air in rather than fuel out/replaced by air. You may not see a leak.

Any chance of adding a one-way valve to the fuel line?

Yeah, my M25 does the same thing. It doesn't happen if I have nearly full tanks, because gravity keeps the line slightly pressurized.

[jpstevens]

Thanks Tkeithlu,
The bleed screw on the new secondary filter is also new, but not the bleed screw at the injector pump, although when I bled the system yesterday, I ran the prime pump then started and ran the engine while checking the bleed screw for leaks. Perhaps I should check again.

[jpstevens]

Expanded reply to Tkeithlu:
I'll check that bleed screw, perhaps replace it. Both prime pump and Racor have ball checks. I have looked at the prime pump check, but assumed that the new Racor check was functioning properly. Should I install another check valve? If so, any recommendations?

[tkeithlu]

That's all that came to mind, Jim. If those check valves aren't leaking then the likely leak is above them in the system. That's based on a guess that the stop valves will stop a bubble from migrating up past them.

OK. Let me think about this again. Consider checking for a fuel leak outward. To do that, clean and dry all the hose, wrap it in dry paper towels, and wait for a spot to form.

The edges on the filters are the biggest area sealed by a gasket. A small tear or bit of dirt could create a leak. They need to be clean, inspected, and oiled before assembly. Not too tight - that can create a leak rather than seal one.

[Engine man]

Here's what could be happening. All is fine when the engine is running. When the engine is off, a vacuum / siphon occurs inside the fuel hose that returns fuel to the boat's fuel tank. This results in the injectors being sucked dry of fuel, and you experience the symptoms you describe.

The fix is to route the fuel return hose into a 'U' configuration, with the lowest part of the 'U' lower than the bottom of the fuel tank. You could alternately route the hose vertically from the engine, then downhill to the tank. Either method will ensure the fuel injectors are not drained by suction, when the engine is not running. Joe

Originally Posted by jpstevens

An appeal to the accomplished members of this forum that know far more than I do about the 3GM30 fuel system:
If I prime my 3GM (bleed screw on secondary filter and bleed screw on injector pump, sometimes injectors) it starts instantly, but after two days idle, it will not start until I bleed again. I have replaced lift pump, Racor filter, secondary filter (including CU washers at banjo fittings) and hose from Racor to the Stewart Warner electric prime pump. I checked the pickup tube at the fuel tank for leaks, all connections for leaks, and am now totally without ideas, and would like to tap into the collective wisdom of the forum.

[jpstevens]

Thanks Engine Man,
That's a good idea which makes me hopeful! Hard for me to run the return in a "U", but I can easily run the line up before down to the tank. In fact, I'll do a test by running the existing return hoe into a gallon jug before buying a new hose long enough for the change.

[gonesail]

i ended up replacing all the fuel line banjo washers on my yanmar 3GM. but i never had the problem you are having nor did I have an electric prime pump. good luck.

[jpstevens]

Originally Posted by tkeithlu

That's all that came to mind, Jim. If those check valves aren't leaking then the likely leak is above them in the system. That's based on a guess that the stop valves will stop a bubble from migrating up past them.

OK. Let me think about this again. Consider checking for a fuel leak outward. To do that, clean and dry all the hose, wrap it in dry paper towels, and wait for a spot to form.

The edges on the filters are the biggest area sealed by a gasket. A small tear or bit of dirt could create a leak. They need to be clean, inspected, and oiled before assembly. Not too tight - that can create a leak rather than seal one.

Thanks again, I obviously need all the help I can get. I'll unscrew the top of the Racor and assure that the filter element is sealed and the gasket not leaking. As I replaced with new I assumed that it was functioning properly and that the ball check works as it should.

[jpstevens]

Originally Posted by gonesail

i ended up replacing all the fuel line banjo washers on my yanmar 3GM. but i never had the problem you are having nor did I have an electric prime pump. good luck.

Thanks, gonesail, I did replace the washers on each side of the secondary filter, but not all in the system. Perhaps need to check that.