PITA air in fuel line problem

[Martin S]

I have a Yanmar 4JH3BE fitted with a Separ fuel filter above engine and fuel tank height. standard on engine fuel filter. When motoring the engine has (so far ) never stopped because of air entrainment. However when left over night, in the morning the engine will not start because of air in the system. Closing the fuel valve and opening the top of the Separ filter housing, as you would to change a filter, the fuel level is low and due to air is below the level of the outlet pipe at the top that leads to the on engine fuel filter. When filled with diesel and bled well the engine starts.

I have changed to rubber gasket sealing the top of the fuel filter. Tightened all connections, ensure the bleed values are properly tighter. All with no result.
There is no evidence of a fuel leaking out.

Please, any ideas.

[michaeld]

Martin,

It's definitely sucking air. I had that happen to me. Replacing the hoses & clamps resolved the problem. Those pesky slow air leaks are frustrating. For troubleshooting, you may need to bypass the Separ filter to determine if it is the culprit. Good Luck.

Michael

[Compass790]

Originally Posted by michaeld

Martin,

It's definitely sucking air. I had that happen to me. Replacing the hoses & clamps resolved the problem. Those pesky slow air leaks are frustrating. For troubleshooting, you may need to bypass the Separ filter to determine if it is the culprit. Good Luck.

Michael

What he said,
You could try pressurising it a little bit, vaccuum leaks are hard to find sometimes. If you are getting air in it as described the filter is defintely the culprit.

[Cheechako]

Are you sure your tank vent(s) isn't clogged? It creates a vacuum in the system which may then create the problem after you stop. It may be nearly clogged allowing just enough to continue running. I had tiny bees build a mini nest in the outlets on one boat.

Other than that you could try installing an electric fuel pump before the filters to boost the system , It's handy to have anyway.

[Lepke]

I have found hard to find fuel leaks by pressurizing the fuel line and listen for the leak.

[tkeithlu]

Or, if your hearing is anywhere near as bad as mine, spray with a liquid soap solution and look for bubbles.

[Bycrick]

I had a similar problem years ago with a Volvo MD 11. It turned out that they’d used ALUMINUM crush washers around the fuel fitting and bolts. Apparently these would corrode and develop tiny, tiny leaks. You couldn’t see anything because the washers were hidden. Replacing them with copper crush washers solved the problem.

[Chayal]

I thread seal all the fittings. Make sure hoses are sound. Quality clamps.

Tissue test has helped me find leaks. I use mmo in my fuel so showed up bright bright red. If you have a sharp ear you can pressurize and listen as suggested, that can be a tough one depending on environment

[Martin S]

Latest on this that may help others. I replace all fuel hoses. The old ones were 20 years and were A2 spec to the old ISO 1999 standard. Changed to A1 to ISO 2013 which have much lower porosity. Used good clamps, I like ABA brand. Also replaced the seals on the top of the Separ filter. I can’t be 100 percent sure this is fixed as it was taking 9 days for enough air to accumulate.

[FPNC]

It is possible, when a fuel tank is installed lower than the engine, to have fuel drain out of the fuel return line and into the tank over long periods. This can allow air to migrate backwards into the fuel system.

[Wizo]

And don't forget the copper crush washer on the fuel bleed -anneal it or replace with new to be sure.