"The 20-horsepower Yanmar in my Orion 27 has low hours, but last summer it began to stop about a minute after it started. When I tried a restart, it was clear to me that it was being starved for fuel. I bled the fuel line, and, when I saw air bubbles coming out at the secondary filter, I thought I’d fixed the problem. But when I let the engine sit unused for a couple of days, the start/stop problem reappeared.
Your book Marine Diesel Mechanics has some helpful ideas, but nothing you mention in it seems to explain my situation; I know there are no fuel-line leaks or poorly seated gaskets. I did notice that the problem occurred when the fuel gauge showed half full, and the problem seemed to go away after I topped off the tank. You mention return-line siphoning in your book, and I wonder whether the half-full tank is letting air get into the line."
-- Karl Westman, Ocean City, New Jersey
Nigel Calder replies: If return-line siphoning is the problem, it will occur no matter what the fuel level is. Although I’ve never been able to prove it, I believe that fuel-line siphoning takes place when fuel dribbles down out of the return line over time and lets air work its way up into the system. At least I think that’s what happens. That’s why I’m curious that you don’t get return-line siphoning when the tank is full.
If your fuel tank is located below the engine, the lift pump may be having trouble lifting the fuel when the tank is half full; that could make the engine become starved for fuel. You should also check the lift pump itself to find out whether its valves are leaking. But before you do anything, make sure the fuel-tank vent is not plugged. An insect nest in the vent, for example, will create a vacuum when the fuel level drops, making the lift pump work much harder. A plugged vent will create a vacuum only after the engine has been run for a while—the tank will equalize when it’s at rest—and that’s the opposite of what you’re experiencing. But since it’s so easy to do, start by checking the vent.