air lock

[frlouis65]

I have a 1981 hunter 27 with the original yanmar diesel. I have two questions possibly related.

1. When I pour fuel into the tank it fills very rapidly and I notice fuel coming out of the airvent on the stern. Does this mean the tank is full?

2. After about an hour of operation the motor seems to run out of fuel. I was told that the engine becomes air locked when this happens. After bleeding the engine it runs fine for about an hour again. Does this mean I have ab air leak in the fuel line and how can I find the leak?

 

[Rick Templeton]

Check your vent line. It sounds like you are getting fuel locked in your filler neck by air. Take the vent hose off and blow it out with a compressor. Should clear it. If not, replace.

 

[Ralph Johnstone]

2. After about an hour of operation the motor seems to run out of fuel. I was told that the engine becomes air locked when this happens. After bleeding the engine it runs fine for about an hour again. Does this mean I have ab air leak in the fuel line and how can I find the leak?

This sounds like the most likely cause although you may want to take a look at both fuel filters first.

Thinking out loud about the air leak, try putting some hydrostatic pressure on the fuel line starting from the tank connection by removing the fuel line and adding a short piece of hose. Elevate the hose and fill with fuel so it exerts pressure on the downstream fuel line. Check for leaks.

Sorry, but that's the best I can come up with on short notice.

 

[Saillife]

Finding air leaks can be an absolute pain! First insure that the vent line is clean. You shouldn't have fuel coming out of it unless you've overfilled the tank.

Get a primer bulb for an outboard and some fuel line. Splice this in at a fitting to pressurize the fuel lines. A leak will show up as fuel squirting from the line. If that doesn't work than while at dock run the engine for 15 minuets or so in idle at 2000 rpms or so. Stop the engine and check your filters. I'd start with the one nearest the tank. If they aren't completely full than your leak is at that filter or before it. Check the fittings to make sure they are sealed. If you still have the problem get enough clear line to run from your tank to the first filter. Run the engine and watch the line for air bubbles. It could be the fuel supply line in the tank has a split in it. Replace the clear line with normal fuel line before you go out!

Assuming a small leak the engine will usually run rough / drop rpms for a moment every couple of minuets when a little air gets into the injector.

I had two airleaks this spring when I launched. The first was a 5 minuet job and the second a two hour one. To find the first leak I got a primer bulb and a short piece of hose which I spliced into the fuel line at various fittings. Using the bulb to pressurize the system I finally found the pinhole in one of the fuel lines. That was the five minuet leak. The 2 hour one was in the fittings to my racor filter. Deduced using clear line to insure it wasn't the tank pickup and level of fuel in the filter.

Best of luck
Brian