3GM30 loses power

[Now or Never!]

I was motorsailing, turning about 2000 RPM on a Yanmar3GM30.  Suddenly, the engine lost power.  It wasn't exactly missing but it wasn't delivering power.  I shut down and restarted but with the same symptoms and then wouldn't run at all.  A few days later, it started right up and ran with no problem.  The same thing had happened a earlier, maybe 50-75 motor hours earlier.  Any thoughts?

[Libations Too]

Some ideas: partially clogged fuel filter or over heating.

If it was an over heating problem it may have been a temporary obstruction in the raw water intake...or running without oil/oil pressure.

Good luck!

[Alma]

It also could be as simple as a wasp nest in your fuel vent. Try running with fuel filler cap removed next time it starts to act funny.

One thing's sure- It isn't spark-plugs!

[Dolce_Vita]

In the spring, one thing to watch out for is marine grasses.  A surprisingly small amount caught on the prop will result in a loss of power, and increasing the throttle to compensate, rapidly leads to overheating.  This happened to us last year.

A quick test to try the next time it happens is to momentarily idle down, go to neutral, then to reverse and gun it breifly.  This will unload some (but not necessarilly all) of any grass caught on the prop.  If the power improves when you do this, you've nailed it.  A quick swim over the side may be necessary to clear it fully.

[Now or Never!]

Some interesting thoughts, especially the fouled prop idea.

A little more info, the first time the problem occurred I was approaching a mooring ball at Vero Beach, FL just before Thanksgiving last year.  The second time it occurred I was motor-sailing across the Great Bahama Bank.  I sailed back to Ft Lauderdale and had a friend who is a marine diesel mechanic look at the problem.  Of course, the engine started right up and ran fine.  Kind of hard to diagnose a problem when it doesn't happen!  He suggested polishing the fuel and cleaning the tank and recommended someone.  He also suggested changing the lift pump which I did.  The fuel was clean and there was very little dirt at all in the tank.

[Rusty Pelican]

I would replace the water fuel separator, sometimes they suck in air thus resulting in loss of power.
One of my friends with the same engine had the same problem. It was a Racor with the plastic bowl.
Good Luck

BobG