Yanmar 2GM20F random shutoff + irregular revs

[julianhessel]

Hi all,
Towards the end of last season my Yanmar 2GM20F shut off suddenly with no warning. My first suspicion was a clogged fuel filter, which hadn't been changed in a while. I tried bypassing the filter at sea and connecting the fuel line directly to the engine, but it still didn't start.
I have a small backup engine, which got us back to shore without issues that time.

On the way to haul-out, the Yanmar started again. It ran fine briefly, then developed irregular revs (see video). Below around 2000 RPM it would eventually cut out.
Above that, revs would dip intermittently and recover on their own, without touching the throttle. The fuel line was still bypassing the filter during the video.
Any ideas what could cause this?

 

[merjan]

I would first make sure that there is no air leak into the fuel lines.

 

[julianhessel]

I would first make sure that there is no air leak into the fuel lines.

Thank's I'll start there!

 

[Boater Sam]

You will come to regret running the engine without a fuel filter............................................................................

 

[dansaskip]

Does sound like a fuel supply problem, a blockage, air leak or even a blocked fuel tank breather pipe.

 

[merjan]

Did you bleed the engine and consistently have air-free diesel coming out of the bleeding screws? Also indeed, I would put the coarse filter back in (especially now that you have ruled it out).