Engine RPM's change

[Harry Gafney]

I have an 86 Hunter 31 with a Yanmar 20GM diesel with fresh water cooling. Recently, the engine just slowed down, RPM dropped, without any change in throttle position. Just slows down on it own then comes back up to speed, but after ten or fifteen of these cycles finally stalled. Did not matter whether prop engaged or not, and engine seems fine since will start again.Seems like fuel system. Any suggestion on how to diagnois and service.Thanks, Harry

 

[Bob La Salle]

Same thing happened to me...

... a few weeks ago, on my 1985 Hunter 31. But, my engine then eventually quit and would not re-start. I got towed back to my marina, and a friend and I found a completely clogged fuel filter, the one near the fuel tank, not the one at the engine. I replaced the fuel filter (Fram C1110PL, $8.00 at West Marine), bled the lines (my first time and was not that difficult), and it started right up and has been perfect ever since. I will never trust my yard to change my filters anymore, will do them myself at least once per season. In my case, I bought this boat in September 1998 and it now appears that my yard has not been changing the filters every season as I requested. My friend looked at my old fuel filter and he said it looked like it was in there for years. E-mail me (robertclasalle@cs.com) for any further help and to let me know what you discover. Good luck.

 

[Mike McKendy]

Here's another possibility

Harry, I had a similar problem with my 2GM20F. Here's what happened. At the bottom of the pick up tube (that's the tube the extends into the fuel tank to almost the bottom from which the fuel flows to the filter and, ultimately the engine)there is a small screen. My engine would surge, then eventually stall. I took the tube out and, lo and behold, it was completely clogged with gunk. I removed the small screen and presto it's cured.I heard this same thing happen to numerous people. I also was told by mechanics that this screen should not be here - that's what the primary and secondary filers are for.It's easy to remove the pick up tube. You might want to try this,Mike

 

[Doug Margison]

Could be either

You definitely have a clog in the system somewhere. The intake in the tank is a common problem. Also either filter could be the problem if not been replaced recently, but more likely the primary filter. We recently had a similar fuel problem with our Hunter 30. I change the primary filter from a 110 Racor to a larger 220 Racor and so far have had no further problems. However, I do still plan to take the intake out of my fuel tank and remove the screen if one is found. I have heard that these screens can cause a lot of grief.Regards,DougSV Mokum II

 

[Bob Rutland]

Low Fuel

I know it's a dumb question, but someone has to ask! Are you very low or out of fuel?This will cause your symptoms.  Bob

 

[Jerry Smith]

Had similar problem...

Doug,I recently had the same symptoms in my Hunter 31 with the 2GM 20 Yanmar. I replaced the lift pump, both filters and bled the system of air. The problem continued. As it turned out the problem was in the exhaust elbo. The internal structure had rusted enough for the water pressure to cause the rusted internal metal to partially close off the exhaust. I was O.K. at low RPMs but if I tried to accelerate the engine would surge and eventually die. I replaced the elbo and it runs great. If your RPM changes are at all speeds it is most likely fuel. If the problem seems to appear at a particular RPMs you might check the elbo.ThanksJerry Smith