Losing RPM's

[Edmund Shanahan]

My 1989 40 Hunter 4JHE has recently begun to lose RPM's. I took delivery in late March, and the 90 hours engine time for the delivery trip the engine ran like a clock. I started off the trip be removing almost all the fuel, cleaning the bottom of the tank, and changing all filters. So, several weeks ago when the engine gradually died after an hour in rough water, I figured it was Racor filter time. Upon a real good examination, I discovered with dismay the Racor was filtering fuel in the return line to the tank, not in the fuel line to the lift pump. Don't know how I, the surveyor, and the engine mechanic surveyor all missed that one, but we did. The only filter actually on duty was the Yanmar. I replaced the Yanmar, the entire Racor unit, remounted it, replaced and corrected the corresponding hoses. Ran like a clock for about 8 hours, then started losing RPMs. It would recover after a couple of minutes, and go on another hour, same thing. After several hours of this, it became consistent. Starts and runs just fine up until 2000 RPM. Racor sediment bowl is clear. I am reasonably confident in the fuel, I have gone through quite a bit this year. The suction line is date stamped 1997, so it has been replaced. The return line is still dated 1989. Checked and tightened the clamps on the new hoses. I could not find the Racor fuel fittings when I replaced the Racor and had to settle for regular brass hase barb fuel fittings, using form a gasket on the threads. I am concerned about the lift pump and the potential for damage with probably years of being fed directly without filtration. Before I attack the tank and the intake screen if there is one, I would like to rule out the lift pump, or the governor. My assumption has been the lift pump would be a simple go/no go device, so that is puzzling. Any suggestions would be appreciated...

 

[Gordon Torresen]

4JHE - lost RPM

Your tale of the filter connection mix up is interesting. I can see why surveyors would miss it. Who would think to look for it? We are coming into filter change season and will alert our mechanics to look for a similar error. It would be good to hear if anyone else has had the experience. Your loss of RPM on the 4JHE sounds like air. You state that you substituted fittings on the Racor when you moved it. The seal for the Racor fittings is an O ring. The threads are not designed to be used to make the seal. In fact, I think the threads are slightly different from standard pipe thread.  The 4JHE is a self bleeding system that usually takes care of minor air incursions. However, if the air gets to be too much, it replaces some of the fuel volume so the engine thinks you cut back on the throttle and slows down.Also, the Yanmar filter is very fine and can reduce fuel volume if it gets a slug of water. Your clean tank should rule this out. I would get the proper Racor fittings and see what happens. You could test for air by inserting a section of clear plastic tubing after the Yanmar filter. If air is presen you will see it. If the tube flattens out, the filter is restricting flow. Good luck! Plese let us know what you find.

 

[Edmund Shanahan]

Thanks for the suggestion

Thank you for the suggestion on the clear tube, I will try it.As far as the fuel line mix up, one point that does make it tough to see is the Yanmar return fuel line goes to a banjo fitting on top of the fixed Yanmar filter. This makes it appear on casual glance that it is the incoming line. On this particular boat model, because of the engine box, you also do not have a straight visual on the entire Yanmar fuel filter. You look at one side from the side access, and another side from the front access, so in either case, you really on see one fuel line at a time. The clincher is where the lift pump intake lines goes to. It is easy to miss, and in my case, the primary surveyor wrote up the fuel line as being too close to the engine mount, and the engine surveyor wrote up the Racor as not being secured adequately. So a lot of attention was focused in the right area, and still missed the big one.