Engine overheats

[Doug Margison]

My 18 hp yanmar is supposed to run at 3400 rpm. When I set it to this rpm it gradually backs off and seems to want to settle in at around 3000 rpm. Last time out I decided to try to hold the rpms at a higher setting and after about 15 minutes the engine overheated and the alarm went off. I have fresh coolant and a new thermostat and there is continuous flow of raw water from the heat exchanger. I would like to get maximum performance out of the engine. Is there something wrong or do I simply have to accept a lower rpm than indicated in the manual? Appreciate any comments or suggestions.Regards,Doug, Mokum II

 

[Steve Dion]

Too high an RPM.

Doug:The Yanmar manual recommends that you run at a sustained RPM of 70-80% of the max rated RPM. On the 2GM/3GM engine this is 3600 max or about 2500-3000 RPM's. Running at the MAX rpms is only suppose to be for brief periods of time.Typically you are just belching black smoke and not moving the boat much faster at the higher RPMS. The 2500-3000 range prevents coking of the elbow and provides good fuel economy too.

 

[Paul Akers]

Me too

This happened to me once when I did the same on my 3HM35F. I was doing it to ensure I could get max rpm's out of the engine with the prop that was on it. A mechanic recommended that I do this. After about 10 mins at that rpm (3200 rpm) I got the same signals.The concensus on this board has always been to run max at 75-80% of max rpm's and reserve the rest for sort bursts.