I have had several Yanmars with cooling problems. The first thing I would do is replace the sending unit. DON'T buy the after market sender for a YM1700. Get the proper Yanmar brand part #124450-44902. The after market unit has a lower temp setting. If that doesn't fix it, have the radiator rodded out at a radiator shop. 50-$70. Make sure it has the shroud on the radiator and the fan and belt are in good condition. If everything is right you should not need a water pump. The water hose flush and flow test usually does not work.