Approx 20K on coolant sytem. Having to add water to resevoir about once a week. Temp gauge on dash shows running slightly warmer than it should, If idling too long, approaches overheat. Cools down again by increasing idle. Pressure check of system shows no leaks and doesn't seem to be head gasket. Suspect water pump? Also, what is correct thermostat for this model: 80C, 88C, or 92C or... Anyone know of a defect with the coolant system in this model? Thanks.