I have a 1970 Mercruiser 165. Low hours on rebuilt engine. Has new waterpump (base, oil seals, oil rings, impeller housing cover, etc.) New shift shaft seal and s.s. shift shaft. New bellows, shift cable and bellows. New prop shaft seals (both). Runs at mid-green level on gauge. Decided to swap out thermostat with new and behold, no thermostat was installed. Put in new Sierra thermostat, the right one for the engine. Ran up to temp on lake in low idle and temp now in high red on gauge. Ran it around for a short while to see if temp would go down. It never did. Pulled thermostat back out while on lake and temp went back to mid green and stayed about there for a full day of water skiing. We are in San Diego, CA and water temp in lake is always just right. Motor is very clean. All runs great except without thermostat. I stay on top of all maintenance and I like things to operate the way they should so looking for pointers on troubleshooting overheat with thermostat in problem. Thanks!
