Before taking my boat (2000 Monterey 240/ 5.7 Volvo SX) out yesterday, I replaced the sea pump impeller. Pump was working fine but I had not replaced the impeller since I bought the boat a few months ago. Overall, very simple job. After replacing the impeller, put the muffs on the outdrive and turned on the water to force water into the pump. Then started engine to make sure all was workingproperly. So far, no problems. Approximately 2 hours later, I put the boat in the water at the lake. Engine started as normal but engine temp quickly rose past normal temp (approximately 165) and quickly went past 200 so I shut the engine down. After a few minutes to cool, I restarted the engine to maneuver back to the dock. Engine temp immediately started dropping and settled at 165. We decided to putt around close for a few minutes to watch the temp. Guage never went above 170. We decided to stay at the lake but watch the temp closely. Spent approx 6 hours on the lake, temp guage never went over 170. I don't know why the engine got hot initially- any suggestions?