50Hp Mecury 2Stroke outboard after six months back in the water. After driving for 10 min an alarm goes off. Thewater outlet was running, but the top part of engine was really a lot hotter than the botoom part. Wait 20 min tilt the engine while runnig a couple of times and could feel the outlet water getting warmer and the top part of engine cooling off. The alarm disappears and after runnig hard for 30 minutes it seems the problem is solved. Could it be an air-bubble in the engine-water compartment for that spesific time? How can I prevent something like that happening again? I'm glad an alarm warned me otherwise I would have thought everything is OK , because the outlet cooling water pipe is flowing strong.
Matt

If it only happened one time, you may have caught something on the water inlets that temporarily restricted the water flow. If it reoccurs, you may need to replace the water pump impeller or the pressure relief valve. Water pump impellers generally should be changed every 12 months, and generally will not last for more than 3 years, even less if the engine isn't run regularly.