Mercruiser 3.0 135hp overheating randomly

[bjohansson]

I have just bought a Maxum Bowrider (93) with a Mercruiser 3.0 135hp. During the last trip the engine temperature suddenly rises very rapidly. As I slowed down the temperature remained high, and then suddenly dropped back to normal. I continued driving and after about 10 min (at normal temperature) it suddenly rised again. I slowed down and felt that the in-water hose was very warm and at low pressure. I turned off the engine to let i cool down. Then it ran ok the last 20 min home.

It felt like the cooling stopped working completely, and then started working again. Could it have been problems with the thermostat? Since it's so "random" it does not sound like the impeller. I know that the previous owner recently changed the water pump.

Any ideas what could cause this random overheating?

/Benny

[CDK]

Replace the thermostat. All of these engines develop temperature issues: small waterborne life deposits calcium on the copper thermostat. In some cases it doesn't close anymore, in others it refuses to open until the cooling water reaches boiling temp.

Do not postpone it, the over-temperature rapidly damages the exhaust manifold.

[gonzo]

Those engines had a series of updates on the thermostat housing and water bypass hose (if installed) Sometimes, they have a mis-matched set of castings or the original faulty ones and do that.

[MCDenny]

I had this problem with a Volvo. A fitting was corroded through letting air into the water intake hose when on a plane. Off plane, the hole was under water and the cooling system worked fine.