5.7 Mercruiser 260HP OVERHEATING

Question
I have a 1985 Sea Ray Sundancer 25?the previous user blew the motor.  I bought a used drop in 5.7 Mercruiser 260 HP (I think the motor was made in the early 90抯) in from a used marine dealer in NH, the motor had only been in fresh water.  My mechanic ran the motor on a stand before buying it.  He checked compression, voltage and engine temp with an Infrared thermometer pointed on the Thermostat housing.  Ran the engine for an hour, at idle the temp was right around 165, throttled up it was around 148.  The motor is in the boat now at idle it was heating up.  The temp would shoot up ?of the gauge on the red zone.  While under way at cruising speed the temp would come right down.   I had the impeller replaced in the leg because the boat had been sitting. Before replacing the impeller the port riser and elbow were much hotter then the starboard, they did not seem as hot after the replacement.    I put it in for a test drive, it was heating up more slowly at idle (still getting hot) but underway and as low as 1400 RMS it was running perfect.  I went out the next day and turned the boat on literally 10 seconds later the gauge went to ? I shut the boat down.  So... I am thinking bad gauge, sending unit or thermostat.  The risers look good and came of fresh water; I put a new exhaust Y in before installing the engine

Answer
Hi Scott...thank you for selecting the undersigned to reply to your question.

I'm not a mechanic, however, your symptoms suggest the fresh water intake has been checked for obstructions, of which there were none.

Based on the temp of the port riser prior to replacing the impeller, and the lower temp afterward, I suspect there could be an obstruction within the water journals on the port side. In the event there's no build-up of water in the bilge because of leakage from a fractured blocke, which when purchased was not given an electromagnet test by your mechanic, the problem is obviously not a faulty thermostat or defective electrical devices. So, in all likelihood, its an internal engine problem that requires more testing and exploration.

My main concern is the fact that when the motor is at idle, it heats up, but when run at full throttle or less, there's no problem. It appears there's an intermittent internal blockage occuring and you need to resolve it before stalling and requiring a tow in inclement weather and sea conditions, which could jeapardize you and your passengers.

That's my thought. Check it out.

Safe Boating
Ron