KAD 42 - Interesting Overheating Problem

Dear Gents,

I have Wellcraft Martinique 29 with single Volvo Penta KAD 42-B

When cruising between 1500 RPM - 2500 RPM at around 7 knots - 14 knots, there is no any overheat problem.

However, when at full throttle around 3000 - 3500 RPM, engine starts to heat and then overheat and alarms rings.

After getting the boat Neutral, heating goes down to its normal within 30 seconds or 1 minutes time.

How an overheated engine can go to its normal heat within 30 seconds ?

What sould the problem be ?

Thx

Could be a lot of things. Here is a list to check off.

Salt water impeller - when was it last serviced
Salt water pump - how old is it and even if the impeller is new, when were the wear plates etc last checked
When was the salt water intake last checked and cleaned
When was the exhaust elbow last checked
How are your coolant levels
When was your thermostat last checked
How clean are your props and hull

Those are many of the most common ones, but there are other less common possibilities. How many hours on the engine and how long has the boat been sitting in the water since the hull was last cleaned?

Definately sounds like a restricted flow of water or a weak raw water pump. Have the heat exchangers been serviced lately? Could be a partially blocked intake strainer for the raw water system also.

When it overheats, the temp gauge reads high AND you get the alarm?

I would add the intake hose to your list. I had the same happen on a VP gas engine.
The rubber lost its integrity over the years and if the stern would leave the water in
a large wave situation, it would draw air, and collapse tight.
Then overheat alarm. Throttle back to low idle, the flow in the hose would be restored,
and the temp gauge would return to normal in about 20 - 30 seconds!