240 2006 sea Ray Sundeck overheats at low RPMS

[johnsimeone]

If the problem was in the manifolds, wouldn’t the boat run hot when underway? The engine cools down when at 1000 rpm or above, heats up when going slowly.

 

[alldodge]

We can keep guessing for a very long time, until you do some testing we may never know

 

[johnsimeone]

Understood. I appreciate the help. What’s your opinion about it possibly being the circulation pump?

 

[Lou C]

You need to get an IR temp gun and take readings of the manifolds, elbows and the intake manifold right under the thermostat housing. The manifolds and elbows will start to run hotter as the internal passages clog due to the natural corrosion of cast iron. That produces flaking rust which starts to clog passages. I don't know what's normal temps for a raw water cooled merc exhaust, but I know they have 2 types, the cold manifolds (one hose feeds the manifold and elbow) and warm manifolds (separate hoses feed the manifold and elbow). The cold manifolds system is like what OMC and Volvo used, these run pretty cool (95-100*) until you get up on plane and then the temps will rise to about 140* or so. The warm manifolds don't get cooling water till the engine thermostat opens so they runs about as hot as the engine, maybe a little hotter. The elbows always get cooling water.
See what the temp is on the intake manifold right under the 'stat housing with the engine warmed up. It should be a bit lower than the rating of the thermostat, if you have a 160* stat, it should be about 145-150 or so.
What does your dash gauge read? with a 160 stat it should range from 160-170 or so.
In salt water about the longest I'd go on set of manifolds and elbows is about 8 seasons, and that's under ideal conditions.
On my boat I check the inside of the elbows with a borescope camera for leaks and I monitor the exhaust temps often. As soon as I see anything abnormal I replace them. Costs a bit of money but there is no way around it.
When you replaced the impeller, is there a wear plate inside, and did you also replace that? If they get scored the impeller won't pump well at low rpm. That can happen even from just sucking up sand.

 

[Lou C]

Understood. I appreciate the help. What’s your opinion about it possibly being the circulation pump?

Unlikely, these get replaced usually because the bearing gets noisy or they start to leak at the bearing. I have replaced only 2 in 24 years of salt water boating.
Likely causes:
impeller housing, wear plate, etc
marine growth in the outdrive water intakes (I clean mine out usually 2x a season because it is on a salt water mooring)
rust flakes in the exhaust manifold/elbow
passages in the thermostat housing necking down (getting restricted) due to cast iron rusting
sticky thermostat
suction leaks that allow air in the cooling system (clear hose test)
Bravo drives have a fitting in the transom mount that gets necked down due to corrosion, this will restrict the volume of cool water coming in. (that's Bravoitis)
These systems depend on a high volume of cool water in and warm water out, to keep the engine and exhaust at normal temps. Anything that restricts that, will cause overheating.

Merc has a specification for how much water the raw water pump should be able to pump out, with the boat in the water. It would be a good idea to do that test, that will tell you a lot. You can have problems on the intake side (raw water volume) and/or on the exhaust side