My 260 sundancer 2004 engine heating

The mercruiser 350 MAG MPI engine heating I have checked all the waterlines and replaced the impeller in the sea water pump as well. The thermostat was checked and the water lines were cleaned still the problem exists.I was advice that u need to fill the block with fresh water and the sea water intake is only to cool the fresh water is it correct. Any advice is appreciated.

There are a few different ways your engine could be cooled.
Can you take a picture and post it.
That would help us help you!

hope this picture is ok Thanks for all your input.

That motor is raw water cooled. All of the cooling water is supplied by the raw water pump. No need to fill the motor separately.

how about some maintenance history?
How many hours on the motor?
When were the manifolds and risers last changed?

What is the actual engine temperature? Should be 160° Fahrenheit.

Does it stay cool at idle speed and overheat at higher engine speeds?

That will be a b111 drive so given the age its going to be a restricted water passage through the transom assembly. Common problem! Your engine is fine so look at the supply passage on the transom assy.

2) when was the last time did you replace the manifolds and elbows?

3) I'm assuming you are in saltwater? I know what happens when you assume. Lol

You can check for the restriction fairly easily by checking the water flow rate (amount) out of the sea water pump. Your engine is fairly simular to mine. So you can do that by taking the large hose off the back of that round grey thing at the bottom of the port side of the engine.
Its got the little blue drain plug.

If you pull the plug out with the engine running you should have a decent amount of pressure. If you take the big hose off you should get a lot of flow. Like fill a bucket quickly.
The shop manual specifies how many gallons per min.

Don"t sink your boat. It should stop flowing with the engine off.
If its barely flowing the problem is the pump or the transom restriction or a clogged hose.

If its good flow, all the water must exit thru the manifolds and then the risers and then the exhaust. The risers usually corrode at the point where the water exits into the big rubber hoses.

So...cold water has to get in and the hot water has to get out. If it doesn"t you get over heating. Usually I check both ends and then the middle!

I hope this helps!

You can also remove the plugs on the underside of the exhaust manifold to see if it’s full of crud.

If they are more than a few years old, you’ll probably not have any water draining out with the plug removed. As it’s blocked by mud scale, rust or any combination of those.

look to see if you have a filter in the inlet hose. could be plugged with weeds or dirt. Most people remove them