Mercruiser seacore 5.0 v8 overheat
 

Hi guys hope someone can help!
I've just bought a second hand mercruiser seacore engine, 5.0 V8, 300hp, and had the mechanics start it on ground which they did before putting it in my Larson Senza 226.
The engine has fresh water cooling system.
When testing for the first time in the water, It overheated, alarm went off.
Before dropping it in the boat, ive had the impeller replaced, like I mentioned ran it on ground however im not sure there were temperature gauge attached then.
My outdrive has just been re-done with seals and bearing changed. I have changed thermostat, flushed the water jacket, risers and heat exchanger, ran water from hose in all pipes on engine and had really good flow. Had nil blockage whatsoever. I had change temperature sensor from my previous engine and same temperature reading. However i bought brand new ones which im guna put on it. I have no water in engine oil and oil looks clear and normal..
The temperature just rise from engine startup and doesnt go down at all even when left idle..
Can anyone help?

year model and engine serial number ? is this an Alpha outdrive or Bravo app ? I am taking it this is an ECM 555 controller base Mercruiser being in a 226 Larson - prob around 2006 year model ?

ALSO Merc never had a 5.0 MPI anything that was 300 hp. They did with 5.7 HO MAG MPI --- so do you have a 5.7 MPI with a Bravo outdrive ???

Hi it's a 5.0 MPI 305 cid.. think its a 260hp when checking it online. I have a bravo outdrive with it. Engine serial number is 1A030156. Seacore model.

So you have close cooling ? I have not ran your engine serial as I am busy. Seacore were closed cooled engines Just checking to make sure it is what you are saying it is.

That is correct it is 260 hp for the 5.0 mpi but earlier models were 240 hp.

1A engine serial number should be around a 2008 year model is that correct ? - just going off the top of my head here.

Impeller and housing last was changed ? the Gen VII impeller housing is a pour design and weak material used. They scar really bad causing decrease water flow thru out the engine. But there is more info as well after I know exactly what you have.

Yes. It is a closed cooling system. Uses a heat exchanger which I also had it taken out and flushed. However all the small pipes in it was clean and open.
As for the impeller housing, I only changed the impeller and not the housing.
I believe it is the 2008 model.
Can it be I have a clogged head cylinder and since water cannot go through causing it to heat up as the engine was sitting for a while. But what's weird is when I ran fresh water throughout the engine, there were no blockage nor debris,acually nothing but water. So if the cylinder head was blocked I would have seen some materials of some sort from the engine block aswel right?