MerCruiser 4.5L 250hp NO EC - OVERHEATING!

[leosmdp]

Hi guys im new in this forum looking for someone who can help in this huge problem that I already put many hours into and still happen to me.
If anyone can help or give any tips about this project I would be more than welcome! or maybe know a good mechanic who know about this!,here around north miami / Fort Lauderdale...

I bought this boat used in a deal for a guy who I guess notice the condition had made only a couples flushes in the system in this poorly but nice 2015 Motor MerCruiser 4.5L 250hp NO EC- ( this model doesn't have a Heat exchanger and raw pump in the same motor belt) perhaps is on the alpha sterndrive. I guess this is the oldest version of this kind of motor.
The Motor has only 105 HS. I bought a Vessel View and checked. Compression Ok Start at the moment.
The problem Overheating.

I took all the cooling water systems out and cleaned as much as i could.. Here are the details.

1- Hoses every single one I cleaned from all the metal sediment.

2- I noticed that the RISER AND MANIFOLD were in good condition, perhaps a little rusty inside but I put some rust remover and afterware on them, Ospho Surface Prep.(acid) (they don't have any cracks on the walls inside and the outside are solid double checked with a endoscope camera).
I know is better to change it, but im working on that 2600$ right now
I attached a garden hose in every single hole and water went out without issues at all. and didnt notice any crack and fugue in the exhaust system… Maybe here the flow is restricted but water moved ok and with good flow.

3- FLUSHED every single hose outside and put a hose inside the motor as well to clean it. (lot of dark water came out here.)

4- WATER PUMP cleaned , checked with a camera and all the vanes are there. It looks ok and twists smoothly. (it was hard to check cuz is sealed)

5- THERMOSTAT the old one was in good condition but anyways i bought a new one. 140 F.

6- POPPET VALVE was with some sediment and with signs of use beginning of material fatigue over the rubbers ( so I changed the DIAPHRAGM and origins) and tested from one to another hose the water moves ok.

7- On the Mercury Alpha Gen 2 i replaced the RAW WATER PUMP for a new one and changed the gear lube and anodes as well.

8- Today i took out the CROSSOVER clean deeply, water moved ok but anyways, i did the job.

9- I put CLEAR VINYL HOSES from crossover to the manifolds and notice when I start the motor and when it is on, from the crossover only gas with a thin water flow travel. When I stopped the engine water came back from the check valves to the crossover again…
Question here IS THIS CORRECT? or have to flow full of water from the crossover to the manifold?

i'm guessing that maybe

1 - I need to change the HEAD GASKETS? maybe for the previous overheat they burned and is passing exhaust gas to the cooling system?
2- I need to replace anyways for NEW manifold and riser ( beyond the water flows there but maybe more restricted?)..

Beside that i also changed oil filter, new oil, new spark plugs, new pcv valve, new belt.

If any one over there can give me more tools or knowlodge about this motor to check it would be more than welcome.

 

[leosmdp]

attached more pic here from the process

 

[leosmdp]

im ran tests in salt water and fresh water connected a garden hose to the sterndrive inside a bucket with water and a dual flushed rubber system connected..
here atteched with temp gun some temps.

 

[Golfman25]

So you’re kind of all over the place here. First things first. Replace the raw water impeller. I think they are buried in the alpha drive itself. Then work up the system from there.

 

[PascalG]

It’s been 30’years since I ve had to deal with a raw water cooled gasser but just replace the raw water pump. Housing and impeller. Housing could be worn and you don’t know the history

Water flow thru the risers using a garden hose isn’t a valid test since the hose base a lot more pressure than the pump buried all the way into the drive.

 

[leosmdp]

So you’re kind of all over the place here. First things first. Replace the raw water impeller. I think they are buried in the alpha drive itself. Then work up the system from there.

that was the first thing I did! (it wasn't so bad I guess some one remplace before but anyways I change everything) and afterward the manifolds and riser and all the rest listed

 

[leosmdp]

It’s been 30’years since I ve had to deal with a raw water cooled gasser but just replace the raw water pump. Housing and impeller. Housing could be worn and you don’t know the history

Water flow thru the risers using a garden hose isn’t a valid test since the hose base a lot more pressure than the pump buried all the way into the drive.

that's why I'm thinking of changing it at this point and after all that I did in replacing the manifolds and riser...
or the head gasket (wishing they were ok..)

but my question here is.. from the crossover when the motor is running should travel from the hoses air or water to the check valves?
in my motor when is running travel hot air with a thin water flow and when I stopped water came back from the hose ...
here I attach the Cooling system diagram.

 

[BillK]

What you are calling "crossover" is really just where the water exits the engine block and flows through the exhaust manifolds - there should be a good water flow, not air. 200+ for an exhaust manifold is way too hot, should about 120deg, you can usually lay your hand on them without burning. You have a water flow problem - other place to check for blockage is the power steering cooler. Also, a water hose provides only the minimum amount of of water flow, you can't really run the engine much above 1000-1200 rpm. If it were me my next steps would be to check the PS cooler and then do a water flow test. Unhook the water intake host that runs from the transom to the thermostat housing - run the engine and let the water pump into a bucket - just for a minute or so - you should be seeing a strong flow of water.