4.3 Mercruiser V6 I/O- Cooling help

[Bassknuckles]

Think outside the box here as almost everything has been changed. My 4.3 Mercruiser V6 I/O overheats once it goes above 2,800 rpm and only under load. It doesn't overheat at the dock or under 2,500 rpms. Once I start to over heat and I bring the boat to neutral it cools down in 30 to 45 seconds. I'm not a mechanic so some of my parts or terms may be off but so far and close to this order:
1. New water pump
2. New Thermostat
3. The block is not cracked
4. Risers and exhaust are good (floppers checked also)
5. Hoses are new
6. Removed the closed fresh water cooling system (now cools off salt water)
7. Changed all gaskets
8. New water distribution housing
9. New Thermostat housing

Also the sensors are good as the engine was checked with a heat gun. The timing is correct and no growth or blockage on the outdrive. Any idea what else...

 

[Aquarius]

What does No. 6 mean? Where does salt water cool the system and where does the fresh water cool the system?

 

[Bassknuckles]

The engine had a half closed cooling system on it so the engine block was cooled by anti-freeze and not salt water. It was removed thinking that it was the problem.

 

[AllyKat]

I don't see a new impellor in all the things that are changed.

 

[Bassknuckles]

Yea that was changed also. I'm guessing their is a blockage somewhere. Thanks.

 

[Avenger]

What's the history? Was this sudden or gradual?

 

[Bassknuckles]

It's been a problem the last two seasons. It may run warm for 1 or 2 trips then have issues and run hot. I believe their is a flow issue somewhere. Next trip I'm going to take the heat gun and measure the temp in certain areas of the engine to see if it's only on one side or certain area on the engine

 

[Avenger]

That's what I would be doing next. When you say risers and exhausts are good, when was the last time the risers and/or manifolds were replaced?

 

[Bassknuckles]

Risers were checked last year. New elbows this year

 

[Avenger]

Does it still use the engine circulator pump in addition to the raw water pump? Has that been looked at?

 

[Bassknuckles]

Does it still use the engine circulator pump in addition to the raw water pump? Has that been looked at?

I don't know, I will add it to my list for the mechanic

 

[Old Mud]

Have you changed anything substantial on board since last season ? Changed the prop ? Who installed the impeller ?

 

[Macks]

Do a compression test,could be a head gasket..