Mercruiser 5.7 Alpha One Overheat about 2600PRM

[Rock Hall]

About a year ago I developed a problem with my 89 Mercruiser. Almost every time I run over 2500 RPM or so, the heat alarm goes off. My temp gauge does go up a little when this happens. So far in an effort to solve the problem I have replaced the Seawater Pump, Circulator Pump, Thermostat, both temp sensors, Exhaust Manifolds and Risers. The problem does seem to be a LITTLE better. I was out by myself with little weight on board and I brought her up on plane and ran about 3200 with no alarm. This was immediately after changing out manifolds and risers. Then yesterday with 4 people on board, alarm went off about 3000 RPM. I'm sure the problem is restricted water flow somewhere. I can run all day long at 2000 RPM and she runs nice and cool.
I do have a major issue with marine growth in the slip I am in now. Brackish water in a creek. I'm thinking there is some growth somewhere in the water path between the outdrive and the engine. I'm looking for some advice on what hoses etc I should be checking first or if anyone has any other ideas. Now that I've replaced all the expensive parts, its just a matter of checking the path between the components. Is it possible for the water jacket in the engine to get clogged up?
Mark

 

[starsnstripers]

Re: Mercruiser 5.7 Alpha One Overheat about 2600PRM

Do you have a raw water, fully closed or 1/2 closed system? Do you have the check balls in the T on the therm housing? I doubt the passeges are blocked by any thing being sucke into the drive intake. If ther was a pevious disinergrated impeller there could be blockage from the pieces in the system.

 

[Rock Hall]

Re: Mercruiser 5.7 Alpha One Overheat about 2600PRM

Do you have a raw water, fully closed or 1/2 closed system? Do you have the check balls in the T on the therm housing? I doubt the passeges are blocked by any thing being sucke into the drive intake. If ther was a pevious disinergrated impeller there could be blockage from the pieces in the system.

Raw Water. Do have check balls. they seem to be moving freely. Old impeller looked fine when I replaced it. I am probably going to pull the boat out in the next few weeks. I know there is a hose going from the top of the out-drive to the transom at the pivot point of the drive. this is one of the first things I want to check. While I have it out, I dont want to miss an other likely culprits.

 

[mike456]

Re: Mercruiser 5.7 Alpha One Overheat about 2600PRM

you could be sucking air. Does the overheat happen when you get up on plane? If so it could be a crack in the waterhose that runs through the transom or some issue with the water pump. I had a similar issue. you can check by putttin a piece of clear hose in the section that runs from the power stearing cooler to the engine. Get it at Home Depot for $15. It should be pretty much solid water when running. If you see alot to air bubble when up on plane that this your problem. Now you just have to find the source of the air. Good luck

 

[Rock Hall]

Re: Mercruiser 5.7 Alpha One Overheat about 2600PRM

Thanks for the info Mike. No so much when Im up on plane, but while Im attempting to plane. that is the hose I was planning to replace. from what I can tell, I should be able to replace that hose without tearing everything apart. you might be right on this. Thanks Again!
Mark