2008 merc 5.0 mpi overheating

I just bought this boat 26' cruiser io with a 2008 mercruiser 5.0 mpi
the engine is extremly clean and well cared for. However as soon as i put it in the water it started overheating. We checked raw water intake by removing a line to see if water was coming in. Yes it was, and very well to boot. I hooked up a fresh water hose to intake while closing the saltwater intake. Turned on hose eng temp went from 230 to 170 in minutes. However if while im putting pressureized fresh water through the system i open the salt water intake and the fresh water goes out of it instead of continuing through engine, and it overheats. Now this is a pretty new engine 2008. Could the water pump be a problem? Is there another pump called a raw water pump, and could that be the problem? The boat was winterized correctly. There is no water intake from the outdrive, only discharge.
Please help if you know.

Re: 2008 merc 5.0 mpi overheating

Do you have an Alpha I outdrive, or a Bravo? Do you have a raw water cooled engine, or a closed cooling system with antifreeze and a heat exchanger.

You do have a raw water pump, with a rubber impeller, what I am trying to find out is where your pump is.

Re: 2008 merc 5.0 mpi overheating

I have a salt water intake through the hull that circulates through the engine. No heat exchange or anti freeze. The water pump and thermo are on the front of engine near the middle.

Re: 2008 merc 5.0 mpi overheating

chas9747 said:

I have a salt water intake through the hull that circulates through the engine. No heat exchange or anti freeze. The water pump and thermo are on the front of engine near the middle.

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Hi, What Don would like to know is, Do you know if you have an Alpha 1 sterndrive, or do you have a Bravo style sterndrive, Or do you have an inboard engine with a transmission?

Do you have your engine serial number that you could post up for us?

It sounds like you are confusing the engines water circulating pump that is mounted on the middle of the engine! Up with the sea water pump that has a rubber impeller inside a housing that is used mainly to draw the water up to the engine, Where the circulating pump mainly circulates the water inside the engine.

So depending on what style drive system you have, That would help us tell you a little bit more information about your cooling system.

Ultimately what ever system you do have it sounds like you have a bad rubber impeller that you should be changing out about every one to two years as a preventive maintenance schedule.

But for starters we just need to help you find out where your impeller housing is mounted on your engine is all.

Re: 2008 merc 5.0 mpi overheating

I have a salt water intake through the hull that circulates through the engine.

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If you have a a through hull water pickup, you must have an engine mounted, belt driven raw water pump. It will be mounted on the starboard side of your engine, down low. You also may have one of the quick drain systems, either 3 point drain, or single point drain. If you do, make sure it is closed, especially the air actuated system if you have it (not sure if it's even used with raw water cooling). If the air pressure is not completely released, the raw water pump will suck air and not pump water.
If you have a multi point drain system, check the side of the pump. The picture below has 2 drain plugs circled. The one on the bottom is for the water pickup (inlet) hose. If it was left out during winterization, that would cause the pump to suck air, but when you put the hose directly to it, it worked some.
Whatever you find as the cause of the problem, you still will need to pull the pump and replace the impeller. Running without water will destroy the impeller.

Re: 2008 merc 5.0 mpi overheating

I dont know if it's an alpha or bravo. How do nyou tell the difference? What i do know is it's a 2008 merc 5.0 mpi with the pulley's on the bow end of the motor. I have about 3 to 4 inches between the engine and the wall in front of it. It has an outdrive but does not draw water through it. It does discharge water through the outdrive propellers. It cools off with saltwater drawn from hull inlet. When i put it back in slip i can close off saltwater intake and the hook up a garden hose and run clean fresh water through it for a few minutes to clean it out.