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2005 5.0 MPI overheats. Risers are cool to touch, thermostat…

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2005 5.0 MPI overheats. Risers...
2005 5.0 MPI overheats. Risers are cool to touch, thermostat is out , water temp 60, new impeller in raw water pump. Think block water jacket is filled with corrosion. Anyway to flush it out?
Submitted: 3 years ago.Category: Boat
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Answered in 17 minutes by:
5/27/2015
Marine Mechanic: Jason, Marine Mechanic replied 3 years ago
Jason
Jason, Marine Mechanic
Category: Boat
Satisfied Customers: 18,217
Experience: Degree in Marine Technology. Gas and diesel marine mechanic.
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Hello my name is ***** ***** whom am I speaking with?
These are not automotive engines and believe it or not they will overheat without a thermostat in them. A thermostat acts as a restrictor, and it slows the flow of water though out the block. With it removed water flows through the block to quickly, and it will not take the heat with it.
So it definitely needs a working thermostat in there. Do you have a new one you can install?
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Customer reply replied 3 years ago
yes , had one in and it did the same thing. Took it out hoping to flush out block
Customer reply replied 3 years ago
Difference in water presuure between both engines with no thermostat seams to be the issue. As I said risers are cool so flow is occurring. How can I flush out the block?
Marine Mechanic: Jason, Marine Mechanic replied 3 years ago
Good morning. Did the boat run aground, or did the sterndrive hit the bottom and stir up a bunch of sand? I ask because the engine has to come out in order to flush it out, and I want to cover all other possiblites first.
How hot does the engine run according to the temp gauge?
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Customer reply replied 3 years ago
no top end was rebuilt 2 seasons ago, ran , and i believe was never winterized. Copious amounts of corrosion in intake manifold.
Thought maybe CLR soaking in jacket for a couple of hours might loosen it up enough to allow it to flush out. Figure what do I have to loose for trying. As you said the engine would have to be removed to do it completely.
Marine Mechanic: Jason, Marine Mechanic replied 3 years ago
To flush out the block what has to happen is you have to strip things down a bit so you can get to the core plugs on the sides of the engine. If the engine is out of the boat it is a lot easier. In the boat you would have to pull the exhaust system off to get at them. After that the core plugs would get removed. From there you can force water into the block.
I don't think the CLR would do any good. Besides, you would need about 5 gallons of it to full the engine up.
Do you know how hot your actual risers are getting to, have you measured them with a temp gun? What I would like you to do is install a new thermostat from Merc. Run the engine, and measure the temps of the exhaust and the risers. They can be no hotter than 160F. If they are, they need to be pulled to see if they are plugged up. If they are plugged up that would explain the hot running as well. Does that make sense? --Jason
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