Overheatng Problem

[sweetseason]

have been fighting and running out of money for the past 3 years with an overheating problem. Several mechanics - most of which I will never talk to again, cant seem to figure it out. It is an I/O 470 merc with the heat exchanger. Both water pumps have been replaced. (the one in the outdrive and the one on the motor)
Please help
The engine will run all day out of the water and work fine (hooked up to a hose)
The engine will in the water all day at slow speeds
The engine will overheat if run for about 20 min at plane speeds
The engine coolent is expelled to the reservoir at this time until it is all expelled and overflows onto the engine block.
The thermostat has been replaced and I can watch the temperature gauge and see when it kicks in at slower speeds.
At high speeds I watch it go up and keep going up untill it overheats.
What is going on?
I plan to dissasemble the entire cooling system myself in the spring but I dont know what to look for considering everything That pumps water or coolent has been replaced.
One mechanic suggested I find someone to hook it up to a dynometer? Everytime I take this boat in it is a $400 dollar bill so I have aready spent more than this boat is worth. Bought the SELOC repair manual for myself for christmas but I dont know what I am looking for. Anyone have an answer for me so I can get out at more than 15mph? 1979v20 470 merc
Thank you
Paul

[cterrebonne]

i would call a local merc dealer and ask a tech what he thinks. if its a dealer that has been around they probably have seen that allready.

[macojoe]

Who put the heat exchanger on?? There is a restriction someplace!! Not allowing the water to flow.
I know a guy that put manfold gastets on backwards and pluged holes that would have stayed open!!

[garagenc]

Originally Posted by macojoe

Who put the heat exchanger on?? There is a restriction someplace!! Not allowing the water to flow.
I know a guy that put manfold gastets on backwards and pluged holes that would have stayed open!!

I agree Mac, It sounds like there is a wrong gasket installed or installed wrong or corrosion has caused a blockage, or something has been sucked up in there and is blocking one of the passages. When I took my old motor out I found that my drain plugs were blocked with rust/crud, might be the same thing there.
Can't you bypass the heat exchanger and run the motor that way on plane to see if you can isolate between the motor or exchanger?

[spareparts]

first thing I would do if it hasn't been done(allways start with the obvious) check or replace the resevoir cap(radiator cap), if it doesn't have enough spring pressure it will allow excess coolant out of the engine into the over flow, casuing it to overheat. As I'm writing this i think of something else to check, Do a compresion test!!!! The 470's have unsupported cylinders where they seal on the head, they can and will leak cylinder pressure into the cooling system, usually accompaning low performance, skipping and coolant mixing with the oil( but not allways). One other thing that pops into my head is the exhaust riser, if it is clogged, it will not let enough raw water out of the system for cool water to replace it, especially when running higher speeds, check your exhaust bellows for signs of running hot. Pull the riser and flip it over, inspect the water outlet holes, hook a garden hose to the riser and see how well the water flows out of it More ideas keep coming up, did you check the water pick ups? remove the pickups and make sure there arn't any leftover water pump impeller pieces in there. I ran into a 3.0l that was doing the same thing last summer, tried everything, finally pulled the water pipe and hose out of the transom plate, it had a piece of an impeller lodged in it, we had back flushed the line several times, but untill we ran a snake thru it, it never budged. There are a number of things to check, these were the first that came to me that I've seen in the past causing overheating. Good luck

[THEFERMANATOR]

Like spare suggested, check for compression entering the cooling system. If your losing coolant under load, you may very well be losing compression to the cooling system. Get somebody else to go with you on a run and remove the pressure cap and put it under load. If bubbles appear your losing compression to the cooling system. If you don't see bubbles, then I would disconnect your raw water line from the gimble at the heat exchanger. Then start the engine and see if you have good water flow. Then disconnect your outlet from the heat exchanger to the exhaust elbow and check there. You may also try putting a pressure guage inline after the heat exchanger but before the exhaust elbow and watch the pressure when your problems occur. This will tell you if you have water flowing, or if you have a major obstruction.