Mercruiser 3.7/470 Overheat Problem

[Alterry]

I have an old 470 which I’ve had since new, 39 years ago. It is overheating after running at cruising rpm, say 3500 or so after a few minutes. I’ve replaced the thermostat, sea water pump, and cleaned the heat exchanger. There is water flowing through the exhaust ports and exhaust at the bottom of the bell housing.

The engine will idle at normal temperature all day long, as soon as you take for a run though fluid begins to flow into the coolant recovery tank and the temperature rises

There is no coolant in the oil or oil in the the coolant, and the compression on all cylinders is around 139, there is not much change between cylinders or if the check is made with the engine warm or cold.

However when you perform the running check at cruise that mercruiser recommends, which is to run the boat at 3500 rpm with a clear line attached to the coolant recovery tank and reservoir, fluid does move rapidly through the line. Mercruiser states this is indicative of a head gasket leak at combustion compression pressures that doesn’t show up during a cranking compression test.

I’m wondering if anyone has any further insight on this.

 

[kenny nunez]

You have a head gasket leak. What you are describing is a common problem. Have a valve job done and get the head resurfaced.
You will never see where the gasket is leaking. The cylinders have no lateral support and over time they tend to “walk” from side to side.
I guaranty changing the head gasket will be the fix.
When the head is off be very careful not to scratch the aluminum block surface. 400 grit sand paper on a flat file with mineral spirits will clean the surface.

 

[kenny nunez]

There is 1 other thing you need to check. With the engine running on a flushout or in the water remove the hose from the drive pump and try to stop the flow with your hand. You should not be able to, If you can stop it then the plastic water pocket cover in the upper section of the drive shaft housing is leaking because it is melted. Or the hose between the bell housing and transom plate is the problem.

 

[Alterry]

I have an old 470 which I’ve had since new, 39 years ago. It is overheating after running at cruising rpm, say 3500 or so after a few minutes. I’ve replaced the thermostat, sea water pump, and cleaned the heat exchanger. There is water flowing through the exhaust ports and exhaust at the bottom of the bell housing.

The engine will idle at normal temperature all day long, as soon as you take for a run though fluid begins to flow into the coolant recovery tank and the temperature rises

There is no coolant in the oil or oil in the the coolant, and the compression on all cylinders is around 139, there is not much change between cylinders or if the check is made with the engine warm or cold.

However when you perform the running check at cruise that mercruiser recommends, which is to run the boat at 3500 rpm with a clear line attached to the coolant recovery tank and reservoir, fluid does move rapidly through the line. Mercruiser states this is indicative of a head gasket leak at combustion compression pressures that doesn’t show up during a cranking compression test.

I’m wondering if anyone has any further insight on this.

There is 1 other thing you need to check. With the engine running on a flushout or in the water remove the hose from the drive pump and try to stop the flow with your hand. You should not be able to, If you can stop it then the plastic water pocket cover in the upper section of the drive shaft housing is leaking because it is melted. Or the hose between the bell housing and transom plate is the problem.

Thanks Kenny, I’ll try the hose check and see if that works