Mercruiser 3.7/470 Overheat Problem

A

Alterry

Cadet
Joined
Sep 1, 2014
Messages
27
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.
 
K

kenny nunez

Captain
Joined
Jun 20, 2017
Messages
3,458
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.
 
K

kenny nunez

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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.
 
A

Alterry

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Joined
Sep 1, 2014
Messages
27
Alterry said:
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.
Click to expand...

kenny nunez said:
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.
Click to expand...
Thanks Kenny, I’ll try the hose check and see if that works
 
A

Alterry

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Joined
Sep 1, 2014
Messages
27
kenny nunez said:
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.
Click to expand...
When I remove the hose from the heat exchanger inlet and put my thumb over it I get a little water squirting past my thumb, very similar to what you would have if you put your thumb over a garden hose. This is running at idle rpm
 
A

Alterry

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Joined
Sep 1, 2014
Messages
27
Alterry said:
When I remove the hose from the heat exchanger inlet and put my thumb over it I get a little water squirting past my thumb, very similar to what you would have if you put your thumb over a garden hose. This is running at idle rpm
Click to expand...
A little more info, I just tried it at 2000 rpm, I’m able to almost completely stop the flow with my thumb. It does pump water through, almost a gallon in about thirty seconds
 
A

Alterry

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Joined
Sep 1, 2014
Messages
27
kenny nunez said:
You still should pull the head.
There are 2 types of 470 head gaskets, the ones that are leaking and the ones that haven’t started leaking yet.
Click to expand...
Thanks Kenny, that’s the direction I’m leaning. There’s a local machine shop that will rework the head for $350. The mechanic is familiar with the 470 and they do a lot of engine work so I’m pretty confident they’ll do a good job.

The water hose in the bell housing probably has ten years or more on so it is suspect, but I didn’t see it leaking when I checked. I’ll pull the bell housing while the head is out for repair, might as well get after everything there. Now all I need is time to do it.
 
K

kenny nunez

Captain
Joined
Jun 20, 2017
Messages
3,458
The head comes off pretty easy, however there are some bolts under the intake manifold going into the head which cannot be seen from the top.
Also be sure to tell the machine shop to replace the air injection plugs in the front and rear of the head.
 
kd4pbs

kd4pbs

Petty Officer 1st Class
Joined
Mar 5, 2012
Messages
258
The 470/488 is like sushi. When it is good, it is amazing. When it is bad, it might kill you. Fastest boat I have ever owned has a 470. Amazing power from such a light engine. No wonder they are sought after so much by hot rod builders.
The Ford head is dead simple to work on. I have to wonder if an aluminum aftermarket head would be a worthy investment for someone who has to replace one anyway. I imagine someone has done it and shared the experience.
 
R

ROY WILLIAMS

Chief Petty Officer
Joined
Aug 8, 2022
Messages
443
Alterry said:
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.
Click to expand...
1978 MCM470 ENGINE [WILLIAMS]. heater exchanger 50% water distilled - coolant antifreeze 50%.
engine oil 30wt 5.5qt. ENGINE LUCAS TB ZINC - PLUS ZDDP 4 OZ. outdrive water sea water / water pump. gasoline filter water. gasoline 90 OCTANE. ENGINE 650 RPM IGNITION 4 DEG BTDC.
470 MCM 4cyl piston compression pressure 140lbs.
 
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