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Mercruiser 180 overheating issue

stevekouts

New member
Ive got a 1987 4 brl Merc 180

Ive got a 1987 4 brl Merc 180 3.7L thats been giving me some overheating issues. This is a closed loop 50/50 antifreeze cooling system with a 4" heat exchanger. The engine will overheat under load not a real issue at idle. I am not loosing coolant and the compression is very good. I sent the head out for rebuilding and surfacing no cracks. I had the lower drive unit impeller replaced last season the boat stays in the water. There was plenty of pump presure coming out of the lower unit when I checked it and I can see clearly through the heat exchanger with a light. If no load is placed on the engine and you increase the rpms the engine cools down. Its under a load where it seems to have an issue. The timing is fine as well.

At this point the only thing I can figure is the the exhaust manifold and riser may be bad. They are made of cast iron and might be oem. Not sure what the issue is this is not a very complex cooling system but its got me baffled right at this moment.

Any advice or experience with this issue would be appreciated.

Steve
 
"Have you checked the thermost

"Have you checked the thermostat? ... that's cheapest and easiest, so check that first. Drop it in a pot of boiling water. If it doesn't open all the way, there you go.

I'm assuming the manifold is cooled by the engine coolant circuit (you stated "closed loop"). The riser is cooled by the raw water leaving the Hx. So the riser is the more likely of the two if there is an issue there. Typically when the water passages start to rust, chucks of metal flake off and start to block the passages. The reduction in raw water flow is eventually enough to cause the engine to overheat.

Are you running in salt water or fresh?"
 
"Troy,

There really wasn&#


"Troy,

There really wasn't a lot of coolant flowing that I could see when I pulled off the radiator cap on the coolant tank which is part of the riser assembly. Normally in an auto you see a fair amount of flow once the thermostat opens up. I have been told this is not necessarily the case in this engine but it would make sence if the thermostat was not opening up all the way or at all! I wonder if the mechanic put the thermostat in backwards???

What your saying is the exhaust manifold is cooled by the antifreeze and the riser is cooled by the sea circuit through the Hx. The riser on this unit also bolts to the exhuast manifold and holds the engine coolant tank.

I will double check the Hx for debris and replace the riser if the thermostat isnt the cause.

Steve"
 
"The old backwards T-stat.. it

"The old backwards T-stat.. it's been done before.

WRT to the manifold cooled by the engine coolant circuit, that's the typical set-up in a closed loop system. There will be a gasket between the manifold and the riser that has the coolant passages blocked off to split the closed loop and raw water portions of the system.

It is possible the manifold is raw water cooled as well (not a complete closed loop system). The raw water leaving the Hx goes into the bottom of the manifold then up and out through the riser. You'll have to trace out the hoses to tell for sure, unless there is reason to suspect the hoses aren't connected properly."
 
Circulation pump possiby worn

Circulation pump possiby worn and giving less than optimum flow? Would be more problem when under load.
 
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