1983 Four Winns mercruiser 488 3.7L overheating HELP

[Carter20]

New boat owners- let me preface with that.
We bought a 1983 Four Winns Marquise 180 last summer. Ran great all summer, until it didn’t. Made a loud sound and engine cut off. We thought it was the lower unit, so we replaced that with a used one with a new water pump. Had some electrical issues so it wouldn’t start….. finally got that figured out after replacing distributor cap, spark plugs, and some wiring and got it fired up.
Then took it out, and now it’s overheating. Replaced the thermostat and added 8qts of coolant, but it still overheats. Doesn’t start to overheat until increased rpms on the water, but once overheated, stays hot unless we shut it off.
We’ve checked all the hoses and there are no blockages or leaks, and we are stumped. My husband has been through the entire manual and we cannot seem to figure it out. PLEASE HELP

 

[Scott06]

Inspect the exhaust manifold and elbow for restrictions is this boat used in salt water

 

[Carter20]

Nope freshwater. We’ve looked at that, and it looks good. Is there something we may have missed?

 

[Eric 3.7LX Alpha 1]

You replaced the entire drive with different one and a new impeller. Did you open the old one up and look at the condition of the impeller to see if any pieces were missing? They may be blocking flow to the heat exchanger. These 470 based motors are very sensitive to overheating usually resulting in a blown head gasket due to the different metals and expansion rates of the aluminum block and iron head.

 

[jhamblett]

Take the end caps off the heat exchanger and make sure the tubes are clear.

 

[kenny nunez]

Another quick check is to remove the drive pump supply hose from the heat exchanger, with the flush out or the drive in the water start the engine and see if it is possible to use your thumb to stop the water from coming out.
If you can then the problem is out side the boat. Either in the drive or the water hose between the bell housing and transom plate. You should not be able to stop the flow, if you cannot then as someone suggested the heat exchanger or very possible the head gasket is the problem.

 

[QBhoy]

Hi. Almost certain that your water pump on the lower starboard side of engine will have failed.

 

[Bt Doctur]

Q, thats attached to the cam ans made of aluminum , Something in the used drive or a bad head gasket

 

[QBhoy]

Q, thats attached to the cam ans made of aluminum , Something in the used drive or a bad head gasket

Oh. I might have another engine in mind BT…I thought it was the 3.7 4 pot. With the water pump down there that usually has the issue. Sure they have an indication when they starts losing their seal. Then the thing overheats and warps the head seal, boiling over the internal cooling