1996 OMC Cobra (Ford 302 / VolvoSX collaboration) sometimes runs hot

Lpgc

Petty Officer 1st Class
Joined
Jun 17, 2023
Messages
303
During light throttle / slow cruising (not on the plane) the engine temperature is OK but sometimes (not always) the temp soon creeps up to around 210F when on the plane at any speed, though it seems more likely to creep up at lower planing speeds than at fast planing speeds or full throttle. It doesn't always overheat, sometimes the temp gauge stays at 140/150F even at slow planing speeds.

I've only had the chance to watch/check the engine while the boat was being driven on the plane once (when my daughter drove the boat), I could feel slight pulsing on the widest diameter coolant pipe which connects between what would be the top and bottom radiator hose connectors if it were a car / truck. I don't know if the pulsing I could feel is due to pressure changes in the engine internal cooling system due to the thermostat changing how much raw water it allows into the engine (which would be OK), or boiling/steam in the engine cooling channels (bad) or something else.

What could be at fault to cause the overheat and what could be causing the pulsing in the rubber pipe I could feel? I have not tested pressure from the raw water pump but I did disassemble it last year and it seemed in good condition. The engine water pump also seemed to be in good condition when I rebuilt the engine. The same thing occurred when I used the original thermostat and a new replacement thermostat.

This is the engine I rebuilt last year shortly after buying the boat. I'll post a link to the thread I did regards this boat last year but be warned it's a long read, mostly me asking newby questions and not engine related but there is some engine related stuff on the link https://forums.iboats.com/threads/1996-four-winns-sundowner-with-5-0-cobra-engine-and-drive.766501/

Thanks in advance, Simon
 

kenny nunez

Captain
Joined
Jun 20, 2017
Messages
3,443
Remove the hose from the drive to the raw water pump. Try reverse flowing that hose into the drive with the drive up, look at the plastic through transom fitting, it could be cracked.
Was this happening before the rebuild? The other problem could be a leaking head gasket which builds enough pressure in the water jackets that it overcomes the raw water pump pressure.
 

Lpgc

Petty Officer 1st Class
Joined
Jun 17, 2023
Messages
303
Thanks Kenny,

When I get the time I will do as you suggest.

When I bought the boat the engine was in bad condition with cracked exhausts allowing water into the cylinders and little compression on a few cylinders, so there was engine vibration and I only got it onto the plane a couple of times before the rebuild.... I don't remember if it did the same then but until recently it seemed to run too cold more often due to debris holding the thermostat open.

For winterisation I left the cooling system (engine, exhausts, pas cooler, raw water pump, rubber hoses) full of glycol based coolant instead of leaving it completely empty, I hope that doesn't damage parts of the cooling system?
 
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