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4.3 Mercruiser Overheating


jlogger

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I have a 1996 4.3 liter that has had some cooling system issues since late last season. I thought it was a Zebra Mussel issue but apparently it wasn't. It will run at a consistent 150 when I keep RPMs below 3200 or so. When I run engine at higher speeds, the temp gauge quickly fluctuates between normal temp (150) and completely cold. After a couple of miles the engine then begins to overheat. I installed a new thermostat last season when the problem started and had the impeller and bellows done last fall as maintenance. The problem existed before and after all of this work was done. I searched and saw some postings other places that indicate a possible water pump problem. I am worried that a faulty head gasket compression seal may be pushing compression into the water jacket. I am hoping I am very wrong on that suspicion. Anyone ever experience or hear of this?

 

Thanks

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You have to start with the water pump in the outdrive.

I know you said it was done, but what your describing is very -outdrive water pump- sounding. Boat in the water, pull the hose coming from the outdrive water pump off where it goes into the motor or heat exchanger and stick it in a 5 gal bucket and time it. Then measure how much water it pumped in the time (you can find what it is supposed to do in many places on the net).  Then you will know if the problem is the outdrive water pump or ahead of it.

 

One time the pickup tube from my outdrive water pump got pinched when assembling it and caused the same problems. 

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Get yourself a infrared heat gun. When the gauge shows the temps shoot your exhaust manifolds, and the heads, these numbers should be with in reason, this will detect which side has a issue or there is no issues other than a faulty sending unit or gauge or hopefully a wire issue. Before getting into the bigger things.

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7 minutes ago, L&M said:

Good talking yesterday Jack, let me know how you make out.

 

Your in good hands now!! Best of luck!!

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 I had an overheating problem with my 260hp 350 merc that I tried to figure out for a couple of years. Mine was running hot while at low, trolling speed. I did everything I could think of to fix it. Changed the impeller, thermostat, water pump, temp sensor and even the temp gauge. After a long, time consuming couple of years If found an article about the check valves in the cooling system. it's just a couple of nylon balls with springs on a threaded rod. I'm not sure what they are designed to do. I bought a set for under $15 and took care of my problem. I think the some 4.3s have these. Your problem sounds a little different than mine but might be worth a shot. You can actually adjust them by tightening and loosening the balls. Maybe Hank can tell us what these are.

 

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They restrict the amount of water that flows out thru the risers/elbows at idle/low rpms & helps keep a steady flow going thu the engine when the sea water pump isn't pumping to capacity due to the lower rpm's .

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  • 1 month later...

Well the verdict is in. Apparently my lower housing on the stern drive corroded between the exhaust and water jackets. I was pushing exhaust into the incoming water. Took out my brand new water pump too. Hopefully fixed and back in water by next week sometime. Just figured I would update in case someone ran into similar problem.

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5 hours ago, jlogger said:

Well the verdict is in. Apparently my lower housing on the stern drive corroded between the exhaust and water jackets. I was pushing exhaust into the incoming water. Took out my brand new water pump too. Hopefully fixed and back in water by next week sometime. Just figured I would update in case someone ran into similar problem.

:yes:

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