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Mercruiser overheating

"Need some help with temp gaug

"Need some help with temp gauge fluctuating from 0-140 and then to high 200's alarm goes off and gauge drops back to 0 and so on.The engine is a 250hp 5.7litre with bravo outdrive.I have grounded out temp gauge and it pins over so i don't think its the gauge.I have replaced the raw water pump,thermostat,manifolds and risers.I did find parts of the impeller in the thermostat housing.Engine runs cool at risers and therm housing.Also replaced temp sensor.any ideas please."
 
"Just wanted to let everyone k

"Just wanted to let everyone know who tried to help me with this problem THANK YOU. I finally figured it out. It was the starboard head. I knew I should of changed both heads when I changed port side due to low compression in #1.
The problem, which I also found on port side, was water leaking around the valve guides. #2 cylinder was really bad it may have even been cracked which was putting cylinder heat into the water stream. It seams the 454 has a problem with the guides leaking water. I don't know. Could depend on how previous owner took care of it for the last 22 years, anyways I put a new reman. head on and it now stays at 150 degrees. I can tell you one thing, there is not much I don't know about a merc. open cooling system now
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"Pual, Somehow your post got i

"Pual, Somehow your post got in with mine. Start a new post to get some replys. Did you change the sending unit? Some mercs also have a separate sender for the overheat alarm which also may be going bad."
 
"Paul, the temp gauge sender i

"Paul, the temp gauge sender is on the starboard side and the alarm sender is on the port side, never use any sealant or teflon tape (needs to make a good ground connection) the fact that both are wacked at the same time, could be sticking thermostat. ?"
 
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