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496HO over heating

Old 06-12-2011, 07:39 PM
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Default 496HO over heating

I have a 27' Fountain with a 496 HO and last year it was over heating so I thought??? Mercruiser techs came down and hooked it up and it wasn't it was running 175 instead of 190. They replaced the sensor and run fine, now this year it is doing it again???? WTH!!!! Has anyone had the same issue and what should I look for next.... I can run about half throttle and will run fine, when I hammer down the temp gauge runs up to 190????? Then when I back it down temp comes down to norm... Hammer back down same thing temp comes up... Impeller has been changed so that isn't it??? any suggestions????
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Old 06-12-2011, 11:13 PM
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Isn't the 496 a closed cooling system? 190 is not overheating, 250 degrees is.

Is your engine temp warning horn going off?
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Old 06-13-2011, 08:06 AM
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If you lost an impeller you might have pieces in your heat exchanger and reducing the cooling capability. Backwash the heat exchanger and see if it helps.
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Old 06-13-2011, 08:12 AM
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Yes warning lights and buzzer start going off at a little over 180... They did back flush it..
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Old 07-06-2011, 10:08 AM
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It seems that too many Merc 496 owners have problems after impeller change outs and the problem 90% or more of the time is from just replacing the impeller only on the pump without dealing with the grooving that can and does appear on both inside and plate end of the pumps. these groove is anything other that very slight in depth and quantity will cause the impeller not to seal properly at the ends and hence not be able to properly prime at start up and provide enough water pressure over the entire rpm band to keep the engine cool. There are now a few kits available that install stainless end plates in the worn housings with a special replacement impeller to correct this grooving problem. The only other items that can sometimes make priming difficult are, air leak on intake side of the hose, crimped small hose that comes thru Bravo drive at transom plate, or debris in intake or same Bravo transom water pass thru point
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Old 07-06-2011, 11:14 AM
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Originally Posted by bigdron40
I have a 27' Fountain with a 496 HO and last year it was over heating so I thought??? Mercruiser techs came down and hooked it up and it wasn't it was running 175 instead of 190. They replaced the sensor and run fine, now this year it is doing it again???? WTH!!!! Has anyone had the same issue and what should I look for next.... I can run about half throttle and will run fine, when I hammer down the temp gauge runs up to 190????? Then when I back it down temp comes down to norm... Hammer back down same thing temp comes up... Impeller has been changed so that isn't it??? any suggestions????
Have you checked your water pickup for debris or have you changed your drive recently?
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Old 07-06-2011, 11:30 AM
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Wink Stay Cool

Good info and points Baja Man.
Boaters need to realize that almost all marine engines have a raw water pump and feed system that gets raw(what your boating in) water to the cooling system on the engine to cool the engine and oil. If this pump suffers from pressure and volume loss, the engine will start overheating and the "Guardian" type alarms will start alarming and try to help you save a $12K to $50K engine!!
Try not to blame the engine and the boat for your configuration and operation! Very few performance boats have raw water strainers on the inlet side, a lot of performance boaters boat in "CRUDDY" water full of sand, silt, debris and this can quickly destroy a good working water pump. Beaching boats on sand and heavy silt bottoms also can quickly wipe out raw water pump impellers and housings.
Remember to service and check your cooling systems regularly, pay attention to higher than normal and increasing water temps in your cooling system as they are trying to tell you something before the out right failure occurs!
Three things in a boat to "Keep Cool"
one is the Engine
two is the "Operator"!
and three is the BEER!

Best Regards,
Ray @ Raylar
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