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HP 500 overheating coming off plane

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HP 500 overheating coming off plane


Old 07-14-2017, 09:08 PM
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Join Date: Jun 2011
Posts: 5
Default HP 500 overheating coming off plane

Every time I come off plane the temperature gage goes from 140 to just under 240.

I have tried different thermostats, replaced the impeller housing and impeller, completely flushed out the oil cooler numerous times, and even ran a coathanger wire through every hole to ensure no rocks. Checked the water connection at the transom assembly to make sure there was no corrosion inside that, Flushed out block holes in the bottom of the block. Ran boat on hose and temperature gauge went up to just over 180. Took temp gun and all temperatures seem to be around 140 except just below the (I believe itís the temp sender on stbd side) I shot the manifold and itís around 160-170 Which is a bit hotter but not matching the gauge. The engine overall is definitely not running hot according to the gauge. I had a mechanic replace the sender thinking that was the problem. Now Iím thinking I need to replace the gauge?? Someone suggested I might have a small head gasket leak but I thought that if I did the temperature would not come back down like it does
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Old 07-15-2017, 12:56 AM
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Join Date: Oct 2000
Location: Omaha
My Boat: .......32............. ............Active Thunder....
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If the alarm is not going off, its not at 240*.
The high water temp alarm uses a different sender than the guage sender and the alarm would be going off by 200*.
Most likely a bad guage.
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Old 07-15-2017, 05:30 AM
Join Date: Feb 2012
Location: westville, NJ
My Boat: 1967 34 Hatteras summer house. 2000 Crownline 248ccr her boat. 79 Pantera 24 Sport.
Posts: 3,996

when you ground the temp gauge wire, it believe it goes to the full hot peg. might be a wire moving forward and hitting something as you decelerate. possibly something inside the gauge acting the same way. how long does it stay reading hot?
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Old 07-15-2017, 02:13 PM
Join Date: Aug 2002
Location: Roanoke VA
My Boat: 1986 Formula 357-SR1
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Just a thought- have you checked the pump impeller? Could it be torn and not able to pump enough water to absorb all the heat when you drop the RPM's?
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Old 07-19-2017, 08:02 PM
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Join Date: Oct 2000
Location: Chesterfield Mo/Loto
My Boat: 98 42 fountain twin 500's
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I have I similar problem only in the opposite direction I run at 140 while on plane 2500-4000 come off plane and drops to lowest reading 100 on the gauge as soon as I throttle back on plane it climbs back to 140. im a twin engine boat and the other motors stays right at 140 once warm and may run a little warmer when running hard. any suggestions, don't want to hijack the thread but may be the same or similar situation

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