Mercruiser 496 (8.1L) overheating

I think the concept of Seacore pertains to "salt-proofing" that which hangs in the water all the time (the outdrive). There was a time when Merc coated the internals of the manifolds with some sort of ceramic stuff but that was another bad design that failed.

Merc designed the seawater side of the engine internals (in a closed cooling system) to drain when the engine shuts down. That's why there are no anodes in our cooling system components. Problem is that oxidation can only occur when there is oxygen present. If the system remained flooded with seawater all the time it couldn't oxidize. So yeah, we don't have zincs to change but in a saline environment, the manifolds and risers are inevitably going to rot away in 5-10 years. At least they moved to a stainless exhaust elbow.
 
I'm having a problem with overheating and the engines going into "safe mode". It only happens over time when running pretty well full rpms. The temp gauges read 175 when it drops to guardian mode. I live 100 miles from any mercruiser dealer that can work on my boat. I replaced the water pumps just last year. I just had the boat out and the raw water seems to be fully flushing. I am going to replace the thermostats and try to drain the old (don't know how old) coolant and replace. There seem to be multiple temp sensors. Does anybody know how many sensors and at what temps they are set for? I assume that the sensors in the exhaust manifolds are set much higher than the coolant sensors. Also, does anyone know the "shop" manual for merc 496's (2010 model) that will show how to replace the thermostat and how to drain the coolant. I have the service manual #30.
 
I have 2009 sea ray Sundancer with 8.1s. Move is v drives so can’t be a outdrive issue. This year I had the exhaust manifolds and risers done. Then one motor was running warmer than other. Temp started rising to almost 180. Other engine would stay 168 and all rpm. Had heat exchanger done thought problem was solved for two weeks. Motor was running at 168, then it all came back and running at 180. Mechanic is baffled. Fresh water is not a problem but had pump changed anyway. I am so frustrated and desperate. Any Ideas? Can new heat exchanger clog up in ten hours of use?
 
Verify your gauge is reading correctly. Possible thermostat sticking. Blocked oil cooler......... check all this as any of these can contribute.
 
I have 2009 sea ray Sundancer with 8.1s. Move is v drives so can’t be a outdrive issue. This year I had the exhaust manifolds and risers done. Then one motor was running warmer than other. Temp started rising to almost 180. Other engine would stay 168 and all rpm. Had heat exchanger done thought problem was solved for two weeks. Motor was running at 168, then it all came back and running at 180. Mechanic is baffled. Fresh water is not a problem but had pump changed anyway. I am so frustrated and desperate. Any Ideas? Can new heat exchanger clog up in ten hours of use?

Your thermostat may not be opening enough and restricting coolant flow from the exchanger to the engine itself. Replacing the thermostat will mean removing the exchanger again since it is underneath it.

Another possible issue is the engine's "water" pump which is on the front of the engine and run by a belt driven pulley. It's rare, but if it shears there's no circulation of coolant.

Good luck finding the cause!
 
Your thermostat may not be opening enough and restricting coolant flow from the exchanger to the engine itself. Replacing the thermostat will mean removing the exchanger again since it is underneath it.

Another possible issue is the engine's "water" pump which is on the front of the engine and run by a belt driven pulley. It's rare, but if it shears there's no circulation of coolant.

Good luck finding the cause!

I have been told the circulator pump never goes but I will try the thermostat and then if needed the pump. It has been a process of elimination and I have not found the cause. Very frustrating so far. Thank you confirming next proper steps
 
So did you ever find the cause? Was it the actual manifolds or do you think it was that plugged up fitting?
I’m having the same issue. my temp gauge reads 179 at 3/4 throttle or 3,500rpm when guardian mode kicks in.
runs fine at 3,000 rpm. 172 degrees at gauge.
why is it kicking it into guardian mode when it’s running at 179 degrees???
Error message reads OVERTEMP
 
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So did you ever find the cause? Was it the actual manifolds or do you think it was that plugged up fitting?
I’m having the same issue. my temp gauge reads 179 at 3/4 throttle or 3,500rpm when guardian mode kicks in.
runs fine at 3,000 rpm. 172 degrees at gauge.
why is it kicking it into guardian mode when it’s running at 179 degrees???
Error message reads OVERTEMP
My bet is the engine temp is not kicking you into guardian the exhaust manifold temperature is, and that is not what is displayed on your dash.
 
So did you ever find the cause? Was it the actual manifolds or do you think it was that plugged up fitting?
I’m having the same issue. my temp gauge reads 179 at 3/4 throttle or 3,500rpm when guardian mode kicks in.
runs fine at 3,000 rpm. 172 degrees at gauge.
why is it kicking it into guardian mode when it’s running at 179 degrees???
Error message reads OVERTEMP

A quick and easy thing to check is the end cap gaskets on your engine's heat exchanger. Mine had elevated temps at cruise and excessive steam plus apparent lack of water flow vs the other engine. The water pump looked fine and no blockages evident anywhere in the exhaust track. Thought about it and decided to check if the heat exchanger was blocked. Turned out that the rubber gaskets on both of the exchanger end caps had degraded/expanded so much that it was blocking the seawater flow into the heat exchanger. Bits of the rubber gasket had even broken off and were lodged in some of the cooling tube openings. Once removed and new gaskets installed, both exhaust water flow and temps went back to normal.

Just a thought and easy to check!
 
A quick and easy thing to check is the end cap gaskets on your engine's heat exchanger. Mine had elevated temps at cruise and excessive steam plus apparent lack of water flow vs the other engine. The water pump looked fine and no blockages evident anywhere in the exhaust track. Thought about it and decided to check if the heat exchanger was blocked. Turned out that the rubber gaskets on both of the exchanger end caps had degraded/expanded so much that it was blocking the seawater flow into the heat exchanger. Bits of the rubber gasket had even broken off and were lodged in some of the cooling tube openings. Once removed and new gaskets installed, both exhaust water flow and temps went back to normal.

Just a thought and easy to check!

I changed out that gasket last year. Mine was also expanded. Thanks for the tip. water flow appears to be normal.
the water coming out the rear during backwash seems a bit hot in my opinion though. mom my other boat the water was warm if I remember correctly but on this one it’s hot. Anyone have temp levels this water should be exiting the motor.
 
My bet is the engine temp is not kicking you into guardian the exhaust manifold temperature is, and that is not what is displayed on your dash.
Definitely displayed OVERTEMP (maybe I missed something) because the error code disappeared after it got into idle.
Although the other three or four times that ithe beep went off there was no message on the dash. so you think my sensors are bad at the manifold? I changed the sensors out about 2 years ago.
or blockage in the riser or manifold?
 
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Overtemp but not the closed coolant side that you read 170’s. The manifold sensors will put you into guardian mode I believe at 210 degrees. Need to be hooked up with computer to read those. Usually caused by low water flow through the exhaust manifolds. My 8.1 had that, ended up being a melted water tube coming through the transom from the previous owner running it too long on the flush connection.
 
So did you ever find the cause? Was it the actual manifolds or do you think it was that plugged up fitting?
I’m having the same issue. my temp gauge reads 179 at 3/4 throttle or 3,500rpm when guardian mode kicks in.
runs fine at 3,000 rpm. 172 degrees at gauge.
why is it kicking it into guardian mode when it’s running at 179 degrees???
Error message reads OVERTEMP
Have you put a thermometer on it? I had the a very similar issue with our 280. Temp within reasonable range but going into guardian mode. At the tie I couldn't measure the temp, but I still could put my hands on the water cooler sections of the manifolds so I knew it really wasn't overheating.

In my case it turned out to be a water temperature sensor that went bad and was reporting the wrong temp.
 
So the port side water exiting is very hot compared to starboard side. Much more water coming out on the starboard side. Is this normal? I don’t remember how it was before. it’s my understanding that the port side does run a bit hotter.
while touching the risers at normal 158 degrees temp I can keep my hand on both for a while.
At this point I think I have a water restriction somewhere on port side exhaust water area. thoughts?
 
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So the port side water exiting is very hot compared to starboard side. Much more water coming out on the starboard side. Is this normal? I don’t remember how it was before. it’s my understanding that the port side does run a bit hotter.
while touching the risers at normal 158 degrees temp I can keep my hand on both for a while.
At this point I think I have a water restriction somewhere on port side exhaust water area. thoughts?
Thoroughly check out, cleanout your heat exchanger. The port side of your motor gets the raw water last, so it has already been heated up by the rest of the system. If not heat exchanger, look at Auxiliary coolers. Per steering, oil, etc. If you do not know the condition of your manifolds a risers, need to pull n inspect. That is the entire system. Of course your pump needs to be 100% as well.
 
So the port side water exiting is very hot compared to starboard side. Much more water coming out on the starboard side. Is this normal? I don’t remember how it was before. it’s my understanding that the port side does run a bit hotter.
while touching the risers at normal 158 degrees temp I can keep my hand on both for a while.
At this point I think I have a water restriction somewhere on port side exhaust water area. thoughts?

Yeah. The pump is 100%, impeller just changed a few months ago.
Will have to inspect everything.
thanks.
 
When you did impeller, dud you inspect and correct any scoring issyes?

Yes, scoring inside the pump body can reduce the effectiveness of the pump. There is a kit I've used on one of my pumps that incorporates two stainless wear plates and a slightly shorter impeller (to account for the stainless wear plate thickness). It didn't seem to make a difference in my case since the problem was later found to be the expanded rubber seals on each end of the heat exchanger.

In this person's case, the symptoms (low water flow out of the exhaust with higher temps) seem to point to either a pump not operating properly or a restriction within the raw water system.
 

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