2008 Optimax Pro XS 250 (Temp question) on a Cougar FTD

[Mark5795]

Today on the water I noticed the temp rising from 123 to 134 then go back to 123 then rise again whenever I was at idle speed in a river. Temp running perfectly fin3. 123 to 125. Lots of water pressure and strong stream. I changed thermostats last spring after they froze up. Runs awesome, I have had the boat 15 years and its taken care of meticulously. Impeller has been changed a few times. I have Merc-monitor gauges. I watch the engine very closely each time I run it but I have never seen it do this? Impeller failing or one thermostat sticking a bit?

It never overheats, doesn’t go above 134 and the stats open before I run it. I watch it at cold start up etc. to make sure.

 

Anyone every experience this and what it might be an indication of?

 

Thanks for any input, right now not too worried but wondering if its indicating a failure coming up?

[apdriver]

At idle, the thermostats control your temp. A 130 degree thermostat would open around 130, let cooling water into engine and start closing again. As the temp started to approach 130 again it would open and the cycle repeats. Perfectly normal.

[Mark5795]

I should have said at wake speed, not sitting at idle. I understand and agree with you but I’ve never seen it go from 123 to 125 to 127 to 130 to 132 to 134 then down to 132 to 130 to 128 to 126 to 123 etc then climbs back up again etc etc like a cycle almost perfect timing every minute or so… like clock work but when I get into deeper water and lower the engine to run it stops climbing and falling….  only while moving at wake speed with motor trimmed up some…  just seems different then what I have ever seen before.

[apdriver]

Mark5795:

I should have said at wake speed, not sitting at idle. I understand and agree with you but I’ve never seen it go from 123 to 125 to 127 to 130 to 132 to 134 then down to 132 to 130 to 128 to 126 to 123 etc then climbs back up again etc etc like a cycle almost perfect timing every minute or so… like clock work but when I get into deeper water and lower the engine to run it stops climbing and falling….  only while moving at wake speed with motor trimmed up some…  just seems different then what I have ever seen before.

What’s it doing at idle only? No throttle.

[Mark5795]

Holds pretty steady.

[apdriver]

Mark5795:

Holds pretty steady.

Does it come up to 130ish at idle?

[Mark5795]

It comes up to temp perfectly fine, its what it does at wake speed that is the issue. At idle it holds steady.

[apdriver]

As long as the temps at idle come up correctly, I wouldn’t worry about it. Cold seize can be an issue if it doesn’t, as you know. Running, the poppet valve opens and the temps aren’t controlled by the thermostats then. Mine typically runs at 120 or so on plane but there’s no set value as there are other variables that will change that. The advice the Master Merc Techs give on BBC 3 liter forum is replace the impeller every year and check your compressor water strainer as annual maintenance, also.

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