Overheating at idle

[The Pirate]

I have an '06 1800 SR3, with the 4.3. Had the impeller replaced last year by very competent mechanic, Mercruiser part.
Running at speed, temp stays at 175 all day. Pull back to idle speed and temp will creep up. Giving it any amount of throttle will drop the temp.

Just an annoyance as long as the temp drops...Any ideas what this could be?

Thanks for your input.

[mmwjr]

This is somewhat common if the temp doesn’t go to high. The engine is working hard and produces heat, when decrease rpm the heat is still there but there is also a decrease in cooling water flow that removes the heat and some temperature rise can occur.

[The Pirate]

Thanks for your response. Good to know....

[AACoPD05]

My friend has the same year, same boat, same motor but he gets an overheat alarm at idle too after running fast. Any suggestions on how to improve cooling? He has also replaced the impeller. I read that 140 degree thermostat is an option.

[The Pirate]

With mine, when i first got it, temp was perfect, went to 175 and didn't move. Orig. owner had done zero maintenance. I have had impeller changed twice in 5 years, either by professional shop, or very skilled mechanic.
He indicated there was a lot of rust coming out of it when running on the muffs. Would running it on the muffs, open blue drain plugs help remove rust, sediment? Never heard of anyone actually doing that....

Most of my running time is at 3,000 rpm steady, up and down a fresh water lake. I was hoping just constant, steady use would help flush it out.

It is very manageable, just an annoyance. Does your friends engine have the oil leak up front? I use a drip pan and oil absorbing sheets......Again, very manageable....but annoying.

[mmwjr]

Originally Posted by AACoPD05

My friend has the same year, same boat, same motor but he gets an overheat alarm at idle too after running fast. Any suggestions on how to improve cooling? He has also replaced the impeller. I read that 140 degree thermostat is an option.

The over temperature alarm sensor is around 200*F, he should verify that the thermostat housing is actually getting that hot with an IR sensor. If it is there is a problem that needs to be addressed. He should also verify the temperature while running.

[mmwjr]

Originally Posted by The Pirate

With mine, when i first got it, temp was perfect, went to 175 and didn't move. Orig. owner had done zero maintenance. I have had impeller changed twice in 5 years, either by professional shop, or very skilled mechanic.
He indicated there was a lot of rust coming out of it when running on the muffs. Would running it on the muffs, open blue drain plugs help remove rust, sediment? Never heard of anyone actually doing that....

Most of my running time is at 3,000 rpm steady, up and down a fresh water lake. I was hoping just constant, steady use would help flush it out.

It is very manageable, just an annoyance. Does your friends engine have the oil leak up front? I use a drip pan and oil absorbing sheets......Again, very manageable....but annoying.

Why not fix the leak? Front crank seal?

[AACoPD05]

Not sure about the oil leak but I will mention to him about checking the temp while running, thanks!