350 Mariner Port Engine over heats at idle

[Finest]

I have searched the post but have not been able to find anything that helps as of yet. My problem is that my port engine over heats when I am in gear at an idle, temperature shoots up pass 200 degrees and I have to shut it down. To date this is what I have done.
Raw water side:
Rebuilt Raw water pump, flushed Transmission cooler, Had the heat exchanger removed cleaned tested and re-installed & flushed the raw water system.
Closed loop side:
Replaced thermostat, flushed and replaced antifreeze.

Flushed and changed oil in both engine and transmission.

The unusual thing is it only happens some time and only when I am in gear and at idle to dock or anchour, as soon as I bring the rpms up to 1000 it cools right down to 165. Any help will be great.

[bud37]

Two things come to mind after reading your explanation.Sounds like you have got the raw water side pretty much checked out.

There could be some air trapped in your closed cooling side and the second ,more work, would be that the engine circ ( water) pump has a problem...( impeller vanes corroded,belt slipping,impeller loose on shaft).See if you can verify good water circulation at idle in your closed side first, hopefully something simple.

Another from Penetang , small world.

[bud37]

Also check your engine hoses, sometimes they can collapse on the inside where it is not obvious.With it being only at idle thats what points to a blockage or circulation issue at low rpm.imo.

[Viper]

Do you also have the U cooler at the front of the engine for engine oil cooling? Those often get blocked.

When you rebuilt the raw water pump, was the impeller intact? Did you backflush the system in hopes of flushing out impeller pieces that may have lodged in there from a previous impeller failure?

[After Taxes]

I agree with Viper, because I watched a YouTube video where a boater had your problem, and it was a broken off piece of impeller that was blocking a water passage.