Help needed! Chronic overheating, Mercury 50HP, 2 cycle, 3 cylinder, 2002

[upwindskipper]

Motor will idle all day with temps in the high 120's on the thermostat housing. As soon as I put it in gear and advance throttle to 2000 PRM's, temp starts climbing until it sets off the alarm at 180 degrees. Here's what I have done so far: new thermostat, Mercury service guy rebuilt the water pump with new impellor and base plate, I have run a salt-away cycle, flushed with vinegar, flushed with water down from the T-stat hole and up from the water tube. Signal stream cools off at higher RPM's (90 - 100 degrees) then warms up at idle to 120 degrees.

Am at my wits end, summer almost over and I have yet to go boating. Will appreciate any and all thoughts.

 

[Ant-Knee]

Do you have a good flow of water coming out the tel tale?

I had similar problem, with my 98 merc 150.. Would idle at 120s, and climb as I gave it more throttle.. then over heat alarm would sound...
I had water coming out of tel tale as it should.. When I pulled lower unit off to check out the water pump, Turns out the plastic tube going up to
the metal tube was cracked.. Which lowers the water pressure.. Changed entire water pump ( Impeller, gaskets, and housing) and all is good now...

Just a thought..

 

[upwindskipper]

Telltale stream is good. I replaced that tube with the new water pump kit. The old one looked fine. Top cylinder heats up first so water is just not getting there. Is there anyway to check for a blockage without taking the powerhead apart?

 

[schematic]

best be checking the poppet valve if your engine has one.....

 

[upwindskipper]

I finally found the source of the overheating by pulling the powerhead and replacing the copper water tube. The tube was OK but the rubber centering seal at the top flange had folded in creating a minor blockage that bcame a major blockage over time, was calcified and covered with salt. Of concern is that all of the exhaust channels in the exhaust plate and around the top of the exhaust housing were packed with carbon. I had to chip it out with a screw driver.

Does anyone know what would cause that?

 

[racerone]

Poor quality oil / fuel can result in carbon build up.