Mariner 150 overheating?

[seanfawcett]

I have a 1993 Mercury Mariner 150 Magnum II which has always run really well.

Last time I ran it, however (first outing of the season) the temperature gauge climbed to about 200 degrees in 5-10 minutes at 3000 rpm and the warning horn sounded. I shut the engine down and let it cool. I started the engine again and again, running at 3000 rpm the warning horn sounded after about 5 minutes. I shut it down and let it cool.

Then I tried something else. I started the engine and ran at 4000 rpm and the engine ran really cool, about 140 degrees, and remained at that temperature for the remainder of the outing. I have been able to reproduce this behavior.

What could be causing this?

Thanks

 

[Texasmark]

On my son's 150 of approximate vintage was a water line of about 3/8" coming out of the top of the rear of the engine block and disappearing just below the powerhead. I am told the high flow pop-off valve is located there. On smaller engines it opens at about 2500 RPM and essentially bypasses the thermostat meaning that the engine runs 10 or so degrees above surface water ambient.

Below 2500 RPM 2 thermostats, one at the top of each bank, pellet stamped 143F on his regulate the flow through both sides of the block and the temp runs just above that opening temperature.

First outing of the season and then finding differences in your outcome tells me you have stat and or PO valve age related sticking problems that the first exercise of the season loosened up. I'd change both stats and the PO valve.

 

[faztbullet]

First thing to do is replace the impeller...