Mercury Outboard heating up

[tear-a-bite]

hi all,

my 200HP Outboard (1991) has been running hotter than usual the last 2 trips out, and the last time out the gauge went red-line. i'd been going really slow for 5 minutes or so (finding that magic spot to drop anchor) and that is when it got too hot. when i throttled it up the temperature went back down, though it was still warmer than usual.

there is quite a bit of water coming out of the motor, though not high pressure. so i figured it was the impeller. i replaced the impeller (it looked fine) and everything went back together fine.

the problem is, it is still running hot, and i don't want it ever to get TOO HOT again.

any ideas out there what to try next? any inputs are appreciated.

thanks!

[5 Salt]

Waterjackets or key port in the colling system is clogged.

That's assuming a with replacement impeller did in fact go back together properly

[fishkisser]

Thermostat not opening up properly at temp ...
Replacing it will allow you to take a look at the water passages in one area
to ascertain if you have corrosion or sand blocking passages ...

[MattPark]

My money is on a stuck popoff valve, or something stuck in it holding it open. They are an often overlooked routine maintenance item on the V6 Mercs. Thermostats are a good thing to check as well, but a stuck thermostat would likely heat the engine up at all rpm, not just idle.

It should be closed when idling to allow enough water pressure to reach the top of the block. At higher rpm (and higher water pressure), they open up and bypass water. If it's open at idle, there will be little pressure, and water may not make it to the top of the block. Your engine will idle hot if that's the case, and cool down as you start moving and the rpms come up.

The valve is on the starboard side of the engine, below the cylinders. There are 4 bolts holding it on, the cover is one piece but each side is a circle. You can get it off without removing the lower cowling, but it's easier just to remove it so you have good access.

It could be debris holding it open, but as long as you have it apart, replace it. My guess is it's almost 20 years old, and your engine depends on it.

A water pressure gauge helps troubleshooting these cooling systems. They are easy to install and inexpensive.

[Hunt'nFish]

I guess the first question to answer is ...is it REALLY getting too hot or is the sensor giving false reading?

Assuming the temp gage is reading right, I'd replace the water pump impeller just to eliminate that from the list of suspects. Probably needs changed anyway.

After that timing, motors can run hot if not tuned correctly.
Good luck,
Hunt'nFish

[budnate]

Matt is 100% right, more good motors have been lost to pop off's sticking and thermostats sticking...guys never do them till its to late.

if that Merc see's salt, you should run a good dose of salt away through it and stay with it, the stuff works!!!

[Just-in-Time]

water pumps
t-stats
popetts

every year or 100hrs

[tear-a-bite]

thanks for all the inputs. another question - where is the popoff valve(s) and thermostat located on the powerhead? i'm having trouble locating them, and the manual i have doesn't show them.

thanks again

[Just-in-Time]

www.dougrussel.com

use this to look up diagrams