1994 Mercury 200 2.5 offshore overheat alarm at higher speed.

[garbubba]

Hey guys, had a buddy come over from New Orleans Saturday & we ran out to Horn Island (in the pitch dark) for the night.
On the way out I was creeping along just on plane, maybe 2600 rpms, then when there aren't anymore channel markers to hit, I picked it up to 3000, after a minute the alarm started to sound. I pulled back to neutral checked, pee is coming out fine, hot but not crazy hot, cowling not hot, no smoke smell except 2-stroke, while I'm checking the alarm goes off. We're halfway now so off we go at 2700. No troubles, nice night, breezy morning. On the way home pretty much the same thing, seemed that it might have been getting worse, alarm went off at 2600rpms so I just slowed down for the last bit.

Only change since last trip was a new stator and plugs.

I don't have an oil alarm, that is long gone. So this has to be heat right?

Motor ran like a top.

No restriction at the inlet, pee seems as strong as ever.

I remember having to replace the poppit valves many years ago, I seem to remember something like this being the cause, could that be it?

I'm due for an impeller, I'll put that on my list, but I have doubts that that's it.

Looking for ideas.....

[phatdaddy]

I'd change the t-stats also. Do you have a temp gauge. I'd also recommend using OEM parts, not sierra. When you change the impellor, get the kit with the housing.

[garbubba]

No temperature gauge. I have a laser thermometer, thought about using that, not much help without a baseline.

[THEFERMANATOR]

1st thoughts that came to my mind was bad poppet or weak impeller. At higher rpm's the poppet is supposed to open and essentially bypass the t-stats to cool the engine down under load. It's a pressure based system, so if its not making good pressure, the poppit can't do it's job. As impellers age, they lose there pmiability, and the impellers don't stay out at higher rpm's. Stats wouldn't be a bad idea to do either. I believe you can get the omc stats to go in that engine to get stainless ones instead of the brass steel ones. A water pressure gauge is a GOOD gauge to have on one of these as it tells you right away where to look.

[spareparts]

get a new Impeller kit with upper housing from Mercury, that should fix you up. Thermostats wouldn't be a bad idea if you cant remember when they were last changed. Usually the poppet valve causes overheat at lower rpm(sticks open or allows water to bypass), check the weep hole on the cover to see if any water is coming out

[garbubba]

Thanks guys!

My daughter is in from out of town for a few days so the boats on hold but I'm starting a list.

Thanks for the explanation of the poppet valves Ferm, I didn't understand the purpose.

Have a few more things to check but I'm thinking new thermostats, poppets & a new impeller kit are in my future.

[phatdaddy]

like ferm said, get a water pressure gauge. i'd rather have that than a tach. a new water pump will deliver about 12-14 psi at 4000 rpm's. when mine would fall between 8-10psi, i knew it was time for a new pump.

[steplift20]

So that's what a poppit valve does, mine to the temp hoes way up at slow speed I have to get going to bring gone temp down Thermostats are changed every year impeller is not to old Poppit valve, I did change it about ten years ago Hmm I think I'm due

[garbubba]

I do have a pressure gauge but it's pretty new & I haven't seen it with a new impeller, yet. Therefore I really just use it to double check. It seemed normal, as did the pee stream, I suspect cruded up thermostats and/or poppets, I'll let y'all know.