At what engine temperature should the sensor trigger the overheat alarm?

[HerbieJ]

I have a 1994 Mercury Tracker, 60 HP, three cylinder. Its oil injection system has been removed and I mix @ 50:1. It runs quite good, as long as I stay below 3800 PRPMs. Above that, the overheat alarm sounds. The alarm stops within 20 seconds of idling in neutral. I could run all day below 3800 RPMs. I suspect my alarm may be faulty. I brought it into the kitchen and did the hot water thing. It triggers the alarm @ 170 degrees. I don't know if this is right or not, but my sensor measures metal, not cooling water and I've come across a few references to heat sensors of this type not activating below 260 degrees and 285 degrees. The temperature sensor for my motor ( according to Boats,net) is Mercury part #78799 2. Does anyone know the temperatrure required to trigger this sensor? If not, does anyone know the typical temperature range for a metal temperature sensor?

Any thoughts will be much appreciated.

 

[Barnacle_Bill]

Re: At what engine temperature should the sensor trigger the overheat alarm?

Rebuild your poppet valve.

 

[Faztbullet]

Re: At what engine temperature should the sensor trigger the overheat alarm?

If the impeller has not been replaced in the last 2 years try that first. There are 2 different designs on this series of block as #1 uses a poppet and 143? t-stat and #2uses a just a 110? t-stat. The sensor for your engine triggers(closes) at 190? +/- 8?.

 

[HerbieJ]

Re: At what engine temperature should the sensor trigger the overheat alarm?

Thanks, Bhile and Faztbullet for your responses. I've tried just about everything to address this problem over this past boating season. Thermostat and poppet unit is new, as is the water pump - the complete pump, not just the impeller. Also change the plate and seal between the pump and the exhaust passage. Even tried cutting a loop off the poppet spring to weaken it a bit. When I pulled the boat out last month I pulled the head and replaced the top and bottom exhaust plate gaskets and the big rubber seal near the bottom of the exhaust passage. While in there, I replaced the undamaged, but aged water tube. None of those gaskets / seals appeared to have failed. As far as I could see into the head's water passages, I could find no build ups. In fact, it looked cleaner than I would have hoped for a motor that had been run in Long Island Sound for a while. (This is my first year with it: I have only run it in a fresh water lake.) So, that doesn't leave me many other things to pursue. That's why I'm exploring the heat sensor itself. I'm hoping to discover that it isn't supposed to go off until the temperature is well past 170 degrees. Not sounding like that's the case though. I'm still open to suggestions and other things to try.