1987 40 HP Yamaha overheat alarm?

Question

runs for approx 4 mins. @ idle, then alarms. water is always coming out of telltale. hot to touch but I can keep my hand under it without burning. I noticed that the hose for ttale comes off the lower of the 3 cylinders. Is it representative of the peak H2O temp ? I would have thought it should be a sample of the water that has cooled all 3 cylinders which to me should be off the top cylinder jacket. I had heard the top cylinder is almost always the one that shows the worst heat damage.

What fraction of the total flow is the telltale, by design?

I thought it was the t-stat so took it out.It still alarms .

It did open under hot water but I didn't check @ exactly what temp. Can it be a legit overheat if the telltale is steady stream AND not scalding hot?

Any simple way to check temp sensor ?

Is there a sensor for each of the 3 cylinders?

Lots of questions, thanks for any answers/advice.

Thanks.

Answer 1

Man you are cooking your engine!  That alarm is not an early warning system - it's telling you to shut the engine off *right now*.

The telltale (official term:  "overboard water indicator", also known as "pee stream") is a small fraction of water flow - it only indicates that the water pump is pumping - it doesn't tell you whether water is circulating the powerhead.  It's not an indicator of cylinder head temperature, either.

With the engine idling on muffs (no load), it shouldn't even get warm, much less hot, especially without a thermostat.

Do *not* under any circumstances start this engine again until you have replaced the water pump impeller.  Inspect the inner pump housing and replace it too, if it shows any scoring at all.

Before you put the thermostat back in (and you will install a thermostat - without it the engine will not reach its design combustion temp, resulting in coked up cylinders, sticky rings and eventually, piston failure), put it in a pot of water on the stove with a kitchen thermometer.  It should open at 140º F.

And for pete's sake, get yourself a manual!

No reason you can't do all the maintenance on this motor yourself.

Have fun.

Answer 2

Just me knows his stuff.  I think You have a 2 stroke outboard. It has an "over heat alarm" .  Don't get over confident that the engine is being cooled because of the by-pass hose discharge.  If you are on a flush attachment, Look to replace the water pump impeller. But, at the same time keep your eye out for blockages in the cooling system.  In few cases is the problem related to the thermostat.

Answer 3

1st do you have a temp gauge  if you dont you should  2nd change the thermostats  3rdit could be the sending unit 4th it could be you ned to replace the water impellor

but first thing   check the sending unit

Answer 4

Replace the impeller.. you aren't getting enough flow..  These should be replaced every 2 or 3 years... put fresh gear oil in the lower unit, while its apart...