90 HP Yamaha - overheats 1986

[Tripper]

I have a recently rebuilt 1986 90 HP Yamaha that overheats at low speed or idle. I have taken it back to shop repeatedly. Waterpump and impeller are new. Thermostat is out. No clogs, holes or corrosion in water line. Runs at idle at shop fine for 20 minutes in tank. After 1/2 hour running at 2800 rpm slow to idle for 1 minute and alarm goes off.

Any ideas on what I am missing?

 

[Marc III]

Re: 90 HP Yamaha - overheats 1986

Try holding your hand to the engine head when this happens. If your can hold it there for awhile, your overheat sensor could be going off prematurely. Try to get your hands on one of those digital heat gun thingys. I don't know the exact normal operating temps, but 160 - 200 degrees F should be near normal. Over 200 would be hot. They also make melt sticks (think hi-temp crayons).

If your engine has seen alot of saltwater time, your midsection could be partially restricted. I watched a buddy cut an old, crusty 200hp Johnson in half to save the powerhead and lower unit.

When you get it sorted out, get a new t-stat. They run better, more efficiently, and build up less carbon at proper operating temps.

 

[BruceAML]

Re: 90 HP Yamaha - overheats 1986

You have identified part of the problem. It is the missing thermostat. Without a thermostat the engine can sometimes run hotter because the water is flowing so quickly it doesn't have time to remove the heat. Use a Yamaha part number 688-12411-11-00 thermostat. I recall that there is a pressure control valve and a compression spring under the thermostat cover. Does water flow strongly from the cooling-water pilot hole? What spark plugs are in the engine? (NGK B8HS10) Are they torqued to 14 ft-lbs? What water pump was installed? Is it the correct one?