1998 C90 outboard 2 stroke Engine Temp Alarm

[msd58]

My good running and reliable C90 Yamaha outboard temp alarm goes off sometimes when I run it over 4000 rpms for sustained periods. I have replaced t-stat, pop off valve, water pump and temp sensor and it still does it!!!!!! When it happens I touch motor with cover off at its warm but not hot to touch anywhere. It usually stops sounding alarm after a minute or two. Some times I shut it off other times just let it idle till it goes off. I know its the temp alarm since my motor premix. I know its the temp sensor because when I unplug the alarm wire and alarm turns off. As long as I am 3900 rpms I fine 99 % of the time.

What is the problem here?

 

[99yam40]

Re: 1998 C90 outboard 2 stroke Engine Temp Alarm

my guess is restricted passages not letting enough water flow to area around the temp sensor to cool off the heat generated by the higher RPMs.May need to clean out the passages.
But could break off some bolts if this is a salt water motor, so do some searching for ways to chemically clean internal passages or flush the best way you can

 

[msd58]

Re: 1998 C90 outboard 2 stroke Engine Temp Alarm

Used in some pure salt water conditions but more brackish to fresh more often. Flushed after salt water use.

 

[msd58]

Re: 1998 C90 outboard 2 stroke Engine Temp Alarm

I noticed that the barb fitting that had a small hose connected that was plugged on the forward part of lower unit was snapped off and now gone. Just a broken barb there now. I looked it up and its were the tube for speedometer used to be. (I dont have speedo on my Yamaha instruments set up its separate gauge all together)

Could that cause a drop in in cooling water pressure if that is not plugged? Enough to cause rise in temp making alarm sound ?