1998 C90 outboard 2 stroke Engine Temp Alarm

[whaler82]

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 its warm but not hot to touch anywhere. It usually stop sounding alarm after a minute or two. Some times I shut it off other times just let it idle till goes off. I know its the temp alarm since my motor is premix. I know its the temp sensor because when I unplug the alarm wire it goes off. As long as I am 3900 rpms I fine 99 % of the time.

What is the problem here?

[Sequoiha]

you could have sand or debris in the block,, in my part of the world, usually small engines mean shallow water, try pulling the t stats out and run the engine for several minutes with the t stat covers off, will help in flushing any sand out of the block, might have to pull the head to clean it out...

[whaler82]

you could have sand or debris in the block,, in my part of the world, usually small engines mean shallow water, try pulling the t stats out and run the engine for several minutes with the t stat covers off, will help in flushing any sand out of the block, might have to pull the head to clean it out...

Could this also be salt build up? Would a Salt Away kit help or not do you think?

[Sequoiha]

it could be salt and corrosion build up, but if it is already that bad salt away wont help,, salt away is a preventative not a cure...

[robert graham]

When you replaced the water pump did you do the Entire pump including the wear plate and housing?....Maybe just a new impeller not enough to ensure pumping efficiency? I have the same exact motor, only 1999 model. Good luck!

[whaler82]

When you replaced the water pump did you do the Entire pump including the wear plate and housing?....Maybe just a new impeller not enough to ensure pumping efficiency? I have the same exact motor, only 1999 model. Good luck!

Yes everything was replaced when installing new water pump.

[whaler82]

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 broke 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 ?

[robert graham]

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 broke 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 ?

I doubt that speedo barb/tube would create an overheat problem/alarm....Are you sure your pressure control valve and thermostat are operating freely?...is that rubber grommet at the water pump body that the aluminum water tube fits into O.K.?....that rubber O-ring on the water pump O.K.?....Keep looking....

[whaler82]

you could have sand or debris in the block,, in my part of the world, usually small engines mean shallow water, try pulling the t stats out and run the engine for several minutes with the t stat covers off, will help in flushing any sand out of the block, might have to pull the head to clean it out...

Took boat out with marine tech and ran engine at 4500 rpm while he monitored with motor heat gun. He took readings all over motor to get base line numbers before running rpm's up. As it ran over an 4000 and up to 4500 rpms the temp sensor went off. He said sides of motor were normal the the head cover area was about 30* warmer than it should be. After idling < 2 mins the sensor went off and head cover temps area quickly went back down to normal. Running back under 4000 rpms temps were normal. He feel that there are blockages in the head cover or head and needs to be removed and cleaned.