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Overheat alarm only under load 1998 90hp mercury 2 stroke

Paulie67

New member
I need help. I have 1998 90hp mercury outboard that is sounding the overheat alarm at full throttle than after I back off the throttle a few seconds later it goes off and than back on when under load again never goes off at idle can drive the boat 20 minutes at 5 miles and hour and no alarm only under load. I have changed the water pump, thermostat and pop it valve and still alarm goes off I know it’s the overheat alarm because on my engine there is no warning module so the alarm sound is the same for low oil and overheat so I disconnected the low oil sensor and get the alarm still disconnected the temperature sensor and the alarm does not sound. When it goes off power head does feel a little hotter than I would like so assuming it’s a real overheat issue. One thing to note when I changed the water pump a 10 millimeter bolt in the housing won’t tighten all the way threads are gone I guess and didnt have the gasket for under the water pump plate so I made my own out of permatex also notice a good amount of water coming out from behind the zincs above the lower unit at the mid section and water also coming out as a steady stream from what appears to be a weep hole on the top of the lower unit. Could I have leak at the water pump housing or a crack in the black vent tube that connects the water pump housing to the pick up tube and therefore not getting enough volume of water to the power head under load? Will pull down the lower unit to inspect and redo with new gasket and helicoil the bolt if I have to just seeing if anyone has seen this before and if I am missing something else thanks so much. Oh forgot to mention strong pee steam coming from tel tail in all instances
 
When you disconnected the temp sensor. Was it in the water under load?
As for the stripped bolt on the impeller housing. This also may cause a decrease in water pressure. I would use a temp gun and make sure the temp isn't rising under load.
 
Thanks so much for the response. It was hard to do in the water by myself but I disconnected the temp sensor in the water after it went off hit the gas and got it back on plain under load and no alarm than yesterday I changed the temp sensor in the water and ran it for a few runs with no alarm thought I found the problem and than on the last run it went off again and than the motor shut off had to wait a while for it to fire up again and putted home heat gun did not say it was over 160 but the powerhead felt very hot to the tough so I assumed the heat gun was totally accurate and it was over heating. I am going to take it out tomorrow and redo the water pump and helicoil the bolt to be safe was wondering though I changed the diaphragm on the poppit valve but am not sure it is opening maybe while it is out I will remove the whole water jacket and check for blockages or corrosion just don’t know if you think that is overkill thanks again
 
Checking for possible solutions is ok. As long as your not throwing needless money at it.

i would start at the impeller housing. Repair that and try again.
 
Thanks again that’s my gut as well I will start there and see how it goes I will let you know my progress

So I took down the lower unit but to my surprise everything looks ok except in front of the water pump at the bottom of the void in the outdrive I found a broken piece of hard black plastic and can’t figure out where it goes and if this is the source of my overheating problem here is a picture of it do you know what this is and where it goes thanks again for all your help
 
Bad news the broken piece of plastic that I found in the outdrive is a piece of a coupling that is at the top of the water tube that goes into the power head according to my local Mercury dealer. That means the power head has to come off to replace the 3 dollar part and that is probably outside my skill set and comfort level and the local guy wants 1800 labor to do the job seems to me may be time to repower. It’s a shame engine runs great and looks brand new. Thanks for your help anyway.
 
One quick question is there a way to take out the six bolts and lift the power head up enough with a hoist to change this part without disconnecting everything? Thanks again
 
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