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Jason
Jason, Marine Mechanic
Category: Boat
Satisfied Customers: 16119
Experience:  Degree in Marine Technology. Gas and diesel marine mechanic.
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I have a 2002 90 hp Yamaha outboard that sets off the alarm

Customer Question

I have a 2002 90 hp Yamaha outboard that sets off the alarm when it is run over 4,000 rpm. Thermostat, thermo/switch and water pump have been replaced. What could be the problem?
Submitted: 2 years ago.
Category: Boat
Expert:  Jason replied 2 years ago.

Good morning, my name is ***** ***** am I speaking with?

Have you checked the engines temperature manually with an IR temp gun at the cylinder head to see if it is actually overheating?

Customer: replied 2 years ago.
it showed 142 to 148 degrees F. This doesn't seem hot enough to set off alarm. Head is hot to touch but not what I would expect for overheat. Ran boat for 20 minutes up to 4K rpm and no alarm. Alarm went off quick when running around 4.5K rpm. Could it be head gasket? Load over 4K just too much to cool?
Expert:  Jason replied 2 years ago.

If anything it sounds like is just a faulty temp sensor activating before it should if you are at 148 degrees. It really shouldn't go off until the engine hits 190F. What you want to do is disconnect it and run it, and see if the alarm still sounds. If it stops, then it is just a faulty temp sensor. Does that make sense?

Customer: replied 2 years ago.
I have already put a brand new thermoswitch on the motor. What are the chances the old one was bad and a brand new one. And yes unplugging it did stop the alarm. The readings in the 140s were running with the cups on it at 1200 and 2200 rpms. I'm a pretty good mechanic but this really has me stumped, that why I went to you.
Expert:  Jason replied 2 years ago.

. The readings in the 140s were running with the cups on it at 1200 and 2200 rpms.

Ahhh okay. What you want to do next is pull the cover, run the engine on the water until the alarm goes off. Immediately shut if off and then take the temps again. Is that something you can do?

Customer: replied 2 years ago.
I will be at the lake next Tuesday where I can do an on water test and record temp on head at time of alarm.
Expert:  Jason replied 2 years ago.

Sounds good.

Customer: replied 2 years ago.
I will be glad to rate as soon as I run motor and take temp when alarm goes off. So far suggestions to diagnose has been spot on.
Expert:  Jason replied 2 years ago.

Sounds good, let me know how you make out.

-Jason

Customer: replied 1 year ago.
tested on water, 3500 rpm below-145 degrees, 4000 rpm- 160-175 degrees, over 4000 rpms set off alarm approx. 185+, bottom cyl reading hottest. Could it be clogged water jacket or maybe that water pump the marine dealer put in does not move enough water at high rpms?
Expert:  Jason replied 1 year ago.

A clogged/scaled up water jacket can definitely do it. There is also a grommet at the top of the water tube seal and if that goes bad it will draw air bubbles into the powerhead at higher speeds. If this was brought to me what I would be doing is removing the lower unit and double checking the pump installation. If that looks good I would pull the thermostat out, hold a garden hose against the water tube and see if water flows freely through the block. If it does not, I pull the cylinder head and look for signs of any kind of clogging. If it does, I pull the powerhead and inspect the top of the water tube grommet. Does that make sense?