Yamaha 90hp outboard overheat question

[ollyjscott]

Just replaced the impeller (kit), thermostat and poppet valve (pressure relief valve) in my 1987 Yamaha 90 and I have a question.

I noticed that when first run up it got pretty damn hot, hot to touch the cyl head and the tattle til pee stream was pretty hot too. Never noticed it get this hot before the changing all of the above so I shut the engine down to let it cool off. Then I started it 15 mins later and it's holding a nice normal temp and is hot but not burning hot to touch and piss stream is cold (as usual, prior to changing parts).

Question: is it normal for this motor to get pretty darn hot prior to the Tstat opening? I have not noticed this before, I suspected that the Tstat was stuck open, that why I changed it out here. It was always running on the cool side in my opion.

Help and advice appreciated.

Also - very certain everything I installed was correctly fitted. Double and triple checked. Additionally - had the cyl head water jacket off when changing the Tstat. Not sure if this has any effect. Can you get trapped air in boat cooling systems? As they are open systems I wouldn't think so...

[robert graham]

I'd double check the correct operation of the high temperature alarm buzzer, and if it is working then it should let you know if it's overheating. The cylinder head on my C90 gets pretty darn hot to the touch after a run but I've never checked the actual temperature....

[ollyjscott]

OK - thanks for your answer

I'll buy an IR temp sensing gun today and checkout actual temperatures and report back.

Interesting that yours gets hot to touch when run - mine never did that prior to Tstat change - I could hold my hand on it all day long without discomfort, both when idling when warming up and also after a long 1 hr drive at 4000rpm.

I'll let you know how it goes and what readings I get.

[99yam40]

If the stat was stuck open and you boat in cold waters, it was running too cool in the past.
we have no way of telling where you run the boat and motor
I believe Robert runs in warm water being where he lives

[robert graham]

The thermostat is designed to open at specific temperatures and close at specific temperatures....you can check it by hanging it in a pot of water on the stove and a thermometer and you can see the unit open and close....As I recall they open around 160 degrees F. And close around 140 degrees F.....or something like that....the manuals have the exact numbers...