Merc 90 Cooling Question

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CharlieZulu

Petty Officer 2nd Class
Joined
Jun 2, 2008
Messages
122
Hi Folks:

Late '80s Merc 90, oil injected. My water flow starts right from the get-go..... does this mean my thermostat is stuck open, or is this normal? If it isn't normal, typically how long does it take for water to flow from a cold start?

DSC03043.jpg
 
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electricjohn

Petty Officer 1st Class
Joined
Dec 16, 2006
Messages
228
Re: Merc 90 Cooling Question

Mine has always flowed fast from start up, and faster up to WOT. A gage threaded into my head shows 7 psi at idle, and 14 at WOT. The water never gets hot, but an infra red thermometer shows my heads at around 140 to 145*F when engine is at normal operating temps.
 
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Hottip

Cadet
Joined
Apr 9, 2011
Messages
20
Re: Merc 90 Cooling Question

The water from the telltale should start flowing as soon as the motor starts to turn over.
 
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Maxz695

Guest
Re: Merc 90 Cooling Question

I believe the is a bypass that allows water to flo even if the thermostat is closed.
 
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Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
27,149
Re: Merc 90 Cooling Question

Charlie, Depending on the model of that motor, you may not have a thermostat. Most/all inline sixes didn't have thermostats..... Telltale water flow was after a few seconds
 
Faztbullet

Faztbullet

Supreme Mariner
Joined
Mar 2, 2008
Messages
15,618
Re: Merc 90 Cooling Question

Thats a 3 cylinder 90 and it has a t-stat and poppet valve. The tattle tale is fed off the exhaust plate before the t-stat so it will pee within 10-20 seconds of engine start.
 
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Maxz695

Guest
Re: Merc 90 Cooling Question

mine runs off the exhaust plate does that mean the thermostat has nothing to do with the tattle tail?
 
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CharlieZulu

Petty Officer 2nd Class
Joined
Jun 2, 2008
Messages
122
Re: Merc 90 Cooling Question

Yes indeed, it's a 3-cyl.

Thanks, guys for the great info.
 
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