'91 Mercury 150 BlackMax overheat

[britre]

I have a '91 Mercury 150hp BlackMax outboard engine. A year ago I replaced the water pump. This year I replaced both thermostats at 143degrees.<br /><br />When the engine runs at full throttle temp indicators run in the middle. However, at a fast idle temp reaches very high. Is this common for this engine? What else can I try to do to keep the temp from running high?

 

[clanton]

Re: '91 Mercury 150 BlackMax overheat

Hotter at idle is normal.

 

[britre]

Re: '91 Mercury 150 BlackMax overheat

How hot should this run? The pin reaches almost all the way over to the other side of the dial.

 

[clanton]

Re: '91 Mercury 150 BlackMax overheat

I would guess about 160 degrees, that is 20 degrees higher then thermostat. You could check the block temp with temp stick, laser temp gun, ot temp probe. Check bypass valve.

 

[Rex]

Re: '91 Mercury 150 BlackMax overheat

TEMP. SENSOR

 

[Rex]

Re: '91 Mercury 150 BlackMax overheat

IF NOT THE TEMP. SENSOR THEN GO THE LOWER<br />UNIT AND PUT A NEW IMPELLERE.<br />BUT FIRST CHECK THE WATER TEMP.<br />FROM THE ENGINE FIRST.

 

[britre]

Re: '91 Mercury 150 BlackMax overheat

Sorry but I am new to outboard engines. How do you check the block temp and bypass valve?