b91 Mercury Black Max running hot at idle

[Brian Trent]

" I have a '91 Mercury 150

" 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.
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? "

 

[tonyob]

"Brian,

The temperature sho

"Brian,

The temperature should not climb high at fast idle. There has to be a problem with your cooling system.

My best guess is that there is a restriction which is preventing full circulation at fast idle. The problem goes away at full throttle because there is a pressure relief valve at the bottom of the block on the starboard side. When pressure builds in the cooling system the valve opens up and allows more flow. This is intended to allow more cooling at high rpms.

There must be something interfering with the flow of the water when the valve is closed.

You might get lucky if the restriction can be dealt with by removing the cover plates and relief valve. This is a simple job and I think it is the only chance for an easy solution. You will probably want to replace the two gaskets and the diaphram if you decide to remove the valve.

Direct your attention to the path the water must take when the valve is closed. You might be able to insert a flexible tool of some sort ( wire, plastic tie, etc. ) and clear things up. Otherwise you might have to remove the powerhead to get to the problem.

"

 

[sparky]

blown head gaskets will cause

blown head gaskets will cause overheating.

 

[brian]

thank you for your advise

thank you for your advise