Low Water Pressure. WOT Overheat.

[Themanofsteel]

On my Mercury Blackmax 150 outboard, I installed a pressure gauge. At WOT pressure increases to about 20 then drop drastically to about 12-15psi and hold there. My overheat alarm eventually triggers.

The impeller has been changed more than once.
The poppet and all parts are new.
Replaced the cylinder head gaskets and the exhaust manifolds as well.

I extended the drive shaft housing, water tube & drive shaft to make it a 25”. I confirmed the water tube is the correct length FYI.

I cannot figure out why I have such low pressure. Been dealing with this issue for some time.

 

[Chris1956]

Gee, that water pressure sounds pretty normal for a Merc V6. You might check your head bolt torque, as leaking compression can cause overheat.

 

[Faztbullet]

What year motor???

 

[Themanofsteel]

1982 Blackmax V6. I read that the PSI should be around 20-25 but maybe I read wrong. I torqued it to 30lbs. I also have the heads surfaced.

 

[Faztbullet]

Should be 17-22psi @5000 RPM

 

[Themanofsteel]

It pretty much remains at around 12-13psi at a steady 5000 rpms.

 

[Themanofsteel]

Does the build up on the right of the 2 bottom cylinders mean anything.

 

[Faztbullet]

I would be sniffing around the top one as it looks rich/washed...

 

[Themanofsteel]

Carburetor Rich? On my engines carbs, how do you adjust the mixture. I couldn’t find a mixture screw. 1982 Mercury Blackmax 150hp

 

[Dukedog]

think Faz is on ta something.. no screws and/or adjusting anything with th WH series.. all done with jetting and timing.. idle and max.

 

[Themanofsteel]

Help me understand why the top cylinder being clean means possibly running rich. Also, wouldn’t running rich have damaged the engine by now?