Replacing Thermostats Optimax 150 HP

[ordonyez]

Motor- 2005 Mercury 150 HP Optimax

Had the boat out on the weekend for the first time after winter storage. It ran great for the first rip (1/2 hour) went out later and motor stopped pumping water and began to overheat and beep.

I towed the boat home and I just changed the impeller / water pump as I thought this was the issue but this has not fixed the problem.

I have good water flow when a hose is hooked up on muffs or by flushing the system but when I put the motor in a water bucket it will not pump water. It is like it needs the assistance of the hose to get it going which is no good on the lake.

Could this be a thermostat problem? If so they seem like it will be hard to get them out. Is there any tricks?

 

[Texasmark]

On a barrel, for proper operation of the pump, the water level has to be higher than the location of the water pump which is just above where the LU separates from the mid section.

You didn't say anything about your operational conditions when the condition occurred or prior which might help with a diagnosis.

On the earlier model 150's you had 2 mechanisms for cooling: The thermostat which controls block temperature at around 140-160F when operating below 2500 rpm and a high (water) pressure relief (poppet) that is forced off it's seat by the increased water pressure above 2500 (serv. manual number), which is a larger diameter valve, and is not temperature controlled, allowing increased water flow that is not temperature controlled. The OT alarm on those engines goes off at 195F.

For a 10 year old engine a good rule of thumb would be to replace those valves (2 stats, one popoff if like older engines) and the impeller in the water pump.

On the older engines the stats were at the top of each bank of cylinders identified by a ⅜" water tube connecting them together and the discharge. I don't know where the popoff is located but I think it is just below the block in the center rear. Faztbullet can tell you.

 

[ordonyez]

I will try finding a bigger tub to start the engine in and report back.

 

[Texasmark]

I will try finding a bigger tub to start the engine in and report back.

While you are running like that, keep your eye on the tub internal temp as a lot of heat goes into that water from the engine.