posted 08-13-2007 08:07 PM ET (US)
In any outboard there are usually three or four components in the cooling system:--a water pump;
--a temperature regulated flow device, e.g., the thermostat;
--a pressure regulated flow device, often called a poppet valve;
--a throttle regulated flow divertor, common in OMC engines.
If you have problems with overheating, I suggest checking these devices, and in the order listed above.
THE WATER PUMP
The fundamental source of engine cooling is the flow of water from the water pump. If there is any deficiency in the operation of the water pump there will be an increase in the tendency for the engine to overheat, particularly at high speed operation where the greatest amount of cooling is needed.
Outboard motors use raw water cooling, and because they have a literally unlimited source of relatively cold cooling water, the designers of outboard motors try to employ various methods to raise the engine operating temperature into a reasonable range, typically around 165-degree-F. At lower speeds the supply of cooling water will generally be too great, so some restriction is employed to reduce cooling water flow in order to permit the engine operating temperature to rise. However, at higher engine speeds, the restrictions in the flow of cooling water must be removed to prevent over-heating of the engine.
The modulation of the flow of cooling water is based on three parameters:
--water temperature
--water pressure
--throttle setting
The THERMOSTAT is a device in the cooling system which opens or closes in response to the temperature of the water in which it operates. Typically a thermostat is closed at lower temperatures, then when the water rises in temperature the thermostat opens. The flow of water is usually arranged so that when the thermostat opens more cooling water flow is provided. If the thermostat become inoperative and stuck in the "cold" mode, it can cause overheating in the outboard motor.
The POPPET VALVE is a device in the cooling system which opens or closes in response to the pressure in the water cooling system. Generally the pressure in the cooling system increases with increasing engine speed. The poppet valve--a name which really just describes the shape of the valve and not the fact that it is associated with a pressure controlled response in the cooling system--is usually arranged so that when the pressure rises the valve opens and permits greater flow of cooling water. If this device is stuck in the "low" pressure mode, it can cause overheating in the outboard motor.
The THROTTLE VALVE is a device which is linked to the throttle control input and senses the position of the throttle. When the throttle is advanced the flow of cooling water is increased. This is a technique used in certain OMC motors, and it may not be applicable to your Mercury 150-HP BlackMax.
There is also a possibility that there are other restrictions in the cooling system which are reducing the flow of cooling water. Engines which operate in saltwater may have deposits of salt or other minerals built up in particular places in the cooling system. This is also possible in freshwater motors, but not as common. Check your cooling system for obstructions.
Most outboard motors have an aspirator which functions to expel air from the cooling system and also to act as an indicator of flow. These are commonly called "tell tales" or confidence streams (as well as other vulgar names). Usually these aspirators include a nozzle to expel the water, and you can make some judgment about the function of the cooling system by the volume and strength of the stream of water expelled at the aspirator nozzle. However, it is very common that debris can block the nozzle path and reduce the output, and this may give a false indication of reduced flow when in fact the system is operating normally save for the restriction in the path to the aspirator nozzle.