Re: Trying to figure out if 1998 40ELPTO has trim sending unit
A water pressure gauge indicates the water pressure in the engine block. As the block is an open system and water will flow out of the engine block at the exhaust, the gauge indicates how effective the water pump is. As the pump efficiency drops (with age or a plaky bag round the intakes
) the water pressure in the block will also drop.
For engines with thermostats and a Poppet valves the gauge will indicate when the poppet opens, by dropping from about 15psi to about 7-10 psi (Depending on the engine), and also indicate the efficiency of the pump.
By knowing the characteristics of the water pressure when the impeller is new and in good condition you have an excellent indicator for the water pump. It will also indicate when the impeller is not working, a long way before the engine would get hot enough to sound the alarm.
Just as an indication of how important Merc thing knowing the water pressure is, all the Mercruiser stern drives MPIs have a water pressure sensor in the inlet hose to the engine, right by the power steering oil cooler, and if the engine management computer sensors a loss of water pressure it will sound the alarm (BIG TIME!!!) and reduce the available power from the engine, depending on the pressure loss, down to as low as 6% of maximum.
Chris........
