3408, 3412 Industrial and Marine Engines
3408 and 3412 Industrial and Marine Engines equipped with a 3N7714 (3408) or 3N7790 (3412) Auxiliary Pump Group (Sea Water) use a special elbow at the water pump outlet. The special elbow has an orifice to control water flow and keep a back pressure in the pump. These pumps must operate under a specific back pressure for maximum efficiency, and for this reason, the orifice in the elbow must be the correct size.
On some engines, a modification has been made in the field by servicemen to cut the orifice larger in the pump outlet elbow. The purpose of this modification was to increase pump flow, but the result was a reduction or complete loss of pump flow. The reduction in pump flow is caused by cavitation (air pockets) in the pump when the back pressure is lower than design specifications.
For this reason, it is important that no modification be made to the size of the orifice in the pump outlet elbow. The 3N7251 Elbow used with the 3N7714 Pump Group on 3408 Engines has an orifice diameter of 17.50 mm (.688"). The 3N8410 Elbow used with the 3N7790 Pump Group on 3412 Engines has an orifice diameter of 23.75 mm (.935").
These pumps must be primed (filled with water) before they are operated for the first time. It is not necessary to prime the pump again unless it has been removed and drained for any reason. During normal use, enough water will be trapped (held) in the pump each time the engine is stopped to prime the pump when the engine is started.