Water Pump By-Pass

Figure 1:

Figure 2:

On the 7.4L engine there is a short heater hose between the front of the intake manifold and the water pump (Figure 1). The function of this hose is to allow coolant to circulate within the engine during warm-up while the thermostat is closed. Circulating the coolant allows for a more rapid and even warm-up of the engine. After the thermostat opens some coolant flow will continue through this by-pass.

On the 5.7L engine the by-pass is a passageway that goes from the right side of the water pump (Figure 2) into and up through the block and into the right cylinder head. It functions in the same manner as the 7.4L.


If this bypass were to become plugged, the coolant would not circulate during warm-up. As a result, there may be some areas of the engine that get warmer than others. If the instrument panel temperature gauge sensor is located in one of these areas, the gauge would indicate a warmer temperature. This reading would not reflect the overall engine temperature. Once the thermostat opens, the engine temperature will begin to even out. However, even with the thermostat open, there will be a reduced coolant flow back to the water pump. This reduced flow may result in higher overall coolant temperatures.


Verification of by-pass operation can be performed on the 7.4L engine by removing the bypass hose and checking for coolant flow or obstructions.


Verification of by-pass operation can be performed on the 5.7L only by removing the water pump from the engine. With the water pump removed, check for an obstruction in the passageway in both the engine block and pump.