The Subaru Impreza is a compact car that Subaru has manufactured since 1993. The most common engine in a late-model Impreza is a 4-cylinder, 2.5-litre engine, which has the oil pump on the front of the engine. The function of the oil pump is to keep the engine lubricated by pumping oil through the engine. The primary portion of the oil pump replacement procedure is the removal of the additional components so you can access the oil pump.
Remove the cable on the negative battery terminal with a socket wrench to prevent the engine from starting while you replace the oil pump. Place a container under the radiator drain and open the radiator drain cock. Allow the coolant to drain into the container and store the coolant for later use. Replace the radiator drain cock.
Raise the vehicle with a jack and support it with jack stands. Place a second container under the drain for the engine oil, then remove the drain plug with a socket wrench. Allow the oil to drain into the container, then fasten the drain plug.
Disconnect the covers for the timing belt with a socket wrench, then remove the timing belt assembly. Remove the bracket for the timing belt tensioner and detach the engine coolant pipe from the water pump. Disconnect the water pump assembly from the engine. Remove the mounting bolts for the oil pump, then pry the oil pump from the engine block with a small pry tool. Discard the seal and O-ring for the oil pump.
Mount the new seal on the oil pump, then apply a thin layer of sealant to the oil pump's mating surface. Mount the new O-ring to the oil pump and install the oil pump to the engine block. Tighten the mounting bolts for the oil pump to 56 inch-pounds with a torque wrench.
Install the water pump to the engine with a socket wrench and attach the engine coolant pipe to the water pump. Install the timing belt assembly and the covers for the timing belt.
Lower the vehicle. Fill the vehicle with the proper amount of coolant and oil. Attach the cable to the negative battery terminal with a socket wrench.