Mercury outboard motors are one of the mainstay brands of the boat motor industry. Providing top notch service and easily sourced parts has been a mainstay of Mercury's success and remains true today. One particular part that often wears out on all outboard motors is the water pump. The water pump is a device that utilises a rubber impeller to draw water up through the engine housing and into the engine compartment to cool the engine. Water pump replacement is a common maintenance practice for Mercury outboards and can be performed in a matter of minutes by a seasoned pro or under half an hour by a novice boat mechanic.
Remove the bolts from the lower gear housing (the portion of the motor that has the prop attached). Be sure the transmission is in neutral and that all four bolts are removed from the housing.
Slowly, but firmly pull the lower housing away from the upper housing. The only things holding the two parts of the housing together at this point will be the driveshaft, transmission shaft and wiring harnesses. Disconnect the wiring harnesses and pull the lower housing the rest of the way off.
Mount the lower housing in an engine vice with the driveshaft pointed up or parallel to the ground. Remove the four bolts holding the water pump onto the transmission using the ratchet set. Carefully slide the water pump housing up over the driveshaft.
Using a screwdriver, pry the impeller out of the housing and examine it for cracks or inconsistencies. Also inspect the interior of the water pump housing for cracks, gouges, or general wear.
Remove the water pump gaskets still attached to the top of the transmission housing. There should be three in this order: rubber gasket, metal gasket, rubber gasket. Replace with the new gaskets from the water pump repair kit (make sure to lube the gaskets with the engine oil prior to mounting).
Slide the new impeller down the driveshaft and then seat the water pump housing on top of it. Give the driveshaft a slight clockwise turn while pushing down on the water pump housing in order to bend the impeller fins in the correct direction. Reinstall the four water pump bolts and slide the transmission housing back into place with the motor housing.
Reconnect the wiring harnesses, mount the upper and lower housings together and replace the housing bolts. Test the water pump by lowering the motor into the water and turning it on. Water should spray in a stream out of the water purge hole if the pump is working correctly.
Lubing all of the replacement parts with the engine oil will be reassembly easier on you without affecting performance.
Make sure the power switch is turned off when performing this maintenance and have a partner help you remove the transmission housing for larger, bulkier engines.