Evinrude outboard engines have to cool themselves internally, just as all other marine engines do. The engine does this by pumping salt or fresh water up into the exhaust manifold area from a water pickup outside the craft. The rubber impeller inside the water pump housing uses rotating vanes to circulate the water. Over time, the rubber impeller vanes become stiff, crack and lose their profile, drastically reducing the cooling water pressure and volume. Overheating results from worn or defective water pump impellers, which must be replaced to restore proper cooling water to the system.
Position the motor so you can work on the side case and from underneath the lower unit. Disconnect the negative battery cable. Loosen and remove the bolt that holds the upper shift lever arm, just underneath the carburettor, if so equipped. Use a socket and wrench to loosen and remove the small outer side case. Shift the transmission lever into "forward." Remove the secondary plate inside the outer side case hole with a socket. Loosen and remove the single bolt inside the case.
Remove the lower unit outside casing to loosen and remove the lower unit top mounting bolt, if so equipped. Use a socket and wrench to remove the lower unit bolts that connect the lower unit to the upper unit. Pull the lower case down and place it in a vice, topside up. Remove the upper shaft O-ring and lower shaft O-ring, if so equipped with both. Use Emory paper to sand down the driveshaft, then lubricate it with grease.
Use a socket and wrench to loosen the water pump housing bolts. Pry the water pump housing loose with a flathead screwdriver and slide it up off the shaft. Make sure the water tube remains behind and in a straight-up position. Don't loose the key from the driveshaft groove. Turn the water pump upside down. Note the orientation of the impeller vanes -- either clockwise or counterclockwise.
Pry the impeller out with a screwdriver, or use pliers. Clean the inside of the impeller bore with a rag and carburettor cleaner. Lubricate the bore with some grease. Install the new impeller into the bore in the same orientation the old one came out. Reinsert the key back into the shaft, holding it in place using some grease. Clean the gasket surface on the water pump and the lower unit mount with a gasket scraper and carburettor cleaner. Install the new water pump gasket. Slide the water pump housing back down over the shaft.
Align the water pump housing to the keyway shaft, making sure the key sits in the shaft groove. Insert the water pump housing bolts and tighten them with a socket and wrench. Remove the lower unit from the vice. Clean the gasket surface between the lower and upper unit with a gasket scraper and carburettor cleaner. Install a new lower unit gasket. Install the lower unit back onto the upper unit by aligning the shaft and water tube with their holes and pushing upward. Turn the propeller by hand to engage the shaft splines with the upper unit.
Insert the bolts in the lower unit as an assistant helps hold the unit up. Tighten the bolts with a socket and wrench. Insert the uppermost lower unit bolt, if you removed one, and use a socket and wrench to tighten it. Replace the upper case and bolts, if you have removed the upper case, and tighten the bolts with a socket. Clean the inner and outer case plate gasket surfaces with a gasket scraper and carburettor cleaner. Use some grease to stick new gaskets on both surfaces.
Screw the innermost single bolt into the inner case and tighten it with a socket and wrench. Replace the inner case and tighten the inner bolts to it with a socket and wrench. Replace the outer case and screw the bolts on with a socket. Reconnect the upper shift lever arm and tighten the bolt that holds it in place with a socket. For the other model Evinrude engine, reconnect the shift lever arm at the carburettor and tighten the single bolt with a socket.
Reconnect the negative battery cable. Place a water-filled 55-gallon barrel under the lower engine unit. Start the engine and look for water spray out of the discharge-exhaust port.
Determine where your shift lever linkage must be disconnected, depending up the Evinrude model. Some disconnect at the carburettor linkage, while others disconnect through the side case. Refer to your boat owner's manual for the proper location.