Water Coolant Problems in a Yamaha 4-Stroke 9.9 Horsepower

A water coolant problem in the Yamaha 9.9-horsepower outboard requires little in the way of tools. Before you resort to the toolbox, though, you first need to ensure the water that comes from the lake, stream, river or ocean in which the motor is immersed can get into the engine. After that, drag out the toolbox. If the water inlets for the motor are clean, you need to replace the water pump. The project is best undertaken with the outboard in the operating position on the back of the boat, or with the outboard on an upright storage stand.

Things You'll Need

  • Jackknife
  • Thin cloth
  • Adjustable wrench
  • Open-end wrench, 9/16-inch
  • Socket and ratchet, 3/16-inch
  • Waterproof marine grease
  • Pump replacement kit
  • Torque wrench
  • Box-end wrench, 5/16-inch
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Instructions

  1. Simple Stuff and Removing the Lower Unit

    • 1

      Wrap the tip of a jackknife in a thin cloth and remove any vegetation or wildlife you find occupying the water inlets. The inlets are located just below the top rim of the outboard's lower unit.

    • 2

      Shift the outboard into "forward." Remove the prop using an adjustable wrench -- something you should do every time you go boating -- and remove any fishing line wrapped around the propeller shaft. Doing this before you start maintenance has a happy side effect: If by some accident the engine tries to start, you won't be disfigured, maimed or killed by a spinning propeller.

    • 3

      Reach under the front lip of the motor case and slide your hand a few inches back. Your hand will run into two rods: one horizontal and one vertical, held together by a clip. Snap the clip loose to disconnect the shift shaft.

    • 4

      Remove the four 9/16-inch locknuts and washers from the lower unit studs that protrude upward through the rim of the lower unit, using an open-end wrench. There are two studs on the front, two on the back. Pull straight down on the lower unit carefully, because you're pulling the driveshaft and the shift shaft away from the engine.Try not to bump either on anything.

    Replacing the Water Pump

    • 5

      Remove the four 3/16-inch bolts that surround the water pump's cover, holding it to the top of the lower unit, using a socket and ratchet. The pump cover is easy to spot -- the driveshaft runs through it. Pull the impeller and pump cover up and over the top of the driveshaft, and catch the key that links the impeller to the driveshaft.

    • 6

      Pull the impeller out of the pump housing. Grease the inside of the pump housing with waterproof marine grease. Dip the tip of the key in the grease and slip it into its recess near the bottom of the driveshaft. On larger Yamahas, you have to turn the driveshaft to set the impeller vanes, but that isn't necessary with the 9.9-horsepower outboard.

    • 7

      Push a new impeller -- from the pump replacement kit -- into the pump housing. Spread a small bit of thread locker onto the 3/16-inch water pump cover bolts. Slide the impeller and pump body back down the driveshaft, over the key and down to the top of the lower unit. Thread the bolts into place and tighten each to 18 foot-pounds using a torque wrench and a crossing pattern.

    • 8

      Lift the lower unit straight back up into the body of the outboard. Guide the studs on the rim of the lower unit through the holes on the "leg" of the outboard. Replace the washers and the locknuts, tightening them with your fingers. Torque them to 105 foot-pounds