Thread: 2 Stroke Mercury 40 Water Pump Replacement

Well winter is here so I need my shallow water rig working. I pulled my Tin boat out this fall with the first super low tides hit and hooked it up to run on the hose. Well the Tattle-Tail had zero water coming out. I thought no worries, I would get a piece of wire and clear the hose, did that and still no water. I removed the hose from the output block fitting and still no water. I next removed the fitting itself and STILL NO WATER .

So I pulled the Lower Unit, Thermostat Housing, the 2 Drain Plugs in the back of the block, and the Service Manual out.



The old Thermostat was not the correct one so I ordered the right one along with a new thermostat Seal. I needed to clean out the Water Bypass Port as well as where the Thermostat sat. Also cleaned the gasket area up with a Stainless Wire Brush on a drill.



Now that everything is cleaned up time to test the Thermostat. You need to always check a new Thermostat before installing it. They do arrive bad and won't open.



This is a 133 degree Thermostat and at 129 degrees it was closed but at 135 it was wide open.



All cleaned up and the Thermostat confirmed operational I closed this area up.



I removed the rear drain plus to check for any buildup of crud in the Block and noticed lots of crud in the threads. I needed to reinstall the plugs and get a seal in the threads so I cleaned the threads up with a Tap before reinstalling the plugs.



Now with the plugs reinstalled using a little Aviation Gasket Maker it was time to move on to the Water Pump.



When you remove the old Water Pump Impeller make sure you don't loose any parts.



All the Impellers I have changed have some kind of Key locking it to a notch in the Driveshaft, do loose it. A lot of engines use Woodruff Keys.



The Housing is fine so I cleaned it up and set to the side while unpacking the Water Pump Kit. Buy the Kits, this is not a job where once into it you find you needed something else. Check out this old Impeller.........See ANYTHING WRONG? Whoever installed this one didn't know you must turn the driveshaft while reinstalling the housing. No I've run this engine for a while without a problem but you can see one of the Blades was folded over while another just ran backwards.



The old Lower Housing Plate wasn't sealed well at all on the bottom which made cleaning the Lower Unit Casting harder to clean up before installing the new one.



Since the casting was a bit corroded I used more of the Aviation Gasket Maker to guarantee a good seal. I will hate myself the next time I have to tackle this job.



With the Lower Gasket & Plate in place I slid the two washers on with the new Impeller. Look at the second picture, always make sure the Key is inside the Impeller. A lot of the Yamaha Impellers are blind on top so you can not see the Key from the top. The Upper gasket goes in with the Seal Ring facing up.



On a little engine like this carefully put a little of the Quicksilver Teflon or Engine Assembly grease under the impeller but not out where it will get under the Upper Gasket as well as inside the Impeller Housing. This particular engine the Driveshaft must be turned Clockwise while pressing down on the Impeller Housing so all the Impeller Blades fold the correct way. If you do not lubricate below the Impeller the Bottom Plate & gasket rotates with the Impeller.



With the Water Pump back together the Output seal needs some lubricant where the Output Tube slides in so it doesn't get pinched.



Once the Lower Unit was back on a quick test run and Success..........we have a good stream of water now.