Mercury 40HP 4-Stroke Outboard Fuel System Rebuild

Older outboards are notorious for encountering problems with their fuel systems, such as contaminants in the fuel or problems with water mixing with ethanol. Keeping an older outboard healthy means occasionally servicing and rebuilding the fuel system.

Watch the video above and follow the steps below to do a fuel system rebuild on a Mercury 40HP 4-stroke outboard.

Tools Needed – Mercury 40HP 4-Stroke Fuel System Rebuild

• Ratchet and sockets
• Crosshead screwdriver
• Flathead screwdriver
• Pliers
• Side cutters

NOTE: The parts you’ll need will depend on which parts of the fuel system you’re rebuilding and the condition of the various components. The list of parts we used for our 2003 Mercury 40HP 4-stroke outboard fuel system rebuild can be found here.

Replacing Mercury 40HP 4-Stroke Primary Fuel Pump

Step 1. Cut the cable ties and disconnect the four hoses attached to the fuel pump assembly, then remove the two mounting screws and the fuel pump assembly from the powerhead.

Step 2. Apply a drop of red threadlocker to the mounting screws and install the new fuel pump assembly.

Step 3. Attach the incoming water hose to the brass barb at the bottom-left of the water pump, and the outgoing water hose to the brass barb at the top-left of the water pump.

Step 4. Attach the incoming fuel hose to the brass barb on the right of the water pump, and the outgoing fuel hose to the brass barb at the top-right of the water pump.

Step 5. Secure the four hoses with new cable ties.

NOTE: If you aren’t using the OEM Mercury cable ties, make sure you use quality cable ties, because cheap ties can quickly break down, allowing fuel leaks and creating the potential for a fire.

Watch the clip below to see the fuel pump replacement segment of our Mercury 40HP 4-stroke fuel system video.

Replacing Mercury 40HP 4-Stroke VST Fuel Pump

Step 6. Disconnect the two hoses and the electrical connector from the top of the vapor separator tank (VST).

Step 7. Remove the three 8mm mounting bolts from the VST. 

NOTE: You’ll need to remove the bottom cowling (seven 10mm bolts) to access the mounting bolt at the bottom of the VST.

Step 8. Disconnect the drain hose from the bottom of the VST, followed by the two hoses from the bottom of the fuel cooler assembly, and unclip the high pressure hose from the bottom of the fuel cooler assembly.

PRO TIP: Note which hose was attached to which barb to make reassembly easier.

Step 9. Remove the five screws holding the VST cover to the VST and the single bolt holding the fuel cooler assembly to the VST. Next, remove the cover and cooler assembly from the VST.

Step 10. Thoroughly clean the inside of the VST with contact/brake cleaner and remove all traces of dirt and debris.

Step 11. Remove the screw holding the float assembly, and the float assembly itself, from the VST cover.

Step 12. Inspect the float valve needle. 

NOTE: If the rubber tip is damaged or the rubber has hardened, replace the float valve with a new one.

Step 13. Remove, clean and inspect the float valve seat, or replace it with a new one.

Step 14. Disconnect the two wires from the top of the VST fuel pump and remove the pump from the VST cover.

Step 15. Remove the three screws holding the fuel baffle plate, and the plate itself, from the VST cover.

Step 16. Thoroughly clean the inside of the VST cover with contact/brake cleaner and remove all traces of dirt and debris.

Step 17. Reinstall the fuel baffle plate and the float assembly.

Step 18. Install the new VST fuel pump and reconnect the two wires to the terminals on the pump.

Step 19. Install a new VST gasket, then reinstall the VST cover to the VST. Next, reinstall the bolt that holds the fuel cooler assembly to the VST.

Step 20. Reconnect the two hoses and the high pressure hose to the bottom of the fuel cooler assembly.

Step 21. Reconnect the drain hose to the bottom of the VST.

Step 22. Reinstall the VST to the powerhead (three 8mm bolts), followed by the lower cowling (seven 10mm bolts), then reconnect the electrical connector and the two hoses to the top of the vapor separator tank.

Watch the clip below to see the VST rebuild segment of our Mercury 40HP 4-stroke fuel system video.