Mercury Outboard Engine Won’t Start (Troubleshooting Guide)

When an outboard motor will not begin operating, it’s important to identify the type of staring problem its experiencing. There are four types of starting problems:

• Engine will not start
• Engine cranks slowly but will not start
• Engine cranks but won’t start
• Engine will start but stalls shortly after

This troubleshooting guide will help you diagnoses a Mercury outboard engine that won’t start. But keep in mind that there could be more than one cause for why a Mercury outboard fails to start up.

Mercury outboard will not start

1. Batter switch is not turned ON or faulty switch. Turn battery switch on or test switch.
2. Battery discharged. Check batter for state-of-charge. Charge or replace if necessary.
3. Battery cables or connections loose or corroded. Clean connections and tighten if needed. It’s also a good idea to check for voltage drop in starter circuit.
4. Bad connection or ground, or possibly a managed electrical harness. Check all connections and grounds, inspect wiring for cut or fraying wires.
5. Fuse (10 A). Inspect wiring thoroughly, then replace faulty fuse.
6. Wiring harness. Check for voltage (12 V) at terminal “B” of key switch.
7. Key switch. Visibly check key switch for defects.
8. Starter solenoid. Check wiring connections and test starter.
9. Seized gearcase or powerhead. Check and repair as needed.

Outboard cranks slowly but will not fire up

1. Discharged battery. Test batter, recharge or replace.
2. Battery connections or cables. Ensure cable connections are clean of corrosion. Torque cables to factory specs. Test for voltage drop in starter circuit.
3. Powerhead hydro-locked. Check the engine’s cylinders for water.
4. Partially seized gearcase or powerhead. Perform inspection and repair as needed.
5. Starter or drive gears. Inspect drive gears and starter.

Outboard cranks over but will not start up

1. Stop circuit grounded. Check emergency stop switch and wiring. Make sure the safety lanyard is properly installed and the stop circuit is not grounded.
2. In gear—tiller models. Place Mercury outboard motor in neutral.
3. Fuel contamination. Check fuel filter for water. Also sample gasoline from fuel return manifold.
4. Poor fuel quality. Replace fuel.
5. Fuel supply restricted. Inspect anti-siphon valve, fuel pick-up and primer. Use primer to fill the vapor separator with gasoline.
6. Low fuel pressure or no fuel pressure to injectors. Follow procedures in factory service manual for checking fuel manifold pressure.
7. Incorrect spark plug heat range, fouled or worn spark plugs. Replace with new spark plugs.
8. Fuel injectors not working. Use multimeter to check for voltage at injectors.
9. Flooded engine or excessive oil. Check injectors for leakage, leaking vapor separator vent, leaking. Refer to system analysis in factory service manual.
10. Ignition. Refer to repair manual for ignition system testing and capacitors.
11. Low or no compression. Perform compression test.
12. Water in engine. Check condition of cylinders and spark plugs.
13. Internal powerhead damage. Check engine in repair as necessary.
14. Natural switch, tiller models. Check or test neutral switch operation.
15. Leaking crankcase/cylinder, reed valves or intake manifold. Inspect and repair as needed.

Outboard starts but stalls shortly after or runs poorly

1. Weak battery. Test battery, replace or recharge.
2. contaminate or low quality fuel. Check gas and replace if necessary.
3. Air in fuel system. Check fuel supply manifold for air in it.
4. Incorrect, broken or fouled spark plugs. Replace with new spark plugs.
5. Low fuel pressure. Check fuel pressure, see repair manual for checking instructions.
6. Low or no alternator output. Check for voltage at circuits. Refer to electrical and ignition system analysis in repair manual.
7. Weak or erratic ignition operation.
8. Crankshaft position sensor or CPS. Check CPS air gap and EMM LED indicators.
9. Low compression. Perform cylinder compression test.
10. Restricted or leaving fuel injector(s). Check injectors, refer to fuel system analysis in repair manual.

Outboard won’t start (carbureted models only)

1. Incorrect starting procedures. Review starting procedure outlined in maintenance section within service manual.
2. Fuel tank empty or incorrect mixture of fuel. Possible fuel contamination; e.g. dirt, water, etc. Check fuel tank for gasoline. Replace if necessary.
3. Blockage to fuel line or loose fuel line connection. Check fuel lines and replace if necessary. Ensure all connections are torqued properly.
4. Enrichment valve or choke solenoid valve not operating correctly. Inspect choke solenoid, enrichment valve and wiring. Replace parts if needed.
5. Carburetor needle and seat is either stuck open (flooding) or closed (no fuel). Refer to carburetor disassembly in factory repair manual.
6. Carburetor out of adjustment; e.g. incorrect carburetor jet, idle mixture screw or restricted jet. Refer to carburetor adjustment procedures in factory repair manual.
7. Incorrect float level. Refer to carb adjustment in factory repair manual.
8. Fuel pump pressure too low. Disassemble the fuel pump and perform inspection of its components.
9. Anti-siphon valve malfunction. Test outboard motor without valve in fuel system or simply inspect valve.
10. Carburetor has a loose connection on intake manifold. Check torque specs or carburetor nuts.
11. Reed block loose or gasket defective. Use a pressure oil can of 2-cycle oil and apply it around the reed block housing/crankcase housing matching surfaces and the base of the carburetor. If the outboard engine RPM changes, torque nuts/bolts to factory specifications or replace defective gasket.
12. Bleed hose has blockage or incorrectly routed. Refer to the powerhead section in factory service manual for diagram of bleed hose routing.
13. Fuel pump diaphragm defective. Disassembly and inspect the Mercury outboard fuel pump components.
14. Carburetor mixing hammer cover could possibly be leaking air. Make sure all screws are tight or replace gasket.
15. Off Idle hose clogged. Use compressed air to remove blockage.
16. Damaged reeds. Refer to the appropriate section in your factory repair manual.
17. Incorrect spark timing. Reset timing to the specifications listed in the factory service manual.
18. Debris preventing inlet needle from seating correctly. Clean out inlet seat and needle.
19. Worn-out inlet needle. Replace if needed.
20. Float seated incorrectly. Reseat float to factory specifications.

It’s highly recommended to obtain a repair manual if your Mercury outboard will not start. There is additional troubleshooting information in the book. The information provided on this page is considered basic. More advanced troubleshooting is located in the manual.