Mercury Outboard Hard To Start

When a Mercury outboard is hard to start up, it’s a clear sign that something is wrong with the motor. An outboard motor should never require a great deal of effort to activate and begin to work. The motor should turn on effortlessly and without extended cranking. 

Sometimes an outboard motor is hard to start but runs fine when started, or is hard starting when the engine is hot or cold, or it’s hard to start after its been sitting for awhile. Whatever the case may be, it’s important to establish the cause of a hard starting engine so as to restore the motor back to working order. Use this troubleshooting guide to get your Mercury outboard to begin operating normally again.

Outboard Hard To Start—Possible Causes And Solutions


  1. Battery lacking power. Ensure battery is fully charged. Replace battery if it will not hold a charge.
  2. Battery terminals, cables and connections. Clean battery terminals, tighten connections and look for damaged cables. Test for voltage drop on high amperage circuit.
  3. Starter or drive gears. Test starter, starter solenoid and its circuitry. Inspect drive gears.
  4. Water contamination inside fuel system.
  5. Take a sample of fuel from fuel return manifold test port or fuel filter/water separator.
  6. Bad or poor quality fuel or fuel contamination. If in doubt, replace fuel. Refer to service manual for fuel specifications.
  7. Air trapped in fuel system. Check for trapped air in fuel supply system. Refer to service manual for inspection procedures.
  8. Spark plugs. Check for fouled spark plugs (e.g. broken insulator), correct heat range or for worn plugs.
  9. Low fuel pressure. Use a fuel pressure tester to check the outboard’s fuel pressure. Refer to factory service manual for testing procedures.
  10. Low or no alternator output. Check the alternator power output. Check voltage output at circuit. Refer to factory repair manual for testing procedures.
  11. Circuit wiring or capacitor. Check capacitor and for loose wiring connections. Refer to electrical and ignition analysis in service manual for checking procedures.
  12. Restricted or leaking fuel injector(s). Check for signs of a leaking fuel injector. Inspect fuel injection wiring. Refer to fuel system check and testing procedures in service manual.
  13. Fuel supply restricted. Check fuel system for air leaks, check primer bulb, anti-siphon valve and fuel pick-up.
  14. Erratic or weak ignition timing operation. Check and test ignition timing. Refer to ignition and electrical system analysis in service manual.
  15. Excessive oil or fuel, engine flooding. Check for signs of a leaking injector, leaking vapor separator vent, or leaking fuel or oil lift pump. Refer to fuel system analysis in service manual.
  16. Internal engine damage. Inspect and repair as needed. If the engine has compression within factory specifications on all cylinders, yet it’s hard to start and runs poorly, check for damage to powerhead, damage to pistons and or worn or stuck piston rings or the possibilities of scored cylinder walls.
  17. Leaking crankcase/cylinder, reed valves or intake manifold. Check and repair as needed.
  18. Crankshaft position sensor CPS. Check and test CPS.
  19. Faulty enrichment circuit. If the enrichment circuit is defective, while the engine is cold, the motor will be hard to start. Likewise, if the enrichment circuit stays active during normal engine operating temperatures, the mercury outboard engine will flood making it difficult to start the motor.
  20. Choke stuck in open position. If the choke is stuck open, the engine will be hard to start.

If you have a boat with an outboard motor, and the engine is hard to start, be sure to use these troubleshooting steps to diagnose the problem. It’s also recommended to download the factory service manual. Advance troubleshooting procedures are listed in the book.

4 thoughts on “Mercury Outboard Hard To Start

  • January 25, 2017 at 3:01 am

    Which mercury outboards do these repair instructions apply to?

    • January 25, 2017 at 3:03 am

      For the most part, these instructions apply to all mercury models while other models don’t apply. It’s recommended you download a repair manual for your specific year and model outboard. But here is a list of models:

      10 HP Mercury 2-Cyl. 2001
      115 HP Mercury Optimax 3-Cyl. 2004-2005
      115HP Mercury 4-Cyl. 2001-2005
      125 HP Mercury 4-Cyl. 2001-2005
      135 HP Mercury Optimax V-6 2001-2005
      15 HP Mercury 2-Cyl. 2001-2005
      150 HP (XR6, Classic) Mercury V-6 2001-2005
      150HP EFI Mercury 6-Cylinder 2001-2005
      150HP Mercury Optimax V-6 2001-2005
      175 HP EFI Mercury V6 2001-2002
      175HP Mercury Optimax V6 2001-2005
      1965-1968 3.9 HP Mercury (Model 39) 1-Cylinder
      1965-1969 35HP Mercury 2-Cyl. (Model 350)
      1965-1978 9.8 HP Mercury 2-Cylinder (Model 110)
      1968-1972, 1973-1990 20 HP Mercury 2-Cyl. (Model 200)
      1968-1987 6 HP Mercury 2-Cylinder
      1969-1974 4 HP Mercury (Model 40) 1-Cylinder
      1969-1978, 1979-1985 7.5HP Mercury 2-Cylinder
      1970-1971 40HP Mercury 2-Cyl. (Model 400)
      1971-1974 40 HP Mercury 2-Cyl. (Model 402)
      1975-1978, 1979-1985 4.5HP Mercury 1-Cylinder
      1976-1980 4HP Mercury 2-Cylinder
      1979-1983 40 HP Mercury 2-Cyl. (Model 40)
      1979-1985 9.8HP Mercury 2-Cylinder (Model 9.8)
      1980-1982 3.6 HP Mercury 1-Cylinder
      1980-1983 25 HP Mercury 2-Cyl.
      1981-1983 18 HP Mercury 2-Cylinder
      1983-1985 3.5 HP Mercury 1-Cylinder
      1984-1985 18HP Mercury 2-Cyl. (Model 18XD)
      1984-1985 25HP Mercury 2-Cyl. (Model 25XD)
      1984-1989 2.2HP Mercury 1-Cylinder
      1984-1989 35 HP Mercury 2-Cyl. (Model 35)
      1986-1987 4HP Mercury 1-Cylinder
      1986-1989 25HP Mercury 2-Cyl.
      1986-1990 8HP Mercury 2-Cylinder
      1986-1991 9.9 HP Mercury 2-Cylinder
      1988-1990 15 HP Mercury 2-Cylinder
      1990-1991 2.2 HP (Model 3.0) 1-Cylinder
      1992-1996 Mercury 150 HP Pro Max
      1992-1997 Mercury 150 EFI
      1992-1997 Mercury 150 Magnum III
      1992-1997 Mercury 175 HP
      1992-1998 Mercury 135
      1992-1998 Mercury 150
      1992-1998 Mercury 150 XR6
      1992-1998 Mercury 150, 200, 225 Horsepower Super Magnum
      1992-1998 Mercury 175 EFI
      1992-1998 Mercury 200 & 200 EFI
      1992-1998 Mercury 200/225 Pro Max
      1998-2000 Mercury 135 hp Optimax DFI Direct Fuel Injection
      1999-2000 Mercury 150 hp Optimax DFI Direct Fuel Injection
      2.5 HP Mercury 1-Cylinder 2001-2005
      20 (25) Jet Mercury 2-Cyl. 2001-2003
      20 HP Mercury 2-Cyl. 2001-2003
      200 HP Mercury V6 2001-2005
      2002-2005 Mercury 40 hp 4-Stroke
      2002-2005 Mercury 40/50/60 ELPT 4-Stroke
      2002-2005 Mercury 40/50/60 ELPT Bigfoot 4-Stroke
      2002-2005 Mercury 50 hp Four-Stroke
      2002-2005 Mercury 60 hp 4-Stroke
      200HP EFI (3.0L) Mercury 2004-2005
      200HP Mercury Optimax V-6 2001-2005
      225 HP EFI Mercury V6 2001-2005
      225HP Mercury Optimax V6 2001-2005
      25 HP Mercury 2-Cyl. 2001-2005
      250 HP EFI Mercury V-6 2001-2005
      3.3 HP Mercury 1-Cylinder 2001-2005
      30 HP Mercury 2-Cyl. 2001-2002
      4 HP Mercury 1-Cylinder 2001-2005
      40 HP Mercury 2-Cyl. 2001-2003
      40 Jet (60) Mercury 3-Cyl. 2001-2005
      40HP Mercury 3-Cyl. 2001-2005
      5 HP Mercury 1-Cylinder 2001-2005
      50 HP Mercury 3-Cyl. 2001-2005
      55 HP Mercury 3 Cyl. 2001
      6 HP Mercury 2-Cyl. 2001-2003
      60 HP Mercury 3 Cyl. 2001-2005
      65 Jet (90) Mercury 3-Cyl. 2001-2005
      75 HP Mercury 3 Cyl. 2001-2005
      75HP Mercury Optimax 3-Cyl. 2004-2005
      8 HP Mercury 2-Cyl. 2001-2005
      80 Jet (115) Mercury 4-Cyl. 2001-2005
      9.9 HP Mercury 2-Cyl. 2001-2005
      90 HP Mercury 3-Cyl. 2001-2005
      90HP Mercury Optimax 3-Cyl. 2004-2005
      Mercury 100 HP
      Mercury 115 HP
      Mercury 125 HP
      Mercury 15 HP
      Mercury 6 HP
      Mercury 65 HP Jet
      Mercury 75 HP
      Mercury 75 HP Jet
      Mercury 75 HP Marathon
      Mercury 75 HP Sea Pro
      Mercury 8 HP
      Mercury 8 HP Sailmate
      Mercury 80 HP Jet
      Mercury 9.9 HP
      Mercury 9.9 HP Sailpower, XR10/MAG 10/Viking 10
      Mercury 90 HP Jet
      Mercury 90 HP M
      Mercury Sea Pro/Marathon 10
      Mercury Sea Pro/Marathon 15

  • January 25, 2017 at 3:12 am

    My Mercury 150 Black Max will crank over but will not start. What would cause this?


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