MerCruiser Cranks but Wont Start or Hard to Start 5 0L MPI 350 MAG MPI MX 6 2

Models these procedures apply to:

Sterndrive (MCM)

5.0L MPI Alpha and Bravo, 350 MAG MPI Alpha and Bravo, 350 MAG MPI Alpha and Bravo Horizon, MX 6.2 MPI, MX 6.2 MPI Horizon

Inboard and Tow Sports (MIE)

350 MAG MPI Inboard, 350 MAG MPI Horizon Inboard, MX 6.2 MPI Inboard, MX 6.2 MPI Horizon Inboard, 350 MAG MPI Tow Sports

Year models these procedures apply to:

2001 2002 2003 2004 2005 2006

Important Information

1. Fist figure out what engine system is causing the issue. For an engine to work, basic needs must be present, spark (ignition), fuel and compression for the engine to operate. If one of these three components are not present or not functioning correctly, the engine will not run.
2. If possible, use a diagnostic tool to determine if faults or codes are present. If codes or faults are found, fix the issue before continuing. 
3. Check and see if fuel is reaching the Schrader valve which is located directly behind the throttle body.
4. Extract the spark plugs, ensure the proper heat range, type and the spark plug is being used and is not burned, fouled or oil contaminated. 
5. Perform a compression test on the motor and make sure the motor is mechanically sound. 

Engine cranks over but will not start or starts hard

Causes

1. Flame arrestor possibly clogged or blocked.
2. Fuel pump pressure is excessive.
3. Fuel tank is empty.
4. Fuel shut off valve possibly closed.
5. Vapor lock.
6. Lean fuel system.
7. Gasoline grate to low or stale, sour fuel.
8. Fuel contaminated with water.

Action

1. Clear and clean the flame arrestor.
2. Inspect for the cause of the excessive fuel pressure, repair as necessary.
3. Add fuel to the fuel tank.
4. Turn the fuel shut off valve to open position.
5. See repair manual for vapor lock condition.
6. Clean and clear the fuel system.
7. Drain the fuel from the tank and refill.
8. Use fuel treatment to remedy the contaminated fuel. If the issue continues, drain the gas from the fuel tank and refill the tank.

NOTE: Never attempt to service your MerCruiser without a factory service manual to help guide you through service procedures. 

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Flushing MerCruiser Engine with Boat In or Out of Water

FLUSHING COOLING SYSTEM WITH BOAT IN WATER

1. Position the drive unit to its full UP position.

2. Next install the flush attachment and position it over the water pickup holes located on the gear housing. See picture below for details.

3. Now attach a simple garden hose. Attach it between the water tap and the flushing attachment.

4. Position the drive unit, or lower the unit to its full IN position.

WARNING: Never rung the MerCruiser motor above 1500 RPM while flushing. Doing so may cause the seawater pump to collapse the flushing hose and cause the motor to overheat. Always keep an eye on the temp gauge to make sure that the engine doesnt overheat.

5. Turn the water tap open roughly half maximum capacity. Never use full water pressure.

6. Before starting the motor, position the remote control in neutral, idle speed position, then start the motor.

7. Run the motor at idle speed while in neutral for ten minutes, after 10 minutes shut the engine down.

8. Completely shut of water tap.

9. Position the drive unit up to its full UP, then remove the flushing attachment from the gear housing.

WARNING: Make certain the area is clear in the area of the propeller, and that no one is in the vicinity. The propeller may be removed to prevent possible injury. 

IMPORTANT: If flushing the engine while the boat is in the water, dont open the water inlet valve until the motor is ready to be restarted. This will prevent contaminated seawater from entering back into the motor. Make sure to leave the water inlet hose disconnected and plugged if the boat isnt fitted with a valve. 

FLUSHING COOLING SYSTEM WITH BOAT OUT OF WATER

MCM Stern Drive

1. Position the flushing attachment directly over the water pickup holes located on the gear housing, see illustration for more details.

2. Attach a simple garden hose between the water tap and the flushing attachment.

WARNING: Make certain the area is clear in the area of the propeller, and that no one is in the vicinity. The propeller may be removed to prevent possible injury.

CAUTION: Never run the motor over 1500 RPM while flushing the motor. Suction from the seawater pump could collapse the flushing hose and this will cause the motor to overheat and damage internal parts.

CAUTION: Always keep an eye on the water temp gauge to ensure the motor doesnt overheat.

3. Turn on the water tap to roughly half maximum capacity. Never use full water pressure while flushing the engine.

4. Before starting the engine, position the remote control in neutral, idle speed position, then start the motor.

5. Run the motor at idle speed (and in the neutral position) for ten minutes, or until the water discharge appears to be clear in color. Then stop the motor.

6. Turn off the water tap, detatch the flushing attachment and the hose.

MIE Inboard

CAUTION: If flushing boat while in water, the seacock (also known as the water inlet valve), if equipped, must be left closed until the motor is to be restarted, to avoid water from entering back into the engines cooling system and/or the boat.

If the boat does not have a seacock (not fitted with one), the water inlet hose has to be left disconnected and plugged to prevent water from entering back into the engines cooling system and/or boat.

Take extra precaution and place a reminder tag to the ignition switch or to the boats steering wheel with the warning message "Open the seacock or re-connect the water inlet house before attempting to start the motor."

These service procedures pertain to the 1993 1994 1995 1996 1997 MerCruiser Inboard MCM 5.0L Alpha 305 CID, MCM 5.0LX Alpha, MCM 5.7L Alpha 350 CID, MCM 350 Magnum Alpha, MCM 5.7L Bravo, MCM 5.7L EFI Alpha, MCM 5.7L EFI Bravo One, Two and Three, MCM 350 EFI/MP Bravo One and Three, MIE 350 EFI/MP Magnum Tournament Ski,  MIE 5.7L, MIE 5.7L Competition Ski, MIE 350 Magnum Tournament Ski, MIE 350 EFI Magnum Tournament Ski, MCM 350 MPI GEN+ Bravo and Black Hawk, MCM 350 Magnum Gen + Alpha + 5.7LX EFI GEN+ Bravo 4 BBL TBI, 350 Magnum EFI Gen + Tournament Ski, 350 MPI Gen + Tournament Ski Black Scorpion, MIE Inboard 5.7L EFI Gen +, 5.7L Bluewater GM V8 8-cylinder motors. 

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