40020 27 Mercury NUT, CAM FOLLOWER TO CARBURETOR

Probable Causes
Diagnostic codes
Throttle position sensor
Air intake and exhaust system
Fuel supply
Fuel rail pump
Low compression (cylinder pressure)
Individual malfunctioning cylinder
Electronic unit injectorsRecommended Actions
Diagnostic Codes
Check for active diagnostic codes on the electronic service tool. Troubleshoot any active codes before continuing with this procedure.Throttle Position Sensor
Use the electronic service tool and observe the signal for the throttle position sensor. Make sure that the throttle reaches the 100% raw position and the calibrated position.
If the signal is erratic, refer to Troubleshooting, "Analog Throttle Position Sensor Circuit - Test" or Troubleshooting, "Digital Throttle Position Sensor Circuit - Test".
If the engine has a 10 position throttle switch refer to Troubleshooting, "Throttle Switch Circuit - Test".
If the repairs do not eliminate the fault proceed to "Air Intake and Exhaust System". Air Intake and Exhaust System
Check the air filter restriction indicator, if equipped.
Ensure that the air filter is clean and serviceable.
Check the air intake and exhaust systems for the following defects:
Blockages
Restrictions
Damage to lines or hoses
If the repairs do not eliminate the fault proceed to "Fuel Supply". Fuel Supply
Visually check the fuel tank for fuel. The fuel gauge may be faulty.
Ensure that the fuel supply valve (if equipped) is in the full OPEN position.
If the temperature is below 0 °C (32 °F), check for solidified fuel (wax).
Check the primary filter/water separator for water in the fuel.
Check for fuel supply lines that are restricted.
Check that the low pressure fuel lines are tight and secured properly.
Check the fuel filters.
Check the diesel fuel for contamination. Refer to Testing and Adjusting, "Fuel Quality - Test".
Check for air in the fuel system. Refer to Testing and Adjusting, "Air in Fuel - Test".
Ensure that the fuel system has been primed. Refer to Testing and Adjusting, "Fuel System - Prime".
Check the fuel pressure. Refer to Testing and Adjusting, "Fuel System Pressure - Test".
If the repair does not eliminate the fault refer to "Fuel Rail Pump". Fuel Rail Pump
Note: The fuel rail pump that is installed by the factory is a nonserviceable item. If any mechanical fault or any electrical fault occurs within the fuel rail pump then the fuel rail pump must be replaced.
Use the electronic service tool to select the correct screen display. Refer to Troubleshooting, "Troubleshooting with a Diagnostic Code".
If the fault is not eliminated, refer to "Low Compression (Cylinder Pressure)". Low Compression (Cylinder Pressure)
Perform a compression test. Refer to Testing and Adjusting, "Compression - Test ".
If low compression is noted on any cylinders, investigate the cause and rectify the cause. Possible causes of low compression are shown in the following list:
Loose glow plugs or blanking plugs
Faulty piston
Faulty piston rings
Worn cylinder bores
Worn valves
Faulty cylinder head gasket
Damaged cylinder head
Perform all necessary repairs.
Ensure that the repairs have eliminated the faults.
If the repair does not eliminate the fault refer to "Individual Malfunctioning Cylinders". Individual Malfunctioning Cylinders
With the engine speed at a fast idle, use the electronic service tool to isolate one cylinder at a time. Note if there is any reduction in engine speed. If a reduction in engine speed is not noted, the isolated cylinder is not operating under normal conditions. If the isolation of a particular cylinder results in a reduction of engine speed that is less than normal, this may indicate that the cylinder is operating below normal performance. Investigate the cause of the fault on any cylinder that is not operating. Investigate the cause of the fault on any cylinder that is operating below normal performance.
Rectify any faults.
If all cylinders have been checked and no faults were detected proceed to "Electronic Unit Injectors". Electronic Unit Injectors
With the engine speed at a fast idle, use the electronic service tool to isolate one cylinder at a time. Note if there is any reduction in engine speed. If a reduction in engine speed is not noted, the isolated electronic unit injector is not operating under normal conditions. If the isolation of a particular cylinder results in a reduction of engine speed that is less than normal, this may indicate that the electronic unit injector is operating below normal performance.
Remove the electronic unit injector from the suspect cylinder. Refer to Disassembly and Assembly, "Electronic Unit Injector - Remove".
Install a new electronic unit injector. Refer to Disassembly and Assembly, "Electronic Unit Injector - Install".
Repeat the test in 1. If the fault is still apparent, remove the replacement electronic unit injector and install the original electronic unit injector. Refer to Disassembly and Assembly, "Electronic Unit Injector - Remove" and Disassembly and Assembly, "Electronic Unit Injector - Install".
If the fault is not eliminated, check for active diagnostic fault codes.