SHAFT 8D5121 - Caterpillar

Probable Causes
Cold cylinder
Ignition system
Incorrect engine timing
Fuel supply
Incorrect air/fuel ratio
Air inlet restriction
Exhaust restriction
Plugging of the catalyst substrate
Throttle actuator and fuel actuator
Incorrect valve lash
Cylinder head and related componentsRecommended Actions
Note: If the symptom only occurs under certain operating conditions (high idle, full load, or engine operating temperature), test the engine under the conditions. Troubleshooting the symptom under other conditions can give misleading results.Cold Cylinder
Use a 251-0030 Infrared Thermometer to check for a misfiring cylinder at the exhaust manifold.If there is an indication of a misfiring cylinder, check the following components for problems:
Spark plugs
Spark plug wires
Ignition transformers
Primary wiring for the ignitionInspect each of the spark plug wires for signs of pin holes and/or of electrical arcing.Refer to Troubleshooting, "Ignition Secondary - Test".Ignition System
Inspect the ignition transformers for the following conditions: loose connections, moisture, short circuits and open circuits. Inspect the primary wiring and the secondary electrical connections.Ensure that the correct transformers and spark plugs are installed on the engine. Inspect the spark plug wires for signs of pin holes and of electrical arcing. Check the spark plugs for fouling, correct gap adjustment, and moisture.Maintain the spark plug according to the Operation and Maintenance Manual.Incorrect Engine Timing
Verify that the base timing is correct. Refer to Testing and Adjusting, "Timing - Calibrate".Fuel Supply
Verify that the fuel pressure is correct. When possible, interview the operator in order to determine if fuel quality is in question. Try to determine if the source of the fuel has been changed.Ensure that fuel is supplied at a sufficient pressure that is stable. Check that the size of the fuel line is large enough to provide the correct volume of fuel. Inspect the fuel system for leaks.The following conditions can affect starting:
Low fuel pressure
High fuel pressure
Poor fuel qualityInspect the following fuel system components:
Fuel filter
Gas pressure regulator
Gas shutoff valve
Fuel actuatorVerify that the system components are operating correctly.Repair suspect components. Replace the fuel filter, if necessary.Incorrect Air/Fuel Ratio
Use a reliable exhaust analyzer to verify that the exhaust emissions are correct. Ensure that the carburetor is adjusted correctly. Refer to Testing and Adjusting, "Carburetor - Adjust" for the correct adjustment procedure.A change in the fuel energy content will change the air/fuel ratio. Obtain a fuel analysis.The fuel supply pressure must be adequate and stable.Strike the gas pressure regulator with a soft hammer. If the engine speed changes, inspect the internal parts of the gas pressure regulator for wear. Inspect the diaphragm for the gas pressure regulator for leaks. Ensure that the valve moves freely. The valve and the valve seat must be in good repair.Ensure that the fuel metering valve is operating correctly. Refer to Troubleshooting, "Fuel Actuator - Test" for details.Air Inlet Restriction
The maximum allowable air inlet restriction is 3.75 kPa (15 inches of H2O). If the air inlet restriction is higher than the specification, clean the filter element or install a new filter element. Check the restriction again. If the restriction remains too high, inspect the air inlet piping for obstructions.For more information, refer to Testing and Adjusting, "Air Inlet Restriction