Engine Misfires, Runs Rough or Is Unstable
- Active codes and logged codes
- Engine oil level
- Fuel system
- Fuel injectors
- Proper lubrication
- Low compression
- Valve train components
Note: The procedures have been listed in order of probability. Complete the procedures in order.
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| Troubleshooting Test Steps | Values | Results |
|---|---|---|
1. Active Codes and Logged Codes A. Establish communication between Caterpillar Electronic Technician (ET) and the Electronic Control Module (ECM). Refer to Troubleshooting, "Electronic Service Tools", if necessary. B. Download the "Product Status Report" from the engine ECM before performing any troubleshooting or clearing diagnostic trouble codes. Troubleshoot any active codes before continuing with this procedure. |
Codes |
Result: There are active codes. Troubleshoot any active codes before continuing with this procedure. Result: There are no active codes. Proceed to Test Step 2. |
2. Check the Engine Oil Level and Obtain an Oil Sample A. Check oil level and obtain an oil sample. If the oil level is high, then take an oil sample to check for fuel dilution. Refer to Troubleshooting, Oil Contains Fuel. |
Oil |
Result: The engine oil sample shows action required. Make the necessary repairs. Result: The engine oil sample and oil level are OK. Proceed to Test Step 3. Result: The engine oil level is not at the correct level. Determine root cause of why the oil level was high or low. |
3. Check the Fuel System Refer to Systems Operation, Testing and Adjusting for additional information. A. Visually check the fuel level in the fuel tank. Do not rely on the fuel gauge only. B. Ensure that the vent in the fuel cap is not filled with debris. C. Inspect the fuel system for external leaks. D. Check that there is no air in the clear primary fuel filter. Refer to Systems Operation/Testing and Adjusting, Low-Pressure System - Test. E. Change the primary fuel filter and the in-line filter for the fuel transfer pump. F. Check fuel quality. Refer to Testing and Adjusting, Fuel Quality - Test for the proper procedure. Check the fuel tank for debris or foreign objects which may block the fuel supply. G. Check for the proper operation of the fuel transfer pump. Replace the pre-filter if excessive contamination or if the maintenance interval is exceeded. Refer to Testing and Adjusting, Low Fuel Pressure System-Test for test information. |
Fuel System |
Result: The fuel system is OK. Proceed to Test Step 4. Result: The fuel system is not OK. Make the necessary repairs. |
4. Fuel Injectors A. If service has been performed on the engine fuel system recently, verify that the injector trim codes in the Engine ECM match the injector for each cylinder. If injectors were moved between cylinders or if an injector was replaced and the injector trim codes were not updated in the Engine ECM, this can cause an engine miss. B. Access the "Cylinder Cutout Test" in the "Diagnostic Tests" under the "Diagnostics" menu. Cut out each cylinder one at a time to listen for improvements in the engine. If engine sound improves while cutting out a specific cylinder or cylinders, then check that the injector(s) are working properly by using 440-6907 Injector Kit. Note: The injector deliver quantity is not a reliable indicator of a failed injector. It is not recommended to use that test in determining if an injector is failed. |
Fuel Injectors |
Result: If injector(s) trim codes for each cylinder do not match what is in the Engine ECM, then refer to Special Instruction, REHS9707 for the procedure to install the new injector QR codes within the Engine ECM and on-line. Confirm that this resolves the issue. Result: If injector(s) are found to be defective, then replace the injector(s). Refer to Disassembly and Assembly, Electronic Unit Injector - Remove and Install. Refer to the Special Instruction, REHS9707 for the procedure to install the new injector QR codes within the Engine ECM and on-line. Confirm that this resolves the issue. Note: If two or more injectors are not working properly, this is likely a result of water in the fuel, bad fuel, or contaminated fuel. Contaminated fuel can occur by pre-filling the fuel filter or contamination during a previous repair of the fuel system. The root cause needsto be determinedto prevent a repeat occurrence. Result: There is no previous known service on fuel system and there was no noticeable improvement during the cylinder cut out test. The customer could have ordered injectors through parts and did not change the injector trim codes in the Engine ECM. Visually confirm that the injector trim code for each cylinder matches what is in the Engine ECM. Refer to Special Instruction, REHS9707 for the procedure to install the new injector QR codes within the Engine ECM and online. |
5. Low Compression A. Perform a compression test. Refer to Testing and Adjusting, Compression – Test. B. Possible causes of low compression: • Faulty piston • Faulty piston rings • Worn cylinder bores • Worn valves • Faulty cylinder head • Damaged cylinder head |
Compression |
Result: Low compression is not present. Proceed to Test Step 6. Result: Low compression is present. Make the necessary repairs. |
6. Valve Train Components A. Check the valve lash. Refer to Troubleshooting, “Valve Lash Is Excessive”. B. Check for damage to valve train components. Remove the valve cover from the suspect cylinders. Check the following items for damage: • Camshaft • Valve springs • Camshaft followers • Rocker shaft • Bridges • Pushrods • Injectors Refer to the Disassembly and Assembly for additional information. C. Check for valves that do not move freely. Remove the cylinder head and inspect the valves. Refer to the Disassembly and Assembly for additional information. |
Valve Train |
Result: The valve train components are damaged. Make the necessary repairs, Verify that the repair eliminated the problem. |
Contact your Cat dealer Technical Communicator (TC). For further assistance, your TC can confer with the Dealer Solutions Network (DSN).
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