Cause
• Engine oil (aerated, incorrect grade, low oil level, extended drain interval)
• Poor fuel quality
• Low fuel pressure
• Aerated fuel
• Electronic control system faults (ECM and IDM)
• Injection control pressure system problems
• Fuel injectors not working properly
• EGR valve stuck open
• Power cylinder problems
• Valve train problems
• Engine or flywheel balance problems
• Exhaust system to cab/chassis contact
• Loose/worn engine mounts
Procedure
1. Verify complaint. Confirm conditions when complaint is present.
When does engine rough idle occur?
• Hot – operating temperature
• Cold
• After high speed operation
• Over entire engine speed range
• Combination of the above conditions
• Is there chassis vibration or any other conditions/observations present when engine idles rough.
2. Inspect exhaust system for contact with cab, frame or body of vehicle.
Engine exhaust pipe contact with cab may transmit engine vibrations to cab, especially on acceleration or engine shifts. This condition may be incorrectly diagnosed as a rough idle complaint.
Complete the following tests on the Performance Diagnostic form. See “Performance Diagnostics” – Section 6 (page 205) in this manual for specific details on each test.
3. Do Test 1 (Diagnostic Trouble Codes) on Performance Diagnostics form. Intermittent
sensor, injector or wiring harness faults can affect engine idle conditions. The ECM may have detected and recorded these conditions.
4. Do Test 2 (KOEO Standard Test) on Performance Diagnostics form. This test will verify electrical operation of actuators.
5. Do Test 3 (KOEO Injector Test) on Performance Diagnostics form. This test will verify that the injectors are working electronically.
6. Do Test 4 (Engine Oil) on Performance Diagnostics form. Check engine oil level. Verify correct oil grade for ambient temperature. See Engine Operation and Maintenance Manual, Engine Lubrication Requirements section.
Confirm oil meets correct API specification for your model and year of engine.
7. Do Test 5 (Fuel) on Performance Diagnostics form. Verify quality and quantity of diesel fuel. Poor quality fuel or low cetane rating can cause white smoking, engine misfire, and low power. See Engine Operation and Maintenance Manual, Fuel Requirements section for your model and year of engine to determine minimum necessary fuel grade and cetane rating.
8. Do Test 6 (Fuel Pressure and Aerated Fuel) on Performance Diagnostics form. Measure fuel pressure at fuel rail (intake manifold). Low fuel pressure, aerated fuel, and fuel inlet restriction will cause the engine to misfire and a loss of power.
9. Do Test 9 (KOER Standard Test) on Performance Diagnostics form. This test will verify the functionality of the injection control pressure system. The engine must be at operating temperature 70 °C (158 °F) to do this test.
10. Do Test 10 (Injection Control Pressure) on Performance Diagnostics form. This test will confirm if the injection control pressure system is functioning properly and verify injection control pressure stability.
11. Do Test 11 (Injector Disable) on Performance Diagnostics form. The primary function of this test is to show the contributions of the individual power cylinders. The test will detect a weak cylinder which could be the result of an injector or base engine problem.
• Test 11 is used in conjunction with Test 12 (Relative Compression) to distinguish between an injector or mechanical problem.
12. Do Test 12 (Relative Compression) on Performance Diagnostics form. This will verify base engine compression.
• Test 12 is used in conjunction with Test 11 (Injector Disable) to distinguish between an injector or mechanical problem.
13. Do Test 16 (Crankcase Pressure) on Performance Diagnostics form. This test will determine the condition of the power cylinders and base engine.
14. Inspect for engine and flywheel balance. Engine roughness at idle that gets worse with a no load acceleration may be caused by an out of balance condition.
• Following the procedure in the Engine Service Manual remove the vibration damper and inspect the elastomer layer for cracks and misalignment. If no problem is found, install vibration damper following the assembly procedure found in the Engine Service Manual. Verify that locating dowel on the end of the crankshaft pulley is aligned with locating hole in the balancer for proper installation.
• Isolate the engine from transmission by removing the transmission, clutch and pressure plate or torque converter. Start the engine and evaluate for roughness. If the engine runs smooth, replace the torque converter or replace clutch and pressure plate.
• Following the procedure in the Engine Service Manual, remove the flywheel and verify proper orientation. If orientation is correct, replace or rebalance flywheel. When removing/installing the flywheel, ensure that
the locating dowel is in the right place and that the flywheel is located properly on the dowel.
• For new engines only remove the oil pan following the procedure in the Engine Service Manual. Inspect the crankshaft counterweights to ensure balance holes exist.
