4 ENGIN E SY M PTOMS DIAGNOSTICS 135
Rough Idle
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 s tuck open
• Power cylinder problems
• Valve train problems
• Engine or fl ywheel balance problems
• Exhaust system to cab/chassis contact
• Loose/worn engine mounts
Procedure
1. Verify co mp la int. 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 v ibra tions to cab, e sp ecially
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. C heck 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 Diagn ostics 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 (K OER St an d ard 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.DoTest10(InjectionControlPressure)on
Performance Diagn os tics 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
EGES-270-1
Read all safety instructions in the "Safety Information" section of this manual before doing any procedures.
Follow all warnings, cautions, and notes.
© August 2008 Navistar, Inc.
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