Doosan D20S-5 Operation & Maintenance Manual page 139

ROUGH, UNSTABLE, OR INCORRECT IDLE, STALLING
Checks
DEFINITION: The engine runs unevenly at idle. If severe enough, the engine may shake.
Preliminary Checks
Sensor Checks
Fuel System
Checks
Ignition System
Checks
Additional Checks
None.
Check the Heated Exhaust Gas Oxygen Sensors (HEGO) performance:
• Check for silicone contamination from fuel or improperly used sealant. If
contaminated, the sensor may have a white powdery coating result in a high but
false signal voltage (rich exhaust indication). The ECM will reduce the amount of
fuel delivered to the engine causing a severe driveability problem.
Check the Temperature Manifold Absolute Pressure (TMAP) sensor response and
accuracy.
• Check for rich or lean symptom that causes the condition.
• Drive the vehicle at the speed of the complaint.
• Monitoring the oxygen sensors will help identify the problem.
• Check for a sticking mixer air valve.
• Verify proper operation of the EPR.
• Perform a cylinder compression test. Refer to Engine Mechanical in the Service
Manual.
• Check the EPR fuel pressure. Refer to the LPG Fuel System Diagnosis.
• Check mixer assembly for proper installation and connection.
• Check for the proper ignition output voltage using the spark tester J26792 or the
equivalent.
• Verify that the spark plugs are the correct type and properly gapped.
Remove the plugs and inspect them for the following conditions:
• Wet plugs.
• Cracks.
• Wear.
• Improper gap.
• Burned electrodes.
• Blistered insulators.
• Heavy deposits.
Check the spark plug wires by connecting an ohmmeter to the ends of each wire in
question. If the meter reads over 30,000 ohms, replace the wires.
Important: The LPG Fuel system is more sensitive to intake manifold leakage than
the gasoline fuel supply system.
• Check for vacuum leaks. Vacuum leaks can cause a higher than normal idle and
low throttle angle control command.
• Check the ECM grounds for being clean, tight, and in their proper locations.
Check the battery cables and ground straps. They should be clean and secure.
Erratic voltage may cause all sensor readings to be skewed resulting in poor idle
quality.
Action
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Operation Section