JLG 4394RT Service Manual - Manual (Page 163)

SECTION 3 - CHASSIS, PLATFORM & SCISSOR ARMS
3121249 – JLG Lift – 3-127
Ignition System Checks • Verify that the spark plugs are correct for use with LPG (R42LTS).
• Check the spark plugs. Remove the plugs and inspect them for the following conditions:
- Wet plugs
- Cracks
- Wear
- Improper gap
- Burned electrodes
- Heavy deposits
• Check the ignition wires for the following items:
- Cracking
- Hardness
- Proper connections
Cooling System Checks • Check the engine thermostat for always being open or for the wrong heat range.
Additional Check • Check the transmission shift pattern. Refer to the OEM Transmission Controls section the Service Manual.
• Check for dragging brakes.
Rough, Unstable, or Incorrect Idle, Stalling
DEFINITION: The engine runs unevenly at idle. If severe enough, the engine or vehicle may shake. The engine idle speed may vary in RPM. Either condition
may be severe enough to stall the engine.
Preliminary Check
Refer to Important Preliminary Checks.
Sensor Checks • Check for silicon contamination from fuel or improperly used sealant. The sensor will have a white powdery
coating. The sensor will 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 Heated Exhaust Gas Oxygen Sensor (HEGO) performance.
• Check the Temperature Manifold Absolute Pressure (TMAP) sensor response and accuracy.
Fuel System Checks • 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 LPR fuel pressure. Refer to the LPG Fuel System Diagnosis.
• Check mixer module assembly for proper installation and connection.
Ignition System Checks • Check for the proper ignition output voltage using the spark tester J26792 or the equivalent.
• Verify that the spark plugs are correct for use with LPG (R42LTS).
• Check the spark plugs. 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.
Additional Checks Important: The LPG Fuel system works on a fumigation principle of fuel introduction and 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.
Table 3-16. Symptom Diagnosis
Checks Action