JLG 4394RT Service Manual - Manual (Page 120)

SECTION 3 - CHASSIS & SCISSOR ARMS
3-84 – JLG Sizzor – 3121133
Cooling System Checks ² Check the engine thermostat for always being open or for the wrong heat range
Additional Check ² Check the transmission shift pattern. Refer to the OEM Transmission Controls section the Service Manual.
² 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 ²Check 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 deliv-
ered to the engine causing a severe driveability problem.
²Check the Heated Exhaust Gas Oxygen Sensor (HEGO) performance:
²Check the Temperature Manifold Absolute Pressure (TMAP) sensor response and accuracy.
Fuel System Checks ²Check 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.
²Check for a sticking mixer air valve.
²Verify proper operation of the EPR.
²Perform a cylinder compression test. Refer to Engine Mechanical in the Service Manual.
²Check the LPR fuel pressure. Refer to the LPG Fuel System Diagnosis.
²Check mixer module assembly for proper installation and connection.
Ignition System Checks ²Check for the proper ignition output voltage using the spark tester J26792 or the equivalent.
² Verify that the spark plugs are correct for use with LPG (R42LTS)
²Check 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
²Check 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 man-
ifold leakage than the gasoline fuel supply system.
²Check for vacuum leaks. Vacuum leaks can cause a higher than normal idle and low throttle angle control command.
²Check the ECM grounds for being clean, tight, and in their proper locations.
²Check 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.
Engine Mechanical Check ² Check the engine for the following:
- Broken motor mounts
- Improper valve timing
- Low compression
- Bent pushrods
- Worn rocker arms
- Broken or weak valve springs
- Worn camshaft lobes
Surges/Chuggles
DEFINITION: The engine has a power variation under a steady throttle or cruise. The vehicle feels as if it speeds up and slows down with no change in the accel-
erator pedal.
Preliminary Checks Refer to Important Preliminary Checks.
Sensor Checks ² Check Heated Exhaust Gas Oxygen Sensor (HEGO) performance.
Table 3-14. Symptom Diagnosis
Checks Action