JLG 680S, Service And Maintenance Manual, Page: 259 / 530

SECTION 3 - CHASSIS & TURNTABLE
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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 drive-
ability 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 result-
ing in poor idle quality.
Engine Mechanical Check 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 accelerator pedal.
Preliminary Checks Refer to Important Preliminary Checks.
Sensor Checks Check Heated Exhaust Gas Oxygen Sensor (HEGO) performance.
Table 3-13. Symptom Diagnosis
Checks Action