JLG 400S Service And Maintenance Manual page 212

SECTION 3 - CHASSIS & TURNTABLE
Checks
Additional Check
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
Sensor Checks
Fuel System Checks
Ignition System Checks
Additional Checks
Engine Mechanical Check
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
Sensor Checks
3-154
Table 3-14. GM Engine Symptom Diagnosis
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
Refer to Important Preliminary 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 delivered to the engine causing a severe drive-
ability problem.
²Check the Heated Exhaust Gas Oxygen Sensor (HEGO) performance:
²Check the Temperature Manifold Absolute Pressure (TMAP) sensor response and accuracy.
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.
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.
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.
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.
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
Refer to Important Preliminary Checks.
² Check Heated Exhaust Gas Oxygen Sensor (HEGO) performance.
Action
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