JLG 4394RT Service Manual - Manual (Page 117)

SECTION 3 - CHASSIS & SCISSOR ARMS
3121133 – JLG Sizzor – 3-81
Exhaust System Checks ² Check the exhaust system for a possible restriction:
- Inspect the exhaust system for damaged or collapsed pipes
- Inspect the muffler for signs of heat distress or for possible internal failure.
² Check for possible plugged catalytic converter. Refer to Restricted Exhaust System Diagnosis or Exhaust System in the
GM Base Engine Service Manual
Cuts Out, Misses
DEFINITION: A surging or jerking that follows engine speed, usually more pronounced as the engine load increases which is not normally felt above 1500 RPM.
The exhaust has a steady spitting sound at idle, low speed, or hard acceleration for the fuel starvation that can cause the engine to cut-out.
Preliminary Checks ² Refer to Important Preliminary Checks.
Ignition System Checks ² Start the engine.
² Wet down the secondary ignition system with water from a spray bottle, and look/listen for arcing or misfiring as you
apply water.
² Check for proper ignition output voltage with spark tester J 26792.
² Check for a cylinder misfire.
² Verify that the spark plugs are correct for use with LPG (R42LTS)
² Remove the spark plugs in these cylinders and check for the following conditions:
² Insulation cracks
² Wear
² Improper gap
² Burned electrodes
² Heavy deposits
² Visually/Physically inspect the secondary ignition for the following:
² Ignition wires for arcing, cross-firing and proper routing
² Ignition coils for cracks or carbon tracking
Engine Mechanical Checks ² Perform a cylinder compression check.
² Check the engine for the following:
- Improper valve timing
- Bent pushrods
- Worn rocker arms
- Worn camshaft lobes.
- Broken or weak valve springs.
² Check the intake and exhaust manifold passages for casting flash.
Fuel System Checks ² Check the fuel system - plugged fuel filter, low fuel pressure, etc. Refer to LPG Fuel System Diagnosis.
² Check the condition of the wiring to the low pressure lock-off solenoid.
Additional Check Check for Electromagnetic Interference (EMI).
² EMI on the reference circuit can cause a missing condition.
² Monitoring the engine RPM with a scan tool can detect an EMI.
² A sudden increase in the RPM with little change in the actual engine RPM, indicates EMI is present.
² If the problem exists, check the routing of the secondary wires and the ground circuit.
Hesitation, Sag, Stumble
DEFINITION: The vehicle has a momentary lack of response when depressing the accelerator. The condition can occur at any vehicle speed. The condition
may cause the engine to stall if it' s severe enough.
Preliminary Checks Refer to Important Preliminary Checks.
Fuel System Checks ² Check the fuel pressure. Refer to LPG Fuel System Diagnosis.
² Check for low fuel pressure during a moderate or full throttle acceleration. If the fuel pressure drops below specification,
there is possibly a faulty low pressure regulator or a restriction in the fuel system.
² Check the Manifold Absolute Pressure (MAP) sensor response and accuracy.
² Check LPL electrical connection
² Check the mixer air valve for sticking or binding.
² Check the mixer module assembly for proper installation and leakage.
² Check the EPR electrical connections.
Table 3-14. Symptom Diagnosis
Checks Action