JLG 400S Service And Maintenance Manual page 209

Checks
Additional Checks
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 spit-
ting sound at idle, low speed, or hard acceleration for the fuel starvation that can cause the engine to cut-out.
Preliminary Checks
Ignition System Checks
Engine Mechanical Checks
Fuel System Checks
Additional Check
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
Fuel System Checks
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Table 3-14. GM Engine Symptom Diagnosis
² Refer to Important Preliminary 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
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.
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.
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
Refer to Important Preliminary 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.
Action
Cuts Out, Misses
SECTION 3 - CHASSIS & TURNTABLE
3-151