JLG 450A_AJ Series II Service Manual - Manual (Page 139)

SECTION 3 - CHASSIS & TURNTABLE
3121290 – JLG Lift – 3-99
Engine Mechanical Checks Important: The LPG Fuel system works on a fumigation principle of fuel introduction and is more sensi-
tive to intake manifold leakage than a gasoline fuel supply system.
1. Check for the following:
- Vacuum leaks
- Improper valve timing
- Low compression
- Bent pushrods
- Worn rocker arms
- Broken or weak valve springs
- Worn camshaft lobes
2. Check intake and exhaust manifolds for casting flash.
Exhaust System Checks 1. Check exhaust system for a restriction:
- Inspect exhaust system for damaged or collapsed pipes.
- Inspect muffler for signs of heat distress or internal failure.
2. Check for plugged catalytic converter. Refer to Restricted Exhaust System Diagnosis or Exhaust System
in GM Base Engine Service Manual.
Additional Checks
Cuts Out, Misses
DEFINITION: A surging or jerking that follows engine speed, usually more pronounced as engine load increases which is not normally felt
above 1500 RPM. Exhaust has a steady spitting sound at idle, low speed, or hard acceleration for fuel starvation that can cause the engine to
cut-out.
Preliminary Checks Refer to Important Preliminary Checks.
Ignition System Checks 1.Start engine.
2. Wet down secondary ignition system with water from a spray bottle and look/listen for arcing or misfir-
ing as you apply water.
3. Check for proper ignition output voltage with spark tester J 26792.
4. Check for a cylinder misfire.
5. Verify spark plugs are correct for use with LPG (R42LTS)
6. Remove spark plugs in these cylinders and check for the following conditions:
- Insulation cracks.
- Wear.
- Improper gap.
- Burned electrodes.
- Heavy deposits.
7. Visually/Physically inspect secondary ignition for the following:
- Ignition wires for arcing, cross-firing and proper routing.
- Ignition coils for cracks or carbon tracking.
Engine Mechanical Checks 1. Perform a cylinder compression check.
2. Check engine for the following:
- Improper valve timing
- Bent pushrods.
- Worn rocker arms.
- Worn camshaft lobes.
- Broken or weak valve springs.
3. Check intake and exhaust manifold passages for casting flash.
Fuel System Checks 1. 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).
- EMI on reference circuit can cause a missing condition.
- Monitoring engine RPM with a scan tool can detect an EMI.
- A sudden increase in RPM with little change in actual engine RPM indicates EMI is present.
- If problem exists, check routing of secondary wires and ground circuit.
Table 3-13. Symptom Diagnosis
Checks Action