JLG 460SJ Service Manual - Manual (Page 182)

SECTION 3 - CHASSIS & TURNTABLE
3-130 – JLG Lift – 3120895
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 Check the fuel pressure. Refer to LPG Fuel System Diagnosis.
Check for low fuel pressure during a moderate or full throttle acceleration. If the fuel pressure drops below specifica-
tion, there is possibly a faulty low pressure regulator or a restriction in the fuel system.
Check the Manifold Absolute Pressure (MAP) sensor response and accuracy.
Check LPL electrical connection
Check the mixer air valve for sticking or binding.
Check the mixer module assembly for proper installation and leakage.
Check the EPR electrical connections.
Ignition System Checks Note: LPG being a gaseous fuel requires higher secondary ignition system voltages for the equivalent gasoline operating
conditions. If a problem is reported on LPG and not gasoline, do not discount the possibility of a LPG only ignition system
failure and test the system accordingly.
Check for the proper ignition voltage output with J 26792 or the equivalent.
Verify that the spark plugs are correct for use with LPG (R42LTS)
Check for faulty spark plug wires
Check for fouled spark plugs.
Additional Check Check for manifold vacuum or air induction system leaks
Check the generator output voltage.
Backfire
DEFINITION: The fuel ignites in the intake manifold, or in the exhaust system, making a loud popping noise.
Preliminary Check Refer to Important Preliminary Checks.
Ignition System Checks Important! LPG, being a gaseous fuel, requires higher secondary ignition system voltages for the equivalent
gasoline operating conditions. The ignition system must be maintained in peak condition to prevent backfire.
Check for the proper ignition coil output voltage using the spark tester J26792 or the equivalent.
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.
Check the connection at each ignition coil.
Check for deteriorated spark plug wire insulation.
Check the spark plugs. The correct spark plugs for LPG are (R42LTS)
Remove the plugs and inspect them for the following conditions:
- Wet plugs
- Cracks
- Wear
- Improper gap
- Burned electrodes
- Heavy deposits
Engine Mechanical Check Important! The LPG Fuel system works on a fumigation principle of fuel introduction and is more sensitive to intake man-
ifold leakage than a gasoline fuel supply system.
Check the engine for the following:
- Improper valve timing
- Engine compression
- Manifold vacuum leaks
- Intake manifold gaskets
- Sticking or leaking valves
- Exhaust system leakage
Check the intake and exhaust system for casting flash or other restrictions.
Fuel System Checks Perform a fuel system diagnosis. Refer to LPG Fuel System Diagnosis.
Table 3-12. Symptom Diagnosis
Checks Action