JLG 4394RT Service Manual - Manual (Page 161)

SECTION 3 - CHASSIS, PLATFORM & SCISSOR ARMS
3121249 – JLG Lift – 3-125
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 condi-
tion 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
specification, 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 oper-
ating conditions. If a problem is reported on LPG and not gasoline, do not discount the possibility of a LPG only igni-
tion 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 gaso-
line 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
manifold 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 3-16. Symptom Diagnosis
Checks Action