JLG 4394RT Service Manual - Manual (Page 118)

SECTION 3 - CHASSIS & SCISSOR ARMS
3-82 – JLG Sizzor – 3121133
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.
² Check for the proper ignition voltage output with J 26792 or the equivalent.
² Verify that the spark plugs are correct for use with LPG (R42LTS)
² Check for faulty spark plug wires
² Check for fouled spark plugs.
Additional Check ² Check for manifold vacuum or air induction system leaks
² 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 ² 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.
² Check for the proper ignition coil output voltage using the spark tester J26792 or the equivalent.
² 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.
² Check the connection at each ignition coil.
² Check for deteriorated spark plug wire insulation.
² Check the spark plugs. The correct spark plugs for LPG are (R42LTS)
² 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.
² Check the engine for the following:
- Improper valve timing
- Engine compression
- Manifold vacuum leaks
- Intake manifold gaskets
- Sticking or leaking valves
- Exhaust system leakage
² Check the intake and exhaust system for casting flash or other restrictions.
Fuel System Checks ² Perform a fuel system diagnosis. Refer to LPG Fuel System Diagnosis.
Lack of Power, Sluggishness, or Sponginess
DEFINITION: The engine delivers less than expected power. There is little or no increase in speed when partially applying the accelerator pedal.
Preliminary Checks ² Refer to Important Preliminary Checks.
² Refer to the LPG Fuel system OBD System Check
² Compare the customer' s vehicle with a similar unit. Make sure the customer has an actual problem. Do not compare the
power output of the vehicle operating on LPG to a vehicle operating on gasoline as the fuels do have different drive feel
characteristics
² Remove the air filter and check for dirt or restriction.
² Check the vehicle transmission Refer to the OEM transmission diagnostics.
Table 3-14. Symptom Diagnosis
Checks Action