JLG 450A_AJ Series II Service Manual - Manual (Page 140)

SECTION 3 - CHASSIS & TURNTABLE
3-100 – JLG Lift – 3121290
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 1. Check fuel pressure. Refer to LPG Fuel System Diagnosis.
2. Check for low fuel pressure during a moderate or full throttle acceleration. If fuel pressure drops below
specification, there is possibly a faulty low pressure regulator or a restriction in the fuel system.
3. Check Manifold Absolute Pressure (MAP) sensor response and accuracy.
4. Check LPL electrical connection
5. Check mixer air valve for sticking or binding.
6. Check mixer module assembly for proper installation and leakage.
7. Check EPR electrical connections.
Ignition System Checks Note: LPG being a gaseous fuel requires higher secondary ignition system voltages than equivalent gas-
oline operating conditions. If a problem is reported on LPG and not gasoline, do not discount the possi-
bility of an LPG only ignition system failure and test system accordingly.
1. Check for proper ignition voltage output with J 26792 or the equivalent.
2. Verify spark plugs are correct for use with LPG (R42LTS)
3. Check for faulty spark plug wires
4. Check for fouled spark plugs.
Additional Check 1. Check for manifold vacuum or air induction system leaks
2. Check generator output voltage.
Backfire
DEFINITION: Fuel ignites in the intake manifold or exhaust system, making a loud popping noise.
Preliminary Check Refer to Important Preliminary Checks.
Ignition System Checks NOTE: LPG, being a gaseous fuel, requires higher secondary ignition system voltages for than equivalent
gasoline operating conditions. The ignition system must be maintained in peak condition to prevent
backfire.
1. Check for proper ignition coil output voltage using spark tester J26792 or equivalent.
2. Check spark plug wires by connecting an ohmmeter to ends of each wire in question. If meter reads
over 30,000 ohms, replace wires.
3. Check connection at each ignition coil.
4. Check for deteriorated spark plug wire insulation.
5. Check spark plugs. Correct spark plugs for LPG are (R42LTS)
6. Remove and inspect plugs for the following conditions:
- Wet plugs.
- Cracks.
- Wear.
- Improper gap.
- Burned electrodes.
- Heavy deposits.
Engine Mechanical Check NOTE: 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.
1. Check engine for the following:
- Improper valve timing.
- Engine compression.
- Manifold vacuum leaks.
- Intake manifold gaskets.
- Sticking or leaking valves.
- Exhaust system leakage.
2. Check 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 3-13. Symptom Diagnosis
Checks Action