Gm Engine Lpg Fuel System Diagnosis; Fuel System Description; Diagnostic Aids - JLG 660SJ Service And Maintenance Manual

3.20 GM ENGINE LPG FUEL SYSTEM DIAGNOSIS

Fuel System Description

Figure 3-105. DEPR
The Engine Control Module (ECM) receives information from
various engine sensors to control the Direct Electronic Pres-
sure Regulator (DEPR) and lock-off Valve.
The lock-off Valve solenoid prevents fuel flow unless the
engine is cranking or running.
At Key ON, the DEPR valve receives a two (2) second prime
pulse from the ECM, allowing time for the fuel to flow through
the fuel filter and fuel lines to the DEPR.
Fuel travels from the lock-off to the light duty 2- stage regula-
tor into the DEPR.
Engine cranking generates vacuum which provided lift for the
mixer air valve and is commonly referred to as air valve vac-
uum. Once in the mixer, fuel is combined with air and drawn
into the engine for combustion.
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SECTION 3 - CHASSIS & TURNTABLE

Diagnostic Aids

This procedure is intended to diagnose a vehicle operating on
LPG. If vehicle will not continue to run on LPG, refer to Hard
Start for preliminary checks.
NOTE: Before proceeding with this procedure, verify vehicle has a
sufficient quantity of fuel and that liquid fuel is being deliv-
ered to the LPR. Ensure the LPG tank manual shut off valve
is fully opened and excess flow valve has not been acti-
vated.
Tools Required:
• 7/16 Open end wrench (for test port plugs)
• DVOM (GM J 39200, Fluke 88 or equivalent).
• 12 volt test light
Diagnostic Scan Tool
• Diagnostic Display tool.
Pressure Gauges
• IMPCO ITK-2 Test kit
• Water Column Gauge / Manometer (GM 7333-6 or equiva-
lent).
• 0-10 PSI Gauge
Test Description
Numbers below refer to step numbers on the diagnostic table.
5. This step determines if the LPR requires replacement
6. This step determines if the problems are in the mechanical
side of the Pressure Regulator or the Electronic Voice Coil
10. This step determines if the Mixer requires replacement
14. This step determines if the Lock Off requires replacement
17. This step determines if the Fuel Filter requires replacement.
– JLG Lift – 3-91