GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2000 🢒 GMC T-Series Medium Duty Truck F6/F7/F8 🢒 Bulletins 🢒 Diagnostic Hints/Troubleshooting Guide for Denso Common Rail Fuel System

Diagnostic Hints/Troubleshooting Guide for Denso Common Rail Fuel System

Subject:Diagnostic Hints/Troubleshooting Guide for Denso Common Rail Fuel System

Models:1999-2002 Chevrolet C-Model (T-Series) Medium Duty Tilt Cab Models
2000-2003 Chevrolet WT5500 Medium Duty Tilt Cab Models
1999-2002 GMC C-Model (T-Series) Medium Duty Tilt Cab Models
2000-2003 GMC WT5500 Medium Duty Tilt Cab Models
with 7800 Duramax™ Diesel Engine (VIN 3 -- RPO LG4)
 
1998-2002 Isuzu FSR/FTR/FVR Commercial Medium Duty
2000-2003 Isuzu FRR Commercial Medium Duty
with 6HK1-TC Engine


This bulletin is being revised to update what the Tech 2 displays for Injector Balance Test and firing order. Please discard Corporate Bulletin Number 02-06-04-041 (Section 06 -- Engine/Propulsion System).

The following diagnostic information is an outline of some common conditions that may occur with the LG4 Denso common rail high pressure fuel system. This information may assist technicians in locating and properly diagnosing certain conditions with the LG4 common rail fuel system. Below are some of the common symptoms that may occur.

Low Power, Check Engine Light On, DTC 0226 or 0227 Stored

Condition

Action

The fuel filters may be restricted due to the following conditions:

    • The filter in the water separator may have been overlooked during service.
    • Normal accumulation of debris.
    • The fuel tanks may be corroded.
    • The water separator is full.
    • Use of incorrect aftermarket filter.
  1. Drain the water separator.
    2. 1.1. Clean out the bottom of the bowl.
    1.2. Replace the filter.
  2. Replace both fuel filters (Main and water separator).
  3. Corroded tanks must be replaced.

The pressure limiter valve may be leaking internally.

Replace ONLY the pressure limiter valve.

Cranks but Does Not Start

Condition

Action

Air trapped in the fuel system.

Bleed the fuel system.

The fuel tank ran out of fuel due to the following conditions:

    • A failure of the dual fuel tank transfer pump.
    • Air leaking from the quick-connect fuel tank supply hose (early model).
    • The banjo gaskets may be leaking air into fuel lines or supply pump.
    • A loose fuel line between the tank and the fuel pump.
  1. Inspect the pressure control valves connectors. The yellow identification tape on the wiring connector goes to PCV #1.
  2. Inspect for gasoline in the diesel fuel tank.
  3. Inspect all lines and fittings between the fuel pump and the tank for tightness.

A wiring harness issue due to the following conditions:

    • The harness may be loose at the ECM connector. The retainer bolt may not be seated.
    • The harness may be damaged at the ECM connector or relay connectors due to loose objects behind the passenger seat.
    • Chafing of the harness between the ECM and the engine.

Misfire, Rough Operation, Idle Surge

Condition

Action

Contamination may be present in the injectors due to corroded fuel tanks or poor maintenance.

Do an injector balance test using the Tech 2®.

    • Notice that the Tech  2® displays the firing order in numerical order and not in engine firing order.
    • If DTC 34 is stored, the Q setting was not transferred from the old ECM.

Started but Stalls

Condition

Action

    • The injectors may be shorted to ground.
    • DTCs 158 or 159 may be stored.
    • Sludge may be present on injector terminals.

Remove the valve cover and clean off excess build-up around the injector's terminal.

Unscrew all six injector's terminals off in order to isolate which injector is shorted to ground by using a Digital Voltmeter. It should be an OPEN circuit when check it to ground.

Difficult to Bleed

Condition

Insufficient fuel in fuel tanks.

Improper bleeding techniques. Refer to the Owner's Manual.

Air may be leaking into fuel supply hoses.

Air may be trapped in the fuel hoses or rigid lines.

Air may be trapped in fuel supply pump or rigid lines.

Engine Hesitation/Throttle Delay

 

This condition is normal for the 1998-2002 Isuzu FSR/FTR/FVR and Chevrolet/GMC T6500/T7500 w/ 7800 Duramax™ engine and 2000-2001 Isuzu FRR and Chevrolet/GMC WT5500. A built-in delay is inherent for these models with the electronic "fly by wire" throttle control.

White Exhaust Smoke

Condition

A diesel fuel cetane rating below 37 index may cause white exhaust. A recalibrated engine coolant temperature sensor is available.

Fuel (Dilution)

Condition

Action

A fuel leak may be present under the valve cover from the fuel return line.

Replace with new and improved washers.

A loose injector high pressure inlet fuel line nut under the rocker cover.

Tighten the injector inlet line.

New Available Components

Below are some new components that may aid in correcting the above conditions.

    • Transfer pump control module (it is recommended to replace the transfer pump when replacing the module because many of those operating transfer pumps may be near failure due to old module logic or dry operation)
    • Fan clutch
    • Coolant temperature gauge
    • Washer for fuel return line under valve cover
    • Coolant temperature sensor to control white smoke