UNSTABLE IDLE/POOR DRIVEABILITY IN P/N (NGV)

SUBJECT: UNSTABLE IDLE/POOR DRIVEABILITY IN PARK OR NEUTRAL (NEW PROM, FILTER, INJECTORS)

MODELS: 1992 C-2500 TRUCKS WITH 5.7L ENGINE (RPO KL5) AND DEDICATED NGV FUEL SYSTEM

CONDITION

Some 1992 C-2500 trucks with an NGV fuel system may exhibit one or more of the following conditions:

1. Unstable idle (idle roll) +/- more than 200 rpms with transmission in park or neutral

2. Poor driveability, low power under load

CORRECTION

A PROM, 7 micron low pressure filter, and improved injectors have been released to correct these conditions. A description of these conditions and remedies follows.

EFFECTS OF FUEL QUALITY ON DRIVEABILITY

Field Experience has revealed a significant difference in the cleanliness of natural gas fuel supplies across the nation. The presence of fuel-borne debris can contribute to poor or reduced performance and fuel system contamination. If NGV component replacement has been necessary as a result of fuel impurities, a 7 micron filter should be installed (P/N 12545590). (See Procedure 1)

IDLE FLUCTUATION

Some NGV trucks may have an idle fluctuation in Park or Neutral. The original NGV calibration had an accepted Park/Neutral idle range of 800 rpm +/- 200 rpm. This may not be acceptable under certain applications. For these cases, a new PROM (P/N 16192816) has been released and should be installed (See Procedure 11).

POOR DRIVEABILITY

For vehicles which, after diagnosis, exhibit failed or contaminated injectors, kit #12545589 which includes the injectors, PROM and filter, should be installed (See Procedures 1, 11 & 111).

FUEL PRESSURE REGULATOR

If during driveability diagnosis, the fuel pressure regulator is found to be defective and replacement is required, care should be taken not to alter the outlet pressure calibration on the new unit during the subassembly process. The new regulator is shipped already calibrated. The calibration is set by the angular orientation of the end plug (opposite the coolant line and gauge port end). The orientation is identified by two aligned detents stamped into the plug and regulator body. When inserting the vent fitting, be sure to back up the plug so as not to move it relative to the body. It the plug is moved, and returned to the aligned position, this does not guarantee correct set pressure as internal parts can shift during the rotation.

Note: The 7 micron filter should be a one-time installation unless the fuel is extremely contaminated. Indications of a plugged filter would be extremely high block learn and integrator values with a lack of power and response under high load or fuel demand.

SERVICE PROCEDURE

I. Installation of Filters

1. Close the Quarter Turn Valve located on inside of frame rail.

2. Loosen the fitting on the end of the flexible fuel line and allow any gas contained in the hose to vent.

3. Disconnect the hose fitting.

4. Loosen and remove fitting (Figure 1, Item 1) from the low pressure solenoid valve.

5. Lubricate the male pipe fitting end of filter (Figure 1, Item 2) with sealant (P/N 12346004) and thread filter into low pressure solenoid. While backing up the low pressure solenoid valve to prevent twisting, tighten the filter to 26-30 N-m (19-22 lbs. ft.).

6. Lubricate the male pipe threads on the fitting (Figure 1, Item 1) with pipe sealant (P/N 12346004) and thread into the filter. While backing up the filter to prevent twisting, torque the fitting to 26-30 N-m (19-22 lbs. ft.).

7. Reconnect and torque 26-30 N-m (19-22 lbs. ft.) the flexible fuel line to the fitting. Be sure to support the flexible line such that upon securing and torquing the fitting, no twist is induced to the flexible line and it is permitted to take a bent shape.

8. Re-open the quarter turn valve.

9. Cycle the ignition key.

10. Check for leaks using a soapy water solution or other suitable solution.

II. Install PROM (see 1992 Natural Gas Powered Sierra Service Manual Supplement X-9262, pages 3-91 through 3-92)

III. Install NGV Injectors (see 1992 Natural Gas Powered Sierra Service Manual Supplement X-9262, pages 4-16 through 4-18)

SERVICE PARTS INFORMATION

P/N Description QTY --- ----------- --- 16192816 NGV PROM Updated 1 12545590 NGV Filter (7 micron) 1 12545616 Injectors (Identified by 2 dot on coil) 12545589 Kit (included PROM, Filter, and injectors) 1

Parts are expected to be available from GMSPO on 6/14/93.

WARRANTY INFORMATION

For vehicles repaired under warranty, use:

Labor Op Description Labor Time -------- ----------- ---------- J6364 Replace PROM See Applicable J8065 Install Filter J8085 Install 2 Injectors Labor Time Guide T7651 Install Kit 1.4 hrs.

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.