Fuel - Poor Idle, Driveability (Natural Gas Vehicles)
Number: 92-356-6ESection: 6E
Date: JUNE 1993
Corporate Bulletin No.: 366001
ASE No.: A1, A8
Subject:
UNSTABLE IDLE/POOR DRIVEABILITY IN PARK OR NEUTRAL
Model and Year:
1992 C2 TRUCKS WITH 5.7L ENGINE AND DEDICATED NATURAL GAS VEHICLE
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 I)
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 II).
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 I, II & III).
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. If 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 C/K Service Manual Supplement ST-375-92 NGS, pages 3-91 through 3-92).
III. Install NGV Injectors (see 1992 Natural Gas Powered C/K Service Manual Supplement ST-375-92 NGS pages 4-16 through 4-18).
SERVICE PARTS INFORMATION:
Parts are expected to be available from GMSPO on 6/14/93.
WARRANTY INFORMATION:
For vehicles repaired under warranty, use (see illustration).