| Subject: | Engine Backfire and Related Driveability Issues |
| Models: | 1999-2002 Chevrolet B7 School Bus Chassis |
| 1999-2002 Chevrolet C6-7 Conventional Cab Medium Duty Models |
| 1999-2002 GMC B7 School Bus Chassis |
| 1999-2002 GMC C6-7 Conventional Cab Medium Duty Models |
| with 7.4L or 8.1L Engine (VINs B, E -- RPOs L21, L18) and IMPCO/Quantum
Technologies LPG Fuel System |
This bulletin is being revised to include LPG Fuel System Checks. Please
discard Corporate Bulletin Number 02-06-04-026 (Section
06 -- Engine/Propulsion System).
This bulletin is being issued to assist technicians in efficiently diagnosing
and correcting backfire and other related driveability concerns.
When a vehicle is identified with a backfire condition, it is recommended
that the check procedures listed below be followed in the indicated order.
Important: A performance check of the Alternative Fuel Control System is recommended.
Reference Quantum/Impco Service Manuals for 7.4L and 8.1L engines, as well
as Quantum/Impco Technologies Service Bulletins pertaining to the 7.4L and
8.1L engines.
Background
Unlike a gasoline port fuel injection system, the IMPCO/Quantum LPG
fuel system is a single point gaseous induction system. LPG is metered into
the intake system at a single point upstream of the throttle body. The air
and fuel combine to form a combustible mixture that is transported through
the plenum and intake manifold to the cylinders.
During normal operation, the intake plenum and manifold are always full
of a combustible air-fuel mixture. If the mixture in the intake manifold
is ignited because of an abnormal combustion event, a backfire occurs.
Preliminary Checks
Perform a Diagnostic Circuit Check on the vehicle. Ensure that the MIL
is functioning properly. If the MIL light is illuminated, retrieve any codes
from the PCM and follow the appropriate section of the GM Medium Duty Service
Manual. A severe backfire may set throttle position sensor codes. These codes
must be cleared before the vehicle can be restarted. In addition, perform
the Alternative Fuel On-Board Diagnostic (AF OBD) System Check in the IMPCO
LPG Service Manual Supplement.
Important: After retrieving PCM and ECU codes, verify PCM calibration. Only 2000
model year low NOx vehicles identified with "LNX" on the upper
left corner of the Vehicle Emissions Control Label should have a 3700006X
calibration (ending with 61-69). If PCM reprogramming is required on all other
1999 and 2000 model year LPG MD vehicles, calibration 3700005X (ending with
51-59) is the latest release.
Check the warranty history on model year 2001 8.1L vehicles. Ensure
that Product Safety Campaign Bulletins #01019A (SI Document ID #803944) dated
May 2001 and #01053 (SI Document ID #814259) dated August 2001 have been
performed on affected vehicles. Use labor operation codes associated with
the campaign for reimbursement unless otherwise noted.
In the event of a backfire through the intake manifold, inspect the
entire air intake system for damage including LPG carburetor, MAF sensor
screen, air filter, air filter housing and vacuum plugs and hose connections.
Replace any damaged components as necessary.
Exhaust System
Check for any exhaust leaks ahead of the heated oxygen sensor (HO2S)
including:
| • | Exhaust manifold cracks |
| • | Leaks between the exhaust manifold and the engine cylinder head |
| • | Missing or broken exhaust manifold studs |
| • | Exhaust manifold to takedown pipe "doughnut" seal.
If this seal needs replacement, use P/N 15090919 (refer to Corporate
Bulletin Number 01-06-05-005, dated October 2001). |
Replace parts or repair as required to ensure that there are no exhaust
leaks.
Ignition System
Important: LPG requires higher secondary ignition voltages for equivalent gasoline
operating conditions. The maintenance schedule for spark plugs and spark
plug wires on LPG Medium Duty trucks is more frequent than on gasoline powered
Medium Duty trucks (see IMPCO Medium Duty Owner's Manual Supplement). Spark
plug service is required every 50,000 km (30,000 mi) or 30
months whichever comes first. Ignition wires require replacement at 100,000 km
(60,000 mi) or 60 months whichever occurs first. Severe service may
require more frequent replacement intervals.
Important: Some components on LPG Medium Duty trucks are warranted by Quantum Technologies.
Please contact Quantum Technical Assistance at 1-800-816-8691
for parts, warranty claims or additional information on LPG fuel system components.
| • | Inspect spark plug porcelain between the top terminal and shell
for signs of carbon tracking. If carbon tracking exists, the spark plug and
wires must be replaced together. |
| • | Inspect spark plug wires for evidence of arcing or other damage.
Using an ohmmeter, check the resistance of each wire. Replace any wire that
exceeds 10 kilo ohms. If replacement is necessary, use OEM wires. |
| • | When installing spark plug wires, spark plug porcelain should
be free of dirt, grease and oil. |
| • | New spark plug wires do not have heat shields or plastic conduit
covering. Existing heat shields and plastic conduit covering must be removed
from the old spark plug wire(s) and reinstalled on any newly installed spark
plug wire(s). Ensure spark plug wires are routed away from and not contacting
other wires, hoses, brackets, etc. |
| • | If spark plug replacement is necessary, use AC Delco R42LTS gapped
to 0.030-0.035." Torque to 20 N·m (15 lb ft). |
| • | Inspect for coolant leaks that may drip onto the ignition coils,
spark plugs and spark plug wires. Replace damaged parts and repair leaks as
required. |
Engine Cam Shaft Sensor (if the engine backfires during cranking or
start-up)
Vehicles with 7.4L Engine (VIN B -- RPO L21)
Refer to Corporate Bulletin Number 00-06-04-016,
dated April 2000. (Hard Start or No Start, Possible DTC 1345; Check
Cam Sensor Alignment)
Vehicles with 8.1L Engine (VIN E -- RPO L18)
| • | Refer to Corporate Bulletin Number 01-06-04-012,
dated March 2001. (MIL light on with a DTC P0341, P0342, P0343;
replace cam sensor) |
| • | LPG Medium Duty trucks equipped with the 8.1L engine will not
start without the cam sensor signal. This operation is different than on
a gasoline powered 8.1L engine. |
Engine Crankshaft Sensor
Vehicles with 7.4L Engine (VIN B -- RPO L21)
| • | Check for loose or improper installation, correct if necessary.
Ensure crankshaft sensor is fully seated. During any replacement, fully seat
the crankshaft sensor and do not use the bolt to pull crank sensor into front
cover. Doing so may bend the crankshaft sensor bracket and increase air gap
resulting in an irregular crank signal. |
| • | Check sensor electrical connector and wiring damage, replace as
necessary. |
| • | Check for excessive crankshaft endplay. Endplay should not exceed
0.0110 in. |
Vehicles with 8.1L Engine (VIN E -- RPO L18)
Check for loose or improper installation, correct if necessary. Ensure
crankshaft sensor is fully seated. Refer to appropriate GM Service Manual
for correct seating and installation procedure.
Important: If a crankshaft sensor is replaced, the CKP System Variation Learn Procedure
must be performed in accordance with the GM Service Manual.
Engine Grounds
Inspect battery to frame ground, engine to frame ground, and engine
grounds at rear of cylinder head for damaged terminals and wires or loose
connections. Additional confirmation of engine ground quality can be obtained
using the following procedure:
| • | Inspect ignition coil grounds. Measure voltage drop from terminal
A of ignition coils to negative battery terminal. Reading(s) should not exceed
0.2 Volts DC. |
| • | Inspect PCM grounds. Measure voltage drop from connectors C1 and
C2 at terminals 1 and 40 to negative battery terminal for the 8.1L engine.
Measure voltage drop from connector C1 at terminals 72 and 76 and connector
C2 at terminals 21 and 60 for the 7.4L engine. Reading(s) should not exceed
0.2 Volts DC. |
Heated Oxygen Sensors
| • | The PCM codes for HO2S diagnostics are not enabled for the LPG
conversion. Refer to Quantum/IMPCO Service Manual and Service Bulletins for
proper diagnosis of HO2S codes stored in the ECU. |
| • | Inspect HO2S wiring and connectors for damage. Ensure that connectors
and wires are secured away from the exhaust system. |
| • | HO2S wiring repairs must be performed according to GM Service
Manual procedure. |
LPG Fuel System Checks
Important: For model year 1999 vehicles only, verify that the fuel control solenoid
(FCS) has been updated with part number 100291 per IMPCO Technical Service
Bulletin 99MD-TSB-002.
PCV Port Vacuum Cap (7.4L Only)
Verify the vacuum cap is present. The cap is located on the intake manifold
port behind the throttle body. A backfire may dislodge the cap, creating a
throttle vacuum leak that causes a lean condition that becomes more noticeable
as RPM increases. This may cause excessive crank time, hard start and poor
idle quality. This may also set a HO2S DTC in the PCM and the AF ECU.
LPG Wire Harness Grommet (7.4L Only)
Verify the LPG wire harness grommet is properly seated in the fuel mixer
(carburetor) baseplate.
| • | A backfire may dislodge the grommet, creating an un-metered air
leak between the MAF sensor and the throttle plate. This may also set a MAF
DTC in the PCM and the AF ECU. |
| • | If necessary, secure the grommet using Locite® 380 Black Max
or equivalent. |
Regulator Control Valve (RCV) (7.4L Only)
Check the RCV for damage. The RCV is located on the top center of the
low pressure regulator (LPR).
- A damaged or broken RCV may create a rich condition at idle. This
may also set a HO2S DTC in the PCM and the AF ECU.
- To verify RCV operation with the engine idling, disconnect the
vacuum hose from the RCV and plug the RCV with a finger. If the HO2S cycles
normally after the RCV is plugged, replace the RCV.
LPG Fuel System Operation
Perform a visual inspection of all LPG fuel system components. Verify
that all vacuum and fuel hoses are secure and tight. Remove the fuel mixer
cover and inspect the mixer posts for cracks or damage.
| • | Monitor Heated Oxygen Sensor (HO2S) operation for normal switching
and voltages at idle and at 2500 RPM. |
| - | An insufficient activity HO2S DTC (42 or 63) may indicate a rich
HO2S signal, a lean HO2S signal, or a problem with the HO2S signal circuit. |
| - | A steady reading of 450 milli-volts may indicate a problem with
the HO2S signal circuit. |
| - | A low HO2S reading at idle that improves when engine speed is
raised to 2500 RPM may indicate an exhaust leak ahead of the HO2S. |
| • | If voltages are not within specification: |
| - | Check the LPG fuel control system operation by performing AF Fuel
Control System Diagnosis in the IMPCO LPG Service Manual Supplement. |
| - | Check the LPG fuel system pressures by performing AF Fuel System
Diagnosis in the IMPCO LPG Service Manual Supplement. If a regulator or mixer
is replaced, verify that HO2S readings and FCS duty cycle are within specification
at idle, 2500 RPM and full load. |
| • | Monitor the Fuel Control Solenoid (FCS) duty cycle. Refer to AF
Fuel Control System Diagnosis in the IMPCO LPG Service Manual Supplement for
duty cycle ranges and test connector locations. Check the duty cycle at idle,
2500 RPM, and full load. |
| • | If the FCS duty cycle is not within the specified range (25% to
75%), perform AF Fuel System Diagnosis in the IMPCO LPG Service Manual Supplement. |
AF ECM Grounds
Remove the transmission tunnel cover in the cab and inspect the AF ECM
ground terminal on the back of the right cylinder head. Verify the connection
is clean and tight. Measure the voltage drop from AF ECM connector C001 (Clear)
pins 21 through 24 to the negative battery terminal (8.1L). Measure the voltage
drop from AF ECM connector C001(Blue) pins 22 through 24 to the negative battery
terminal (7.4L). Reading should not exceed 0.2 Volts DC.
Low LPG Fuel Level
Check LPG fuel level and fuel gauge operation.
| • | For proper LPG fuel system operation, the low pressure regulator
(LPR) requires a steady supply of liquid fuel from the LPG fuel tank. If the
vehicle is operated with a low fuel level, the pickup tube inside the tank
may draw vapor instead of liquid. This condition may be most noticeable when
traveling uphill, downhill or over rough roads. |
| • | Operation with low LPG fuel level may cause the vehicle to run
poorly, hesitate, surge or backfire. This may also set a HO2S DTC in the PCM
and the AF ECU. |
LPG Fuel System Restriction
Check the following components for external frosting or freezing:
| • | LPG Fuel Tank Manual Shut-off Valve |
| • | LPG Fuel Lines (Underbody and Underhood) |
| • | LPG Fuel Filter. The filter has a recommended replacement interval
of 12 months 30,000 miles, whichever occurs first. |
| • | Low Pressure Regulator (LPR) |
| • | External frosting or freezing indicates a LPG fuel system restriction
possibly caused by contamination, debris or a plugged LPG fuel filter. This
may cause a fuel pressure drop. Operation with low fuel pressure may cause
the vehicle to run poorly, hesitate, surge or backfire. This may also set
a HO2S DTC in the PCM and the AF ECU. |
Fuel Contamination or Poor Fuel Quality
Check for LPG fuel contamination or poor fuel quality. During LPG refining,
storage and transportation, the potential for contamination exists. This may
be difficult to detect without having an outside lab test a fuel sample. The
following contaminants may be found in LPG:
| • | Water -- may cause fuel system restriction that creates a
lean condition, especially under high load and low ambient temperatures. |
| • | Sulfur -- may cause deterioration of the HO2S. This may send
a false rich signal to the PCM and AF ECM, under-fueling the system, possibly
causing the vehicle to run poorly, hesitate, surge or backfire. |
| • | Rust, Dirt or Hydrocarbon "Heavy Ends" -- may
contaminate and plug the LPG fuel filter, low pressure regulator (LPR), fuel
mixer (carburetor) or control solenoids. |
Bulk Propane Delivery Vehicles
Important: Quantum Technologies does not manufacture or install bulk LPG storage
tanks, or endorse their installation unless completed in accordance with instructions
found on the Vehicle Emission Control Information label located underhood.
The following information is offered as a general guideline only.
Special consideration should be given to propane delivery vehicles equipped
with 2,000 to 3,000 gallon bulk storage tanks.
| • | Propane delivery vehicles may not use a separate LPG motor fuel
tank. Instead, the LPG fuel system is supplied with fuel direct from the bulk
tank. Bulk tanks typically have two fuel supply valves -- a liquid outlet
valve and a vapor outlet valve. Operating the vehicle with both valves open
may create a lean condition. Fuel must be supplied by the liquid oulet valve
ONLY. The vapor outlet valve must be closed. |
| • | A low fuel level in the bulk tank may also create the conditions
and symptoms described above under Low LPG Fuel Level. On a bulk tank, the
liquid pick-up is located on the bottom front of the tank. Although the fuel
level in the bulk tank may appear adequate, fuel sloshing or driving uphill
or over rough roads may cause liquid fuel starvation at the pick-up. |
| • | Because propane delivery vehicles are typically operated off the
bulk tank, there is a greater potential for fuel contamination. The bulk tank
is emptied and refilled at a greater frequency. These vehicles may require
more frequent LPG fuel filter service to prevent the conditions and symptoms
described above Fuel Contamination and Poor Fuel Quality. |
For additional help, contact Quantum Technologies Technical Assistance
at 1-800-861-8691.
