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GASEOUS FUELS ENGINE PERFORMANCE *
(CNG, LNG, LPG)
For every task in Gaseous Fuels Engine Performance, the following safety requirement
must be strictly enforced: Comply with personal and environmental safety practices
associated with clothing; eye protection; hand tools; power equipment; proper
ventilation; and the handling, storage, and disposal of chemicals/materials
in accordance with local, state, and federal safety and environmental regulations.
IX. GASEOUS FUELS ENGINE PERFORMANCE - CNG, LNG, LPG
A. DIAGNOSIS AND REPAIR
A.1 General
1. Interpret and verify complaint; establish a strategy for diagnosis and repair,
document findings. P-1
2. Interface with vehicle's on-board computer, perform diagnostic procedure
using manufacturer's recommended electronic diagnostic equipment and tools (to
include PC based software, DVOM, and/or data scan tools); determine needed repair.
P-1
3. Perform engine vacuum test; determine needed repair. P-2
4. Perform cylinder balance test; determine needed repair. P-2
5. Perform cylinder compression test; determine needed repair. P-2
6. Perform cylinder leakage test; determine needed repair. P-2
7. Diagnose ignition or fuel problem with an exhaust gas analyzer, an oscilloscope,
and/or engine analyzer; determine needed repair. P-1
A.2 Ignition System Diagnosis and Repair
1. Diagnose no-starting, hard starting, engine misfire, poor driveability, spark
knock, power loss, poor mileage, and emissions problems on vehicles with electronic
ignition (distributorless) systems; determine needed repairs. P-1
2. Diagnose no-starting, hard starting, engine misfire, poor driveability, spark
knock, power loss, poor mileage, and emissions problems on vehicles with distributor
ignition (DI) systems; determine needed repairs. P-2
3. Inspect and test ignition primary circuit wiring and components; repair or
replace as needed. P-2
4. Remove distributor, inspect, test, and service as needed; reinstall. P-2
5. Inspect and test distributor ignition system secondary circuit wiring and
components; replace as needed. P-2
6. Inspect and test electronic ignition (distributorless) system secondary circuit
wiring and components; replace as needed. P-1
7. Inspect and test ignition coil(s); replace as needed. P-1
8. Check and adjust (adjust where applicable) ignition system timing and timing
advance/retard. P-1
9. Inspect and test ignition wiring harness and connectors; replace as needed.
P-2
10. Inspect and test ignition system triggering/input devices; replace as needed.
P-1
11. Inspect and test ignition control module; replace as needed. P-1
12. Remove, inspect, and replace spark plugs according to manufacturer's procedures
(procedures differ for alternative fuel vehicles). P-1
A.3 Fuel Systems - Gasoline (Bi-fuel)
1. Diagnose no-starting, hard starting, poor idle, flooding, hesitation, surging,
engine misfire, excessive emissions, power loss, poor mileage, and/or dieseling
problems on vehicles with injection-type fuel systems; determined needed repairs.
P-1
2. Inspect and test fuel pump and pump controls; replace as needed. P-1
3. Perform fuel injector tests (resistance, current, spray pattern, flow, and
pressure drop); clean or replace as needed. P-1
4. Service air and fuel filters. P-1
5. Verify proper operation of fuel pressure regulator; determine needed repair.
P-1
6. Check minimum idle speed. P-2
7. Inspect the operation of the thermostatic air cleaner; determine need repair.
P-3
8. Test the operation of the fuel evaporation control system; determine needed
repair. P-2
9. Inspect and test PCV system; determine needed repair. P-1
10. Test the operation of the EGR; determine needed repair. P-2
A.4 Fuels Systems - Alternative Fuels (CNG, LNG, LPG)
1. Interpret and verify complaint; establish a strategy for diagnosis and repair,
document findings. P-1
2. Diagnose hot or cold no-starting, hard starting, poor driveability, incorrect
idle speed, poor idle, flooding, hesitation, surging, engine misfire, power
loss, stalling, rich or lean mixture conditions and excessive emissions; determine
needed repair. P-1
3. Retrieve and record Diagnostic Trouble Codes (DTC) through the use of recommended
PC base software, DVOM and/or data scan tool. P-1
4. Test input sensors and related circuitry; determined needed repair. P-1
5. Verify output commands from electronic control module (ECM). P-1
6. Test operation of fuel metering device; service or replace as needed. P-1
7. Inspect and test operation of throttle actuator or throttle assembly; determine
needed repair. P-1
8. Inspect and test operation of waste-gate; determine needed repair. P-3
9. Test fuel supply pressure to the fuel metering device; determine needed repair.
P-1
10. Service system fuel filter(s). P-1
11. Inspect and test catalytic converter; determine needed repair. P-3
12. Analyze symptoms and perform diagnostic procedures on vehicles with supplemental
on-board computer support systems. P-1
13. Diagnose and repair intermittent or complete failure of electric, electronic
or mechanical devices (e.g., hour meters, fuel level indicators, fuel selection
devices). P-2
IX. GASEOUS FUELS ENGINE PERFORMANCE
B. PREVENTATIVE MAINTENANCE
1. Perform pre-maintenance safety inspection; report abnormal or unsafe conditions.
P-1
2. Identify specific maintenance needs (engine oil, ignition systems, catalysts)
for gaseous fueled engines. P-1
3. Identify and inspect all fuel system connections, fittings, hoses and tubing
for damage, chaffing, leaks, routing and mounting; perform necessary maintenance.
P-1
4. Identify and inspect low pressure regulator for mounting integrity. P-2
5. Perform recommended regulator maintenance. P-2
6. Identify and inspect fuel metering device(s); perform necessary maintenance.
P-1
7. Identify and inspect throttle actuating assembly (and linkage) for proper
operation. P-1
8. Identify and inspect secondary ignition components; perform necessary maintenance.
P-1
9. Remove, inspect, and replace spark plugs according to manufacturer's procedures
(procedures differ for alternative fuel vehicles). P-1
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