Compression
Ensure battery is fully charged. Warm engine to normal operating temperature. Turn off engine. Purge fuel system. See FUEL PRESSURE RELEASE under FUEL SYSTEM. Disconnect wire at ignition coil to prevent engine from starting. Check engine compression with all spark plugs removed and throttle wide open. See COMPRESSION SPECIFICATIONS .
Check engine mechanical condition with a compression gauge, vacuum gauge, or an engine analyzer. See engine analyzer manual for specific instructions.
| Application | Specification | ||
|---|---|---|---|
| Compression Ratio | 9.1:1 | ||
| Normal Compression Pressure | (1) | ||
| Minimum Compression Pressure | 100 psi (690 kPa) | ||
| Maximum Variation Between Cylinders | (1) | ||
| (1) Information not available from manufacturer. | |||
| (1) | Information not available from manufacturer. |
COMPRESSION SPECIFICATIONS
LEAK CHECK
| WARNING | Do not leave a vehicle with leaks inside an enclosed area. Gas buildup will cause a fire hazard. |
If a leak is suspected at a fuel line, fitting or component, pressurize system (open shut-off valves) and using a commercially available liquid leak tester, check suspect area. Allow 3-5 minutes for bubbles to develop. To check fuel cylinders, see FUEL CYLINDER TESTING in SYSTEM & COMPONENT TESTING - RWD VANS - CNG article.
FUEL PRESSURE RELEASE
| WARNING | Fuel system is under high pressure. ALWAYS release fuel pressure properly before attempting to open system for testing or component replacement. System must be purged before removing or replacing components. |
Fuel Tube Purging With Engine Running
Note. Use this procedure if engine runs and fuel cylinders are not being serviced.
Close all 4 fuel control valves on cylinders. Open manual master fuel shut-off valve, located between two side cylinders. Start engine and run until engine dies. Attempt to start engine 3 more times. Fuel system is now purged. If one way check valve or fuel fill receptacle is serviced, high pressure will remain in line. Slowly open fitting to relieve residual pressure while outdoors. Fuel cylinders still contain high pressure. After repairs, ensure high pressure fittings are checked with a go no-go gauge. Do not open fuel control valves until all high pressure components have been resealed.
Fuel Tube Purging Without Engine Running
Note. Use this procedure if engine does not run and fuel cylinders are not being serviced.
Close all 4 fuel control valves on cylinders. Open manual master fuel shut-off valve, located between two side cylinders. Move vehicle outdoors. Remove shield mounted below cylinders on left side of vehicle. Cycle ignition switch to ON position a few times to activate high pressure solenoid. Turn ignition off. Slowly loosen fuel line at low pressure fuel shut-off solenoid. Fuel pressure will bleed out. Leave line loosened until all pressure is released. System is now purged. If one way check valve or fuel fill receptacle is serviced, high pressure will remain in line. Slowly open fitting to relieve residual pressure while outdoors. Fuel cylinders still contain high pressure. After repairs, ensure high pressure fittings are checked with a go no-go gauge. Do not open fuel control valves until all high pressure components have been resealed.
Fuel Cylinder Purging With Engine Running
Note. Use this procedure if engine runs and a fuel cylinder is being serviced.
Open manual master fuel shut-off valve. Open fuel control valve on cylinder to be serviced. Close fuel control valves on all cylinders not being serviced. Start engine and run until engine dies. Attempt to start engine 3 more times. Fuel system is now purged. Cylinder may be removed for service. After repairs, ensure high pressure fittings are checked with a go no-go gauge. Do not open fuel control valves until all high pressure components have been resealed.
Fuel Cylinder Purging Without Engine Running
Note. Use this procedure if engine does not run and a fuel cylinder is being serviced.
Open manual master fuel shut-off valve. Open fuel control valve on cylinder to be serviced. Close fuel control valves on all cylinders not being serviced. Very slowly loosen fuel fitting at control valve of fuel cylinder being serviced. High pressure will be released. Leave line loosened until all pressure is released. Fuel cylinder is now purged. Cylinder may be removed for service. After repairs, ensure high pressure fittings are checked with a go no-go gauge. Do not open fuel control valves until all high pressure components have been resealed.
IGNITION CHECKS
Note. Ignition system is identical to gasoline-fueled vehicles. For ignition system spark testing and component testing, see BASIC DIAGNOSTIC PROCEDURES - TRUCKS - GASOLINE article.
IDLE SPEED & IGNITION TIMING
Idle speed and mixture are controlled by Powertrain Control Module (PCM). No adjustment is required. PCM controls Idle Air Control (IAC) motor for proper idle speed.
Ignition timing is controlled by PCM and is nonadjustable. If vehicle is equipped with a distributor, DO NOT attempt to adjust timing by rotating distributor. If distributor is rotated, fuel system synchronization will be changed. If distributor is rotated or removed, check fuel system synchronization. See FUEL SYSTEM SYNCHRONIZATION under FUEL SYSTEM in ON-VEHICLE ADJUSTMENTS - TRUCKS - GASOLINE article.
SUMMARY
If no faults were found while performing basic diagnostic procedures, proceed to SELF-DIAGNOSTICS - JEEP, TRUCKS & RWD VANS article. If no diagnostic trouble codes are found in self-diagnostics, proceed to TROUBLE SHOOTING - NO CODES - CNG article for diagnosis by symptom (i.e., ROUGH IDLE, NO-START, etc.) or intermittent diagnostic procedures.