Compression
Check engine mechanical condition using a compression gauge, vacuum gauge, or an engine analyzer capable of performing a relative compression test. See engine analyzer instruction manual for availability and description of relative compression feature.
Check engine compression with engine at normal operating temperature at specified cranking speed, all spark plugs removed and throttle wide open.
| Application | Specification |
|---|---|
| Compression Ratio | 9.1:1 |
| Minimum Compression Pressure | (1) 100 psi (690 kPa) |
| (1) Maximum difference between each cylinder is 25 psi (1172 kPa). | |
| (1) | Maximum difference between each cylinder is 25 psi (1172 kPa). |
COMPRESSION SPECIFICATIONS
Exhaust System Backpressure
Check exhaust system using a vacuum gauge or pressure gauge. Remove O2 sensor or air injection check valve (if equipped). Connect a 1 - 5 psi (6.9 - 34.5 kPa) pressure gauge, and run engine at 2500 RPM. If exhaust system backpressure is greater than 1.75 - 2 psi (12.0 - 13.8 kPa), exhaust system or catalytic converter is plugged.
If using a vacuum gauge, connect vacuum gauge hose to intake manifold vacuum port and start engine. Observe vacuum gauge. Open throttle part way and hold steady. If vacuum gauge reading slowly drops after stabilizing, check exhaust system for restriction.
FUEL PRESSURE
| WARNING | Always relieve fuel pressure before trying to open system for testing or component replacement. High fuel pressure may be present in fuel lines and components. DO NOT allow fuel to contact engine or electrical parts while testing fuel system components. |
Fuel Pressure Release
- Disconnect negative battery cable. Slowly open fuel cap to release fuel tank pressure. Remove protective cap from pressure test port on fuel rail. Fuel pressure test port is located at rear of engine. CAUTION: To minimize fuel spillage, wrap a shop towel around and under pressure test port.
- Use Fuel Pressure Gauge/Hose (C-4799), with gauge removed, with gauge end of hose placed into approved gasoline container. To release pressure, screw other end of hose onto pressure test port. After pressure is released, remove hose from test port and continue fuel system service. When servicing is completed, install protective cap on fuel rail test port.
| WARNING | Before disconnecting any fuel line, or fuel system component during testing, repeat FUEL PRESSURE RELEASE procedure. Perform FUEL PUMP TEST with fuel tank at least half full. |
AUTO SHUTDOWN (ASD) RELAY
Note. If ASD relay is suspected of causing a fuel or ignition system malfunction, use Diagnostic Readout Box (DRB) to diagnose ASD relay. See TESTS W/CODES - 3.9L article in the ENGINE PERFORMANCE section.
Auto Shutdown (ASD) relay provides battery voltage to alternator field winding, electric fuel pump, fuel injectors, ignition coil and O2S sensor heating element. ASD relay is PCM controlled by switching ASD relay ground circuit on and off. ASD relay is located inside Power Distribution Center (PDC) which is located in engine compartment. See label on PDC cover for relay and fuse identification.
PCM determines engine speed and synchronizes injectors from distributor pick-up signal. When distributor pick-up signal is not received when ignition switch is in RUN or START position, PCM turns ASD relay ground circuit off. ASD relay turns off battery voltage to alternator field winding, electric fuel pump, fuel injectors, ignition coil and O2S sensor heating element.
If ASD relay has a suspected malfunction, replace with a known good part and retest. If malfunction is still present, refer to TESTS W/CODES - 3.9L article in the ENGINE PERFORMANCE section.
FUEL PUMP RELAY
Note. If fuel pump relay is suspected of causing a fuel or ignition system malfunction, use Diagnostic Readout Box (DRB) to diagnose pump relay. See TESTS W/CODES - 3.9L article in the ENGINE PERFORMANCE section.
Fuel pump relay is controlled by PCM through the same ground circuit as ASD relay or through ASD relay. When PCM turns ASD relay ground circuit off, fuel pump relay ground circuit is also turned off. See AUTO SHUTDOWN (ASD) RELAY.
IDLE SPEED & IGNITION TIMING
Ensure idle speed and ignition timing are set to specification. Always refer to emission control information label in engine compartment before servicing. If manual and emission label specifications differ, always use specifications on emission label. For adjustment procedures, see ADJUSTMENTS - 3.9L article in the ENGINE PERFORMANCE section.
SUMMARY
If no faults were found while performing BASIC TESTING PROCEDURES, proceed to TESTS W/CODES - 3.9L article in the ENGINE PERFORMANCE section. If no hard codes are found in self-diagnostics, proceed to appropriate TESTS W/O CODES - GASOLINE article in the ENGINE PERFORMANCE section for diagnosis by symptom (i.e., ROUGH IDLE, NO-START, etc.) or intermittent diagnostic procedures.