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 | 8.9:1 | |
| Minimum Compression | ||
| Pressure | (1) 100 psi (690 kPa) at 130 RPM | |
| (1) Maximum difference between each cylinder is 25 percent. | ||
| (1) | Maximum difference between each cylinder is 25 percent. |
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
- Fuel pressure must be fully released before opening fuel system or removing any fuel-carrying components. To release pressure in tank, open fuel tank cap slowly.
- To release remaining pressure in system, disconnect injector wiring harness. Using jumper wire, connect injector harness ground terminal No. 1 to ground. (Scheme 81) Connect injector harness positive terminal No. 2 to positive battery terminal with second jumper wire.
- DO NOT keep injector connector connected to positive battery terminal for longer than 5 seconds. Remove jumper wires. This releases system pressure. Continue fuel system service.
Scheme 81
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 - 2.5L TBI 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.
On 2.5L, 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 - 2.5L TBI 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 - 2.5L TBI 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 - 2.5L article in the ENGINE PERFORMANCE section.
SUMMARY
If no faults were found while performing 2.5L BASIC DIAGNOSTIC PROCEDURES, proceed to TESTS W/CODES - 2.5L TBI article in the ENGINE PERFORMANCE section. If no hard codes are found in self-diagnostics, proceed to 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.