COMPRESSION
Engine mechanical condition can be checked 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.
| CAUTION | Use a remote starter to crank engine during compression test. DO NOT use ignition switch. The fuel injectors on many fuel injected models are triggered through ignition switch, which can cause flooding or crankcase contamination. |
EXHAUST SYSTEM BACKPRESSURE
The exhaust system can be checked with a vacuum gauge or a pressure gauge. Remove O2 sensor or air injection check valve (if equipped). Connect a 1-10 psi gauge and run engine at 2500 RPM. If exhaust system backpressure is greater than 1 3/4-2 psi, exhaust system or catalytic converter is plugged.
If a vacuum gauge is used, connect it to intake manifold vacuum and start engine. Observe vacuum gauge. Open throttle part way and hold steady. If vacuum gauge reading slowly drops after stabilizing, exhaust system should be checked for a restriction.
FUEL PRESSURE
Basic diagnosis of fuel system should begin with determining fuel system pressure. High fuel pressure may be present in PFI fuel system components. Relieve PFI fuel pressure before opening system. DO NOT allow fuel to contact engine or electrical parts. Carbureted and TBI systems do not retain fuel pressure when pump is not operating.
FUEL PRESSURE RELEASE (4.0L PFI)
Disconnect negative battery cable. Remove fuel filler cap. Remove cap from pressure test port on fuel rail. Position shop towels to soak up any spilled fuel. Using a small screwdriver or pin punch, push test port valve in to relieve fuel pressure. (Scheme 3) Install cap over test port.
Scheme 3
SPARK PLUG & COIL WIRE RESISTANCE
Check spark plug wire resistance on suspect wires. Resistance should be no greater than specification. See SPARK PLUG & COIL WIRE RESISTANCE VALUES table.
| Wire Length (Inches) | Ohms |
|---|---|
| 0-15 | 3000-10,000 |
| 15-25 | 4000-15,000 |
| 25-35 | 6000-20,000 |
| Over 35 | 8000-25,000 |
SPARK PLUG & COIL WIRE RESISTANCE VALUES
2.5L & 4.0L
Ensure battery has a minimum of 12.5 volts (9.5 cranking volts) available to operate cranking and ignition systems. If engine cranks, but does not start, see appropriate SELF-DIAGNOSTICS article and check ECU.
4.2L & 5.9L
Turn ignition on. Using voltmeter, check voltage between positive terminal of ignition coil and ground. If voltage is 5.5-6.5 volts, perform IGNITION COIL CRANKING TEST. If voltage is not within specifications, check ignition coil further. See appropriate article in SYSTEM/COMP TESTS .
IDLE SPEED & IGNITION TIMING
Ensure idle speed and ignition timing are set to specification. Always refer to Emissions Control Information Label in engine compartment before attempting servicing. If manual and emission label specifications differ, always use specifications on emission label. For adjustment procedures, see ADJUSTMENTS article.
SUMMARY
If no faults were found while performing BASIC TESTING , proceed to TESTS W/CODES article, if available. If no hard codes are found in TESTS W/CODES , or vehicle does not have a self-diagnostic system, proceed to TESTS W/O CODES article for diagnosis by symptom (i.e., ROUGH IDLE, NO-START, etc.), or intermittent diagnostic procedures.