Engine Controls - Basic Testing: Diagnosis Jeep Cherokee II

VISUAL INSPECTION

Perform a visual inspection of all electrical wiring. Look for chafed, stretched, cut or pinched wiring. Inspect electrical connectors and connections for tight fit and corrosion. Repair as necessary. Inspect all vacuum hoses for proper routing, cuts or pinches. If necessary, see VACUUM DIAGRAMS article to verify routing and connections. Repair as necessary. Inspect air induction system for possible vacuum leaks.

FUEL SYSTEM PRESSURE TEST (2.5L)

1. Remove pressure test port plug from throttle body. (Scheme 1) Install a pressure test fitting in place of test port plug. Connect a 0-30 psi fuel pressure gauge to test fitting. Start engine and idle. Check fuel pressure. See FUEL PRESSURE SPECIFICATION table.
2. If fuel pressure is not to specification, turn adjustment screw on bottom of pressure regulator to obtain correct fuel pressure. (Scheme 2) If fuel pressure is considerably above specification and adjusting pressure regulator fails to correct the problem, inspect fuel line for blockage.
3. If fuel pressure is considerably below specification and adjusting pressure regulator does not correct the problem, momentarily pinch off fuel return line and recheck fuel pressure. If fuel pressure has risen, replace pressure regulator. If pressure has not risen, check fuel filter and supply line for blockage.

Scheme 1

Scheme 2

FUEL SYSTEM PRESSURE TEST (4.0L)

1. Perform FUEL PRESSURE RELEASE (4.0L PFI). Connect a 0-60 psi fuel pressure gauge to test port pressure fitting on fuel rail. Remove vacuum line from fuel pressure regulator. Start vehicle and note gauge reading. See FUEL SYSTEM PRESSURE SPECIFICATION table. Reconnect vacuum line and note gauge reading. See FUEL SYSTEM PRESSURE SPECIFICATION table.
2. If fuel pressure is not 8-10 psi higher with vacuum line disconnected, inspect pressure regulator vacuum line for leaks, kinks or blockage. If fuel pressure is low, momentarily pinch shut hose section of fuel return line. If pressure remains low, inspect fuel supply line, fuel filter and fuel rail inlet for blockage.
3. If fuel pressure rises, replace fuel pressure regulator. If fuel pressure is above specifications, inspect fuel return line for kinks and blockage.

FUEL SYSTEM PRESSURE SPECIFICATIONS

FUEL PUMP PRESSURE TEST (4.2L & 5.9L)

1. Inspect for kinked or bent fuel tubes or hoses or leaks in supply tubes or hoses. Remove air cleaner assembly. Disconnect fuel inlet fitting at carburetor. Disconnect fuel return hose at fuel filter and plug nipple on filter. Remove any spilled fuel from engine.
2. Connect pressure gauge, restrictor and flexible hose between filter and carburetor. Position flexible hose and restrictor so fuel can be discharged into an appropriate graduated container. Operate engine at idle speed and discharge fuel into graduated container by momentarily opening hose restrictor.
3. Close hose restrictor, allow pressure to stabilize and note pressure reading on gauge. See FUEL PUMP PRESSURE SPECIFICATIONS table. If pump pressure is not to specifications, and fuel tubes and filter are in satisfactory condition, replace pump. If pump pressure is to specification, perform capacity test.

FUEL PUMP PRESSURE SPECIFICATIONS

TESTING SPARK AT COIL

1. Remove coil wire from distributor cap. Hold end of cable approximately 1/4" from ground. Crank engine and check for spark from coil wire. Spark should be present and constant. If not, check spark plug and coil wire resistance.
2. If spark is present and constant, continue cranking engine. Slowly move coil wire away from ground. As coil wire is moved away from ground, check for arching at coil tower. If arching at coil tower is present, replace coil.
3. If spark is good and no arching is present, secondary ignition system is producing necessary voltage. Ensure spark reaches spark plugs by checking condition of rotor, cap, spark plug wires and spark plugs. Repair or replace as necessary.

FAILURE TO START TEST

Note. Perform TESTING SPARK AT COIL before proceeding with test.

IGNITION COIL CRANKING TEST (4.2L & 5.9L)

Turn ignition switch to START position while observing voltmeter. Battery voltage should exist. If battery voltage does not exist, check for short or open in wire attached to starter solenoid "I" terminal. Perform IGNITION COIL RESISTANCE TEST. Check starter solenoid for proper operation. See appropriate article in SYSTEM/COMP TESTS .

IGNITION COIL RESISTANCE TEST (4.2L & 5.9L)

1. Remove primary and secondary leads from ignition coil. Using ohmmeter, check primary resistance between (positive) and (negative) terminals of coil. See IGNITION COIL RESISTANCE SPECIFICATIONS (4.2L & 5.9L) table.
2. Check secondary resistance between center tower and either (positive) or (negative) terminals of coil. See IGNITION COIL RESISTANCE SPECIFICATIONS (4.2L & 5.9L) table. Replace coil if readings are not within specification.

IGNITION COIL RESISTANCE SPECIFICATIONS (4.2L & 5.9L)

IDLE RPM TEST

Note. Engine idle set RPM should be tested and recorded when vehicle is first brought into shop for testing. This will assist in diagnosing complaints of engine stalling, creeping, and hard shifting on vehicles equipped with automatic transmissions. Curb idle speed on 2.5L and 4.0L are controlled by electronic control unit and are not adjustable.

1. Using a tachometer, connect Red tachometer lead to negative terminal of coil and Black lead to ground.
2. Turn selector switch to appropriate cylinder position of engine being tested. Start engine and warm to operating temperature.
3. Turn tachometer RPM switch to 1000 RPM position.
4. With engine at operating temperature, momentarily open throttle and release to ensure there is no binding in linkage and throttle lever is fully against its stop.
5. Note engine RPM. See CURB IDLE SPEED table. If engine RPM is not within specification, see appropriate TESTS W/CODES article for 2.5L and 4.0L and see ADJUSTMENTS article for 4.2L and 5.9L.

CURB IDLE SPEED