Engine Controls - Basic Testing Jeep Wrangler I

INTRODUCTION

The following diagnostic steps can help prevent overlooking a simple problem. This is also the place to start diagnosis for a "NO-START" condition.

The first step in diagnosing any driveability problem is to verify the problem exists with a test drive under similar conditions.

Before entering self-diagnostics (if equipped), a careful and complete inspection of several systems is required. Most driveability or "no-start" problems are not related to computerized engine control systems, but are in fact simple mechanical, electrical, fuel or vacuum related faults. Most engine control problems are the result of mechanical breakdowns, poor electrical connections or damaged or misrouted vacuum hoses.

Before considering the computer system as a possible cause of problems, ignition high tension wires, fuel supply, electrical connections and vacuum hoses should be checked. Failure to do so may result in improper diagnosis or lost diagnostic time.

Note. All voltage and resistance tests should be performed with a Digital Volt-Ohmmeter (DVOM) with a minimum 10-megohm input impedance, unless stated otherwise in test procedure.

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.

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.

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 1) Install cap over test port.

Scheme 1

FUEL SYSTEM PRESSURE TEST (2.5L)

1. Remove pressure test port plug from throttle body. (Scheme 2) 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 3) 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 2

Scheme 3

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

CAPACITY (VOLUME) TEST (4.2L & 5.9L)

Operate engine at curb idle. Open hose restrictor for 30 seconds and allow fuel to discharge into graduated container. If volume of fuel is less than 1 pint (470 ml), repeat test using an auxiliary fuel supply and replacement fuel filter. If pump volume conforms to specifications while using auxiliary fuel supply, inspect for a restriction in fuel supply tube from tank and for proper fuel tank venting.

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.

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.

SPARK PLUG & COIL WIRE RESISTANCE VALUES

FAILURE TO START TEST

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

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 .

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 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.

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

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.