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Engine Controls - Basic Testing - 4.0l Jeep Grand Cherokee ZJ

Testing & Diagnostics 2 illustrations ~1325 words

INTRODUCTION

Note. For Grand Cherokee with 5.2L engine, see appropriate BASIC TESTING - 5.2L article in this section.

The following diagnostic steps will help prevent overlooking a simple problem. This is also where to begin diagnosis for a no-start condition.

The first step in diagnosing any driveability problem is verifying the customer's complaint with a test drive under the conditions the problem reportedly occurred.

Before entering self-diagnostics, perform a careful and complete visual inspection. Most engine control problems result from mechanical breakdowns, poor electrical connections or damaged/misrouted vacuum hoses. Before condemning the computerized system, perform each test listed in this article.

Note. Unless stated otherwise in test procedure, perform all voltage tests with a Digital Volt-Ohmmeter (DVOM) having a minimum 10-megohm input impedance.

VISUAL INSPECTION

Visually inspect all electrical wiring, looking for chafed, stretched, cut or pinched wiring. Ensure electrical connectors fit tightly and are not corroded. Ensure vacuum hoses are properly routed and are not pinched or cut. See VACUUM DIAGRAMS article in this section to verify routing and connections. Inspect air induction system for possible vacuum leaks.

Compression

Check engine mechanical condition with a compression gauge, vacuum gauge or engine analyzer. See engine analyzer manual for specific instructions.

WARNINGDO NOT use ignition switch during compression tests on fuel injected vehicles. Crank engine with a remote starter. Fuel injectors on many models are triggered by ignition switch during cranking mode, which can create a fire hazard or contaminate the engine's oiling system.
ApplicationSpecification
4.0L8.7:1
5.2L9.1:1
Compression Pressure
Normal120-150 psi (827-1034 kPa)
Minimum100 psi (689 kPa)
Maximum Variation Between Cylinders30 psi (207 kPa)

COMPRESSION SPECIFICATIONS

Exhaust System Backpressure

The exhaust system can be checked with a vacuum or pressure gauge. Remove O2 sensor. Connect a 0-5 psi pressure 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 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

Basic diagnosis of fuel system should begin with determining fuel system pressure.

WARNINGALWAYS relieve fuel pressure before disconnecting any fuel injection-related component. DO NOT allow fuel to contact engine or electrical components.

Fuel System Pressure Release

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 70) Install cap over test port. Reconnect negative battery cable.

Scheme 70

Scheme 70: Fuel System Pressure Release
Scheme 70: Fuel System Pressure Release

Deadhead Pressure

  1. Relieve fuel system pressure. See FUEL SYSTEM PRESSURE RELEASE under «FUEL PRESSURE»(/jeep/grand-cherokee/zj-1992-1996/remont/testing-diagnostics/#engine-controls-basic-testing-40l__fuel-pressure). Connect a 0-100 psi (0-7.0 kg/cm 2 ) fuel pressure gauge to the pressure test port fitting on fuel rail.
  2. Remove fuel pump relay (located in power distribution center). Using a jumper wire, connect terminals No. 30 and 87. (Scheme 71) Note fuel pressure gauge reading, and momentarily pinch off fuel return line.
  3. Fuel pressure should be at least 75 psi (5.5 kg/cm 2 ). DO NOT allow pressure to exceed 100 psi (7.0 kg/cm 2 ). Release fuel return line.

Fuel System Pressure Test

  1. Relieve fuel system pressure. See FUEL SYSTEM PRESSURE RELEASE under «FUEL PRESSURE»(/jeep/grand-cherokee/zj-1992-1996/remont/testing-diagnostics/#engine-controls-basic-testing-40l__fuel-pressure) . Connect a 0-100 psi (0-7.0 kg/cm 2 ) fuel pressure gauge to the pressure test port fitting on fuel rail.
  2. Remove vacuum line from fuel pressure regulator. Start vehicle and note gauge reading. Reconnect vacuum line and note gauge reading. See «FUEL SYSTEM PRESSURE SPECIFICATIONS»(/jeep/grand-cherokee/zj-1992-1996/remont/testing-diagnostics/#engine-controls-basic-testing-40l) table. Ensure pressure readings are as specified. If fuel pressure is not higher with vacuum line disconnected, inspect pressure regulator vacuum line for cause of no vacuum.
  3. If fuel pressure is high, inspect fuel return line for kinks and blockage. If fuel pressure is low, momentarily pinch off fuel return line. If pressure remains low, inspect fuel supply line, fuel filter and fuel rail inlet for blockage. If pressure rises, replace fuel pressure regulator.
ApplicationPsi (kg/cm 2 )
Vacuum Line Disconnected39-41 (2.70-2.88)
Vacuum Line Connected31 (2.18)

FUEL SYSTEM PRESSURE SPECIFICATIONS

Fuel System Rest Pressure

  1. Relieve fuel system pressure. See FUEL SYSTEM PRESSURE RELEASE under «FUEL PRESSURE»(/jeep/grand-cherokee/zj-1992-1996/remont/testing-diagnostics/#engine-controls-basic-testing-40l__fuel-pressure) . Connect a 0-100 psi (0-7.0 kg/cm 2 ) fuel pressure gauge to the pressure test port fitting on fuel rail. Start engine and note reading on pressure gauge. Shut engine off.
  2. Wait 30 minutes and check fuel pressure. Fuel pressure should be within 20 psi (1.4 kg/cm 2 ) of first reading. If pressure is not as indicated, check fuel system for leaks at fuel pressure regulator, fuel pump check valve or fuel injectors.

Capacity (Volume) Test

  1. Relieve fuel system pressure. See FUEL SYSTEM PRESSURE RELEASE under «FUEL PRESSURE»(/jeep/grand-cherokee/zj-1992-1996/remont/testing-diagnostics/#engine-controls-basic-testing-40l__fuel-pressure). Disconnect fuel line on fuel rail. Place fuel line into a graduated container.
  2. Remove fuel pump relay (located in power distribution center). Using a jumper wire, connect terminals No. 30 and 87. (Scheme 71) Volume of fuel should be one pint. (.47L) in 30 seconds. If fuel volume is not as specified, check for a plugged fuel filter or fuel pump inlet sock.

Fuel Pump Relay

  1. Disconnect negative battery cable. Remove Powertrain Control Module (PCM) harness connector. Reconnect battery cable. Turn ignition on. Check for voltage at Pink wire in PCM harness connector. If voltage is not present, go to step 4). If voltage is present, ground the wire. Fuel pump should work.
  2. If fuel pump does not work, check if relay clicks. If relay does not click, go to step 5). If relay clicks, ensure voltage is present at relay terminals No. 30 (battery) and 87 (fuel pump), and at fuel pump. (Scheme 71)
  3. If fuel pump still does not work, check ballast resistor and fuel pump ground. If ballast resistor and ground are okay, replace fuel pump.
  4. If no voltage is present, ensure wire has continuity to fuel pump relay terminal No. 85. Ensure voltage (from ignition switch) is present at relay terminal No. 86. (Scheme 71)
  5. If all circuits are okay or fuel pump relay did not click in step 2), check resistance between relay terminals No. 86 and 85. (Scheme 71) Resistance should be 70-80 ohms. If resistance is not 70-80 ohms, replace relay.
  6. Disconnect negative battery terminal. Reconnect PCM harness connector. Connect battery cable. Check fuel pump operation. If fuel pump does not operate, see «TESTS W/CODES - 4.0L»(/jeep/grand-cherokee/zj-1992-1996/remont/testing-diagnostics/#engine-controls-tests-wcodes-40l) article in this section and check PCM.

Scheme 71

Scheme 71
Scheme 71

Spark Test

  1. Remove coil wire from distributor cap. Hold end of cable about 1/4" from ground. Crank engine. Spark should be present and constant. If spark is not present and constant, check coil and coil wire resistance. Perform «IGNITION PRIMARY»(/jeep/grand-cherokee/zj-1992-1996/remont/testing-diagnostics/#engine-controls-basic-testing-40l__ignition-primary) test under IGNITION CHECKS.
  2. If coil wire spark is present, ensure spark reaches spark plugs. Check condition of rotor, cap, spark plug wires and spark plugs. Repair or replace if necessary.

Spark Plug & Coil Wire Resistance

Resistance should be 250-1000 ohms per inch.

Ignition Coil Resistance Test

  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»(/jeep/grand-cherokee/zj-1992-1996/remont/testing-diagnostics/#engine-controls-basic-testing-40l) table.
  2. Check secondary resistance between center tower and negative terminal of coil. See «IGNITION COIL RESISTANCE»(/jeep/grand-cherokee/zj-1992-1996/remont/testing-diagnostics/#engine-controls-basic-testing-40l) table. Replace coil if readings are not within specification.
ApplicationPrimarySecondary
Diamond Coil.97-1.1811,300-15,300
Toyodenso Coil.95-1.2011,300-13,300

IGNITION COIL RESISTANCE

IGNITION PRIMARY

Note. Perform SECONDARY SPARK under IGNITION CHECKS before proceeding with test.

  1. Ensure battery has a minimum of 12.5 volts (9.5 cranking volts) available to operate cranking system. Ensure battery cable connections are clean and tight.
  2. Using an analog voltmeter, backprobe the following wires at the distributor. Touch positive lead to Gray/Black wire. Touch negative lead to Black/Light Blue wire.
  3. Set voltmeter to 15-volt DC scale. Crank engine. If voltmeter needle does not fluctuate between 0-5 volts, check related circuits. If necessary, replace camshaft position sensor. If camshaft position sensor is okay, but engine does not start, see «TESTS W/CODES - 4.0L»(/jeep/grand-cherokee/zj-1992-1996/remont/testing-diagnostics/#engine-controls-tests-wcodes-40l) article in this section and check PCM.

SUMMARY

If no faults were found while performing BASIC TESTING , proceed to TESTS W/CODES - 4.0L article in this section. If no hard codes are found in self-diagnostics, proceed to TESTS W/O CODES article in this section for diagnosis by symptom (i.e., ROUGH IDLE, NO START, etc.) or intermittent diagnostic procedures.

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