Engine Controls - Basic Testing Geo Storm I

INTRODUCTION

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 using Digital Volt-Ohmmeter (DVOM) with a minimum 10-megohm input impedance.

PRELIMINARY INSPECTION & ADJUSTMENTS

Note. The following procedures describe how to check individual systems. For a no-start condition, see DIAGNOSTIC CHARTS at end of article.

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 not pinched or cut. See VACUUM DIAGRAMS article in the ENGINE PERFORMANCE Section to verify routing and connections (if necessary). 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.

ENGINE COMPRESSION

Exhaust System Backpressure

The exhaust system can be checked with a vacuum or pressure gauge. Remove O2 sensor or air injection check valve (if equipped). Connect a 1-10 psi pressure gauge and operate 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 SYSTEM

Basic diagnosis of fuel system begins with determining fuel system pressure.

FUEL PRESSURE RELEASE

1. Loosen fuel tank cap to release fuel tank pressure. Remove fuel pump relay from fuse/relay block located in left corner of engine compartment, near the battery.
2. Start engine and allow to idle until engine dies. Crank engine for approximately 10 seconds to ensure residual fuel line pressure is released. Reinstall fuel pump relay.

FUEL PRESSURE

1. Release fuel pressure. See FUEL PRESSURE RELEASE under FUEL SYSTEM. Disconnect negative battery cable. Disconnect fuel line at fuel filter located below brake booster in engine compartment.
2. Install Adapter (J-235957-10) between fuel line and fuel filter. Install Fuel Pressure Gauge (J-34703-1) on adapter. Disconnect vacuum hose from pressure regulator. Install negative battery cable. Ensure ignition has been off for at least 10 seconds.
3. Turn ignition on and check fuel pressure. 2) Install negative battery cable. Check fuel pressure. See FUEL PUMP PERFORMANCE table. For complete Fuel System Pressure Test Procedure See DIAGNOSTIC CHART A - 7. Release fuel pressure and remove fuel pressure gauge. Release fuel pressure and remove fuel pressure gauge. Install fuel line at fuel filter and tighten to 25 ft. lbs. (34 N.m).

FUEL PUMP PERFORMANCE

IGNITION CHECKS

Note. For a no-start condition, see DIAGNOSTIC CHARTS at end of article.

Spark Check

1. Crank engine and check for a strong Blue spark at coil wire and each spark plug wire using Spark Tester (ST-125).
2. If spark exists on some spark plugs, check for defective distributor cap or defective rotor. Check spark plug high tension wire resistance. Replace spark plug high tension wires if resistance exceeds specification. See HIGH TENSION WIRE RESISTANCE table.

HIGH TENSION WIRE RESISTANCE

Ignition Coil Power Source

Turn ignition on. Using voltmeter, check voltage between positive terminal of ignition coil (Black/Orange wire) and ground. If battery voltage does not exist, check for open circuit in Black/Orange wire between ignition coil and 15-amp ignition coil fuse in junction block. Junction block is located behind left kick panel.

Ignition Coil Resistance

Disconnect electrical connector and high tension coil wire from ignition coil. Using ohmmeter, measure primary and secondary resistance. (Scheme 1) Replace ignition coil if resistance readings are not within specification. See IGNITION COIL RESISTANCE table.

IGNITION COIL RESISTANCE Ohms@68°F (20°C)

Scheme 1

Pick-Up Coil/Crank Angle Sensor

1. Measure pick-up coil/crank angle sensor air gap. Adjust or replace distributor if gap is not .008-.016" (.20-.40 mm.)
2. Check pick-up coil/crank angle sensor resistance. See I - SYS/COMP TESTS article in the ENGINE PERFORMANCE Section.

IDLE SPEED & IGNITION TIMING

Ensure idle speed and base ignition timing are set to specification. For adjustment procedures, see ADJUSTMENTS article in the ENGINE PERFORMANCE Section.

CHART A-3, CRANKS BUT WON'T RUN

Electronic Spark Timing (EST) is controlled by ECM. Ignition module sends a reference signal on Yellow/Red wire to ECM when engine is cranking. With engine speed less than 400 RPM, ignition module will control ignition timing. When engine speed is greater than 400 RPM, ECM applies 5 volts on by-pass line (Yellow/Green wire) to switch timing control to ECM (EST mode). In EST mode, ECM controls ignition timing.

Note. Test numbers refer to test numbers on diagnostic chart.

1. If CHECK ENGINE light is on with ignition on, ECM is receiving voltage. ECM is located under left side of instrument panel, left of steering column, and contains a 24-pin and 32-pin connectors. If TPS voltage exceeds 2.5 volts, engine may be in clear flood mode, resulting in starting problems. Engine will not start without reference pulses; Scan tool should indicate engine speed during cranking.
2. If engine speed is indicated during cranking, ignition module is receiving a crank signal; however, if spark is not present during cranking, ignition module is not triggering ignition coil, or secondary ignition problem exists.
3. Test light should flash, indicating ECM is controlling fuel injectors.
4. This determines whether ignition module is not generating reference pulse, wiring is defective, or ECM is defective. When connecting and removing test light with battery voltage on Yellow/Red wire, a reference pulse should be generated. If engine speed is indicated, ECM and wiring are okay.
5. This checks for ignition voltage at fuel injector wiring harness. Disconnect wiring harness before probing Red/Green wire. Reinstall wiring harness connector after checking for voltage.
6. This checks for open in Blue/White or Blue/Yellow wire from connector to ECM. Ensure fuel injector wiring harness is installed.

DIAGNOSTIC AIDS

Water or foreign material in fuel can cause a no-start condition or lack of power. Sticking EGR valve may cause a low air/fuel ratio, resulting in a no-start condition if system does not enter clear flood mode. Low fuel pressure can lower air/fuel ratio, resulting in a no-start condition.

Scheme 2

Scheme 3

Scheme 4

CHART A-5, CRANKS BUT WON'T RUN

With ignition on, ECM activates fuel pump relay and operates fuel pump. Fuel pump operates as long as engine is running or cranking, and ECM is receiving ignition reference pulses. If no ignition reference pulses are received, ECM will turn off fuel pump within 2 seconds after ignition is turned on. The ECM is located under left side of instrument panel, left of steering column, and contains 24-pin and 32-pin connectors.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This checks for battery voltage at fuel pump relay.
2. This checks fuel pump wiring circuit.
3. This determines if ECM is supplying voltage to fuel pump relay.

Visually inspect wiring and connectors if intermittent problem exists.

Scheme 5

Scheme 6

CHART A-7, FUEL SYSTEM PRESSURE TEST

Fuel pump delivers fuel to fuel rail and fuel injectors, where fuel pressure is maintained at 35-42 psi (2.5-2.9 kg/cm 2 ) by pressure regulator. Excess fuel is returned to fuel tank. With engine stopped, fuel pump can be energized by applying battery voltage to fuel pump relay connector with a fused jumper wire. Fuel pump relay is located in fuse/relay block on left side of engine compartment, near battery.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Release fuel pressure before disconnecting fuel lines. Wrap fuel line with shop towel before disconnecting fuel lines.
2. With engine idling, high manifold vacuum is applied to pressure regulator, lowering fuel pressure to approximately 25-30 psi (1.8-2.1 kg/cm 2 ).
3. Applying vacuum to pressure regulator should decrease fuel pressure.
4. If fuel pressure decreases quickly, it may be caused by one of the following: defective fuel pump check valve, fuel lines leaking, leaking pressure regulator or fuel injectors stuck open.
5. Fuel pressure less than 35 psi (2.5 kg/cm 2 ) may be caused by defective pressure regulator or restricted fuel flow. Low fuel pressure may result in hard starting and poor driveability, and Code 44 may be set.
6. Restricting fuel return line allows fuel pump to obtain maximum fuel pressure. With battery voltage applied to fuel pump relay connector, fuel pressure should be greater than 65 psi (4.6 kg/cm 2 ).
7. This determines if high fuel pressure is caused by fuel line restriction or defective pressure regulator.

Improper fuel pressure may cause the following: engine cranks but will not start, engine cuts out, hesitation, loss of power, and Codes 44 and 45 may be set.

Scheme 7

Scheme 8

SUMMARY

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