Engine Controls - Basic Testing Geo Prizm pre-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 (if equipped), 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. Perform all voltage tests with a Digital Volt-Ohmmeter (DVOM) with a minimum 10-megohm input impedance, unless stated otherwise in test procedure.

VISUAL INSPECTION

Visually inspect all electrical wiring, looking for chafed, stretched, cut or pinched wiring. Ensure electrical connectors for fit tightly and are not corroded. Ensure vacuum hoses are properly routed and not pinched or cut. See VACUUM DIAGRAMS article 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 an engine analyzer. See engine analyzer manual for specific instructions.

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 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, exhaust system should be checked for a restriction.

FUEL PRESSURE

Note. Fuel system diagnosis should always begin with checking fuel pressure. ALWAYS relieve fuel pressure before disconnecting any fuel injection-related component. DO NOT allow fuel to contact engine or electrical components.

Fuel Pressure Release

Loosen fuel filler cap to relieve tank pressure. Unplug circuit opening relay, located under radio in instrument panel center console, and attached to ECM. Start engine and run until it stalls, crank engine for additional 30 seconds to ensure pressure relief. Remove ignition key. Slowly loosen cold start injector line at fuel rail. Use shop towel to absorb fuel that escapes as fuel pressure is relieved. When fuel stops leaking, tighten line.

Fuel Pump Circuit Test

Remove fuel tank cap. Turn ignition on and listen for fuel pump operating sound. If there is no sound, check fuse, fuel pump relay, fuel pump and all electrical connections.

Fuel Pressure Test

1. Ensure battery is fully charged and ignition switch is off. Relieve fuel system pressure. See FUEL PRESSURE RELEASE. Remove banjo bolt from cold start injector fuel line.
2. Install Fuel System "T" Fitting Adapter (J-38347) with 2 new gaskets. Install Fuel Pressure Gauge (J-34730-B). Install jumper wire between "B+" terminal and Fuel Pump (FP) terminal of diagnostic connector, located on left strut tower. Check for fuel leaks. (Scheme 1)
3. Turn ignition switch on. Open fuel pressure gauge shut-off valve. Open gauge air-purge valve to bleed trapped air/fuel into container.
4. With fuel pump operating and engine off, fuel pressure should be 38-44 psi (2.7-3.0 kg/cm 2 ). Maximum fuel pressure (fuel line pinched) should be more than 57 psi (3.9 kg/cm 2 ). With ignition and fuel pump off, fuel pressure should remain at more than 21 psi (1.5 kg/cm 2 ) for at least 5 minutes. Start engine to measure fuel pressure with engine idling. Fuel pressure should be 30-33 psi (2.0-2.3 kg/cm 2 ).
5. If pressure is not within specification, check for defective fuel pump, pressure regulator or restriction in fuel system. Repair as necessary. Remove pressure gauge. Start engine and ensure no fuel leaks exist.

Scheme 1

REGULATED FUEL PRESSURE

UNREGULATED FUEL PRESSURE

MAXIMUM FUEL PRESSURE

REST FUEL PRESSURE

Available Spark

1. Crank engine and check for a strong Blue spark at coil wire and each spark plug wire using Spark Tester (ST-125). If spark tester is not available, use spark plug with ground electrode broken off to create expanded gap, or using insulated pliers. Another check is to hold high-tension wire terminal 5/16" (8 mm) from a ground while cranking engine.
2. If spark does not occur, check ignition coil, ignitor, rotor and wiring connections. If connections are good, use an ohmmeter to check resistance of high-tension wires. See HIGH TENSION WIRE RESISTANCE table. If resistance is not within specification, replace wires.
3. Check for voltage at ignition coil positive terminal. If voltage is not present, check wiring between ignition switch, coil and ignitor. If voltage is present, check ignition coil resistance. See IGNITION COIL RESISTANCE table. If ignition coil resistance is incorrect, replace coil.
4. If ignition coil resistance is correct, check pick-up coil resistance. See PICK-UP COIL RESISTANCE table. If pick-up coil resistance is incorrect, replace pick-up coil or distributor. If resistance is within specification, check air gap between reluctor and pick-up coil. See PICK-UP COIL AIR GAP table.
5. If air gap is incorrect, adjust as necessary. If air gap is correct, check ignition signal from ECM. See appropriate trouble code chart in TESTS W/CODES article. If signal is okay, replace ignitor.

Scheme 2

Ignition Coil Power Source (Prizm 4A-GE, VIN 5)

Power source travels from ignition switch along Brown wire through noise suppressor, changes color to Black/Orange wire and then travels to coil. Coil is located inside distributor.

Ignition Coil Power Source (Prizm 4A-FE, VIN 6)

Power source travels from ignition switch along Brown wire through noise suppressor to coil. Coil is located inside distributor. (Scheme 2)

Ignition Coil Resistance

1. Remove distributor cap, rotor and dust cover. Using ohmmeter, check resistance between primary terminals of coil. (Scheme 2)and (Scheme 3).
2. Check secondary resistance between coil secondary terminal and primary positive terminal. Replace coil if readings are not within specifications. See IGNITION COIL RESISTANCE table.

Scheme 3

HIGH TENSION WIRE RESISTANCE

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

Crank Angle Sensor

Computerized fuel injected systems use a tach (RPM) reference signal to determine when vehicle is running and when coil and/or injectors should be triggered. Crank angle sensor is incorporated within pick-up coil unit and is adjusted by same procedure as pick-up coil.

Pick-Up Coil

1. Measure pick-up coil resistance between terminals No. 2 and No. 3. (Scheme 4) If resistance is not within specification, replace distributor. See PICK-UP COIL RESISTANCE table.
2. Measure second pick-up coil resistance between terminals No. 1 and No. 3. (Scheme 4) If resistance is not within specification, replace distributor. See PICK-UP COIL RESISTANCE table.
3. If resistances are within specification, check pick-up coil air gaps and adjust as necessary. Replace distributor if air gap specification cannot be obtained. See PICK-UP COIL AIR GAP table.

Scheme 4

PICK-UP COIL AIR GAP (1)

PICK-UP COIL RESISTANCE - Ohms @ 68°F (20°C)

IDLE SPEED & IGNITION TIMING

Ensure idle speed and base ignition timing are set to specification. If necessary, see ADJUSTMENTS article.

IGNITION TIMING SPECIFICATIONS (Degrees BTDC @ RPM)

SUMMARY

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