Engine Controls - Basic Diagnostic Procedures: Diagnosis Chevrolet Venture I

VISUAL INSPECTION

Visually inspect all electrical wiring. Look 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. If necessary, see appropriate VACUUM DIAGRAMS article to verify routing and connections. Inspect air induction system for possible vacuum leaks.

Diagnosis

1. Start engine and allow it to reach normal operating temperature. Increase engine speed to 2000-2500 RPM and note gauge. If reading exceeds 1.25 psi (.09 kg/cm 2 ), exhaust system is restricted.
2. Check exhaust system for collapsed pipe, heat distress and possible internal muffler failure. If none of these conditions exist, check for restricted catalytic converter. Replace as necessary.

NO START DIAGNOSIS

Note. For terminal and circuit identification, see appropriate wiring diagram in WIRING DIAGRAMS article.

1. Definition No start is defined as engine cranks properly, but does not start. Engine may fire a few times.

General Inspection

1. Ensure proper starting procedure is being used. Visually check vacuum hoses for splits, kinks and proper connections, as shown on vehicle's emission control information label. Check ignition wires for cracking, hardness and proper connections at both coil pack and spark plugs.
2. Remove spark plugs. Check and replace as necessary. In very cold temperatures, ensure oil is proper viscosity and not contaminated with gasoline.

Diagnostic Aids

Check TP sensor for inaccuracy, binding or sticking in wide open position, or internal opens or shorts. Check for low fuel level or fuel contamination. Check for engine mechanical problems i.e., low compression (timing chain failure). Check for skewed MAP sensor. The MAP sensor value should change when cranking engine. Compare any questionable barometer readings with that of a known-good vehicle.

Check PCM ground circuits. The PCM ground circuits will only cause a problem if all grounds are not making a good connection. If a PCM ground problem is suspected, the most probable place to inspect is where grounds meet at engine block.

A cold engine that has not been started should display ECT and IAT temperatures that are relatively close to each other.

Intermittent concerns may be caused by several conditions i.e., poor connections, rubbed through wire insulation and broken wires inside their insulation. Check for poor connections or damaged harnesses. Check PCM harness and connectors for improper mating, broken locks, improperly formed or damaged terminals, poor terminal-to-wire connections and damaged harnesses.

1. Perform OBD system check. See ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK in appropriate SELF-DIAGNOSTICS article. After performing OBD system check, go to next step.
2. If any DTCs are set, diagnose them first. See appropriate SELF-DIAGNOSTICS article. If no DTCs are set, turn ignition on. Using scan tool, command fuel pump on and off. If fuel pump runs, go to next step. If fuel pump does not run, see «FUEL PUMP ELECTRICAL CIRCUIT»(ref-3909-S00920448522000052300000) .
3. Connect fuel pressure gauge. See «BASIC FUEL SYSTEM CHECKS (GASOLINE)»(ref-3909-S32232778352000052300000) . Turn ignition on and check fuel pressure. If fuel pressure is 52-59 psi (3.7-4.1 kg/cm 2 ), go to next step. If fuel pressure is not as specified, diagnose fuel system. See «BASIC FUEL SYSTEM CHECKS (GASOLINE)»(ref-3909-S32232778352000052300000) .
4. Disconnect 24X CKP sensor. Using scan tool, monitor 3X crank sensor parameter while cranking engine. If scan tool indicates 3X reference signal, go to next step. If scan tool does not indicate 3X reference signal, see «IGNITION SYSTEM»(ref-3909-S30865994662000052300000) .
5. Reconnect 24X CKP sensor. Remove INJ fuse (10-amp) located in underhood junction block. Connect Spark Tester (J-26792) to a spark plug wire and connect spark plug wire of companion cylinder to ground. Companion cylinder shares the same ignition coil (i.e., 1/4, 2/5 and 3/6). Crank engine and observe spark tester. Repeat for each cylinder. If spark is present, go to step 7 . If spark is not present, see «IGNITION SYSTEM»(ref-3909-S30865994662000052300000) .
6. Reinstall INJ fuse. Disconnect fuel injector harness connector. Connect Injector Load Tester (J-34730-380) with Adapter (J-34730-410). Observe load tester while cranking engine. If all LEDs for cylinders 1-6 flash, go to next step. If all LEDs do not flash, diagnose fuel injectors. See FUEL SYSTEM (GASOLINE) in appropriate SYSTEM & COMPONENT TESTING article.
7. Check air intake and exhaust systems for restrictions. Check for vacuum leaks and engine mechanical problems. Repair as necessary. If no problem is found, go to step 2 .

1. Ensure proper starting procedure is being used. Visually check vacuum hoses for splits, kinks and proper connections, as shown on vehicle's emission control information label. Check ignition wires for cracking, hardness and proper connections at both coil pack and spark plugs.
2. Remove spark plugs. Check and replace as necessary. In very cold temperatures, ensure oil is proper viscosity and not contaminated with gasoline.

Preliminary Inspection

Note. VCM data is deleted when using scan tool CLEAR INFO function. Before clearing DTCs, use scan tool CAPTURE INFO function to record FREEZE FRAME and FAILURE RECORDS data.

1. Perform OBD system check. See ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK in appropriate SELF-DIAGNOSTICS article. After performing OBD system check, go to next step.
2. Attempt to start engine. If engine starts and continues to run, check for intermittent problem. See appropriate TROUBLE SHOOTING - NO CODES article. If engine does not start, go to next step.
3. Connect scan tool. Monitor VTD FUEL DISABLED parameter while cranking engine. If VTD FUEL DISABLED parameter is active while cranking engine, diagnose anti-theft system. See appropriate ANTI-THEFT SYSTEMS article in ACCESSORIES & EQUIPMENT. If VTD FUEL DISABLED parameter is not active while cranking engine, go to next step.
4. Using scan tool, monitor TP sensor voltage with throttle closed. If TP sensor voltage is greater than 2.5 volts, diagnose using diagnostic procedure for DTC P0121. See appropriate SELF-DIAGNOSTICS article. If TP sensor voltage is not greater than 2.5 volts, go to next step.
5. Using scan tool, monitor ECT sensor. If ECT sensor value is less than -22°F (-30°C), diagnose using diagnostic procedure for DTC P0118. See appropriate SELF-DIAGNOSTICS article. If ECT sensor value is not less than -22°F (-30°C), go to next step.
6. Using Spark Tester (J-26792), check for spark at 2 spark plug wires while cranking engine. If spark is present at both wires, go to next step. If spark is not present at both wires, see «IGNITION SYSTEM»(ref-3909-S37615054182000061700000) .
7. Turn ignition off. Reconnect spark plug wires. Connect fuel pressure gauge. See «BASIC FUEL SYSTEM CHECKS (GASOLINE)»(ref-3909-S32232778352000052300000) . Turn ignition on and check fuel pressure. If fuel pressure is 55-60 psi (3.9-4.2 kg/cm 2 ), go to next step. If fuel pressure is not as specified, see «FUEL PUMP ELECTRICAL CIRCUIT»(ref-3909-S10517198812000052300000) .
8. Turn ignition off. Disconnect fuel injector harness connector at intake manifold. Connect injector test light to each injector circuit one at a time and crank engine. If injector test light flashes for each injector circuit, see «DIAGNOSTIC AIDS»(ref-3909-S09368055862000061700000) . If injector test light does not flash for each injector circuit, diagnose fuel injector circuit. See FUEL SYSTEM (GASOLINE) in appropriate SYSTEM & COMPONENT TESTING article.

If no problems are found in fuel pump circuit or ignition system, check for the following conditions

1. Faulty fuel injectors.
2. Faulty anti-theft system.
3. Fouled spark plugs.
4. EGR valve stuck open.
5. Water in fuel system.
6. Grounded ignition control signal circuit.
7. Restricted exhaust system.

Check ignition system for grounded tachometer circuit, worn distributor cap or worn distributor rotor. Check ignition wires by lightly spraying them with water in order to identify intermittent no starts. Check for incorrect valve timing, loose or broken timing chain, low compression, worn camshaft lobes, or bent or burned intake or exhaust valves.

The VCM incorporates integrated circuits that are fault protected, therefore, if a circuit has failed, the integrated circuit may not be damaged. If this is the case, the system will keep the faulty circuit open until the condition has been corrected. Afterwards, reconnect the VCM and check the circuit. Replace VCM only if circuit is still inoperative.

An intermittent may be caused by a poor connection, rubbed through wire insulation or broken wires inside their insulation. Thoroughly check any circuitry suspected of causing an intermittent complaint. See appropriate TROUBLE SHOOTING - NO CODES article.

1. Ensure proper starting procedure is being used. Visually check vacuum hoses for splits, kinks and proper connections, as shown on vehicle's emission control information label. Check ignition wires for cracking, hardness and proper connections at both coil pack and spark plugs.
2. Remove spark plugs. Check and replace as necessary. In very cold temperatures, ensure oil is proper viscosity and not contaminated with gasoline.

Ignition Relay Diagnosis

1. Perform OBD system check. See ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK in appropriate SELF-DIAGNOSTICS article. After performing OBD system check, go to next step.
2. Turn ignition off. Remove IGN relay from underhood electrical center. Using test light connected to ground, probe IGN relay battery feed circuit at relay cavity. If test light illuminates, go to next step. If test light does not illuminate, go to step 8 .
3. Turn ignition on. Using test light connected to ground, probe IGN relay ignition feed circuit at relay cavity. If test light illuminates, go to next step. If test light does not illuminate, go to step 9 .
4. Turn ignition off. Measure resistance between negative battery terminal and IGN relay ground circuit at relay cavity. If resistance is less than 5 ohms, go to next step. If resistance is not less than 5 ohms, go to step 10 .
5. Turn ignition off. Using fused jumper wire, jumper IGN relay battery feed circuit and IGN relay load circuit together at relay cavities. Using test light connected to ground, probe both sides of the following fuses located in underhood electrical center: ENG I (10-amp) INJR A (15-amp) INJR B (15-amp) If test light illuminates at both sides of all fuses, go to next step. If test light does not illuminate at both sides of all fuses, go to step 10 .
6. Check IGN relay for poor terminal connections. Repair as necessary. If connections are okay, go to next step.
7. Replace IGN relay.
8. Repair open or short to ground in IGN relay battery feed circuit.
9. Repair open in IGN relay ignition feed circuit.
10. Replace underhood electrical center.

Ensure proper starting procedure is being used. Visually check vacuum hoses for splits, kinks and proper connections, as shown on vehicle's emission control information label. In very cold temperatures, ensure oil is proper viscosity and not contaminated. Ensure glow plug system is operating properly.

INJECTOR CIRCUIT DIAGNOSIS

Note. This test applies to vehicles equipped with VCM.

FUEL PUMP FLOW TEST

1. Remove fuse for fuel pump circuit. Disconnect fuel line at fuel pump outlet fitting. Install a hose at fuel pump outlet fitting and place hose in container to collect fuel.
2. Crank engine for 15 seconds. If fuel pump does not supply about a 1/2 pint of fuel in 15 seconds, diagnose fuel system. See «FUEL SYSTEM PRESSURE TEST»(ref-3909-S04610257722000052300000) .

FUEL SYSTEM PRESSURE TEST

Note. For fuel pump circuit testing, see appropriate diagnostic procedure under NO START DIAGNOSIS .

1. Turn ignition off. Install "T" adapter and pressure gauge at injection pump. Start engine and measure fuel pressure.
2. If fuel pressure is not 4 psi (.28 kg/cm 2 ), check fuel lines and fuel tank sending unit for restriction. If fuel lines and sending unit are okay, replace fuel pump.