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Engine Controls - Basic Testing: Other GMC Vandura P3500

Testing & Diagnostics 8 illustrations ~2909 words

Compression

Check engine mechanical condition using a compression gauge, vacuum gauge or engine analyzer capable of performing a relative compression test. If using engine analyzer, see engine analyzer instruction manual for availability and description of relative compression feature.

Because fuel injectors on many models are triggered by ignition switch during cranking mode, DO NOT use ignition switch during compression tests on fuel injected vehicles. Use a remote starter to crank engine to prevent fire hazard or engine's oiling system contamination.

Exhaust System Backpressure

Before replacing any components, check exhaust system for restrictions. Use a vacuum gauge or a low pressure (1-5 psi) pressure gauge to check exhaust system.

If using a vacuum gauge, connect gauge to intake manifold vacuum, and start engine. Observe gauge while holding throttle steady at 2000-2500 RPM. If vacuum gauge reading slowly drops after stabilizing, exhaust system may be restricted.

  1. Check At Air Pipe Remove rubber hose at exhaust manifold AIR pipe check valve, and remove check valve.Install pressure gauge to hose and nipple via Propane Enrichment Device (J26911). Nipple should be inserted into exhaust manifold AIR pipe.
  2. Check At O2 Sensor Remove O2 sensor. Install backpressure tester in place of O2 sensor. After test is completed, coat O2 sensor threads with anti-seize compound.

Definition

No-start is defined as engine cranks okay, but does not start. Engine may fire a few times.

Note. Before performing following tests, check battery condition, engine cranking speed and fuel supply.

Fuel System (TBI)

  1. Before checking fuel system for a no-start condition, check ignition for adequate spark. Check for proper fuel pump pressure (9-13 psi) and capacity (one pint in 30 seconds). See A-7, BASIC FUEL SYSTEM CHECKS (GASOLINE).
  2. Crank engine, and watch for injector spray. If injector spray occurs, go to step 5). If no spray occurs, disconnect injector harness, and check for battery voltage at harness.Battery voltage should be present on one injector terminal.If battery voltage is not present, check for blown injector power fuse. If battery voltage is present on both terminals, check for wires shorted to one another.
  3. If battery voltage is present on only one terminal, connect injector test light (also called a "node" light) to injector harness. Crank engine, and note light. If light flashes, check for stored ECM codes. See DIAGNOSTIC CIRCUIT CHECK. If no codes are present, refer to HARD START symptom in «TESTS W/O CODES»(ref-10262) article in this section. If light does not flash, momentarily touch test light from battery voltage to ECM RPM reference terminal (circuit No. 430).
  4. Each time test light is removed from ECM RPM reference terminal, injector test light should flash. If test light does not flash, check for open in RPM reference wire or injector drive (ground) circuit. If wiring is okay, replace faulty ECM. Prior to replacing ECM, check ECM power and ground circuits.
  5. If injector spray occurred while cranking engine, disconnect injector harness,and crank engine.If injector spray or leakage occurs, a no-start condition could be caused by excessive fuel being delivered during cranking. Repair faulty injector or injector seal. If no spray or leakage occurs, refer to HARD START symptom in article «TESTS W/O CODES»(ref-10262) in this section.

Fuel System (4.3L Turbo PFI)

  1. Before checking fuel system for a no-start condition, check ignition for adequate spark. Check for proper fuel pump pressure and capacity. See A-7, BASIC FUEL SYSTEM CHECKS (GASOLINE).
  2. Disconnect injector harness. Turn ignition on, and check for battery voltage at each injector harness. Battery voltage should be present on one side of each injector. If battery voltage is not present, check for blown injector power fuse. If battery voltage is present on both injector terminals, check for wires shorted together.
  3. If battery voltage is present on only one terminal,connect injector test light to injector harness. Crank engine, and note light. Repeat on other injector connectors.If light flashes, check for stored ECM codes. If no codes are present, refer to HARD START symptom in the «TESTS W/O CODES»(ref-10262) article in this section.
  4. If light does not flash, disconnect distributor 4-wire HEI-EST connector. Momentarily touch test light from battery voltage to ECM RPM reference wire (circuit No. 430) of 4-wire connector. Each time test light is removed from ECM RPM reference terminal, injector test light should flash. If test light does not flash, check for open in RPM reference wire or injector drive (ground) circuit. If wiring is okay, replace faulty ECM.

Scheme 186

Scheme 186
Scheme 186

Scheme 187

Scheme 187
Scheme 187

Scheme 188

Scheme 188
Scheme 188

Scheme 189

Scheme 189
Scheme 189

Scheme 190

Scheme 190
Scheme 190

Model 700 TBI Unit (2.5L)

Place transmission gear selector in Park (Neutral on manual transmissions). Set parking brake, and block drive wheels. Loosen fuel filler cap to relieve tank pressure. Disconnect 3-terminal electrical connector at fuel tank. Start engine, and allow to run until engine stalls due to lack of fuel. Crank engine 3 seconds to remove any fuel pressure left in fuel lines. Fuel lines are now safe for servicing.

Model 220 TBI Unit (Except 2.5L)

WARNINGFuel pressure is relieved and drops to zero when ignition is turned off. To minimize risk of fire and personal injury, cover area to be disconnected with a shop rag.

Port Fuel Injection (4.3L Turbo)

Fuel system is under pressure. Pressure must be relieved prior to servicing fuel system. Fuel pressure may be relieved by one of following methods.

  1. Disconnect fuel pump at rear body connector. Start engine, and run it until it stalls. Crank starter 3 seconds to remove remaining fuel from fuel lines.Reconnect rear body connector.
  2. Install Fuel Pressure Gauge (J-34730-1) on fuel pressure connector of fuel rail. Wrap shop towel around pressure connection when installing fuel pressure gauge to absorb fuel leakage. Install gauge bleed hose in container. Open bleed valve to bleed fuel pressure.

TBI

  1. Turn ignition off for 10 seconds. Turn ignition on, and listen at fuel tank for fuel pump operation. Pump should run 2 seconds (20 seconds on models equipped with fuel module).If fuel pump runs, go to next step. If fuel pump does not run, go to step 7).
  2. If fuel pump runs, turn ignition off. Verify fuel tank has fuel. Relieve fuel pressure. See «FUEL SYSTEM PRESSURE RELIEF»(ref-10260-S27856027492001110200000) . Remove air cleaner, and plug air cleaner vacuum ports (if equipped). Disconnect fuel line between throttle body and fuel filter. Install Fuel Pressure Gauge (J-29658A) and Adapter (J-2968A-85) between steel fuel line and flexible hose, ahead of in-line fuel filter.
  3. Turn ignition on, and note reading on pressure gauge. If fuel pressure is 9-13 psi (.63-.91 kg/cm 2 ), no problems are present. If pressure is less than 9 psi (.63 kg/cm 2 ), go to step 5). If pressure is greater than 13 psi (.91 kg/cm 2 ), turn ignition off, and bleed fuel pressure. Disconnect fuel return line downstream of pressure gauge. Insert return line into a gasoline container.
  4. Turn ignition on. If pressure is now 9-13 psi (.63-.91kg/cm 2 ), correct restriction in fuel return line between disconnected point and fuel tank. If fuel pressure is greater than 13 psi (.91 kg/cm 2 ), check for restricted return line (including fuel filter) downstream of pressure gauge. If no restrictions are present, replace fuel pressure regulator (TBI 700) or fuel meter cover/pressure regulator (TBI 220).
  5. If fuel pressure was less than 9 psi (.63 kg/cm 2 ), check for restricted line between in-tank fuel pump and pressure regulator. If fuel line is okay, disconnect injector connector. Turn ignition on. Gradually pinch fuel pressure gauge outlet hose. Note pressure. CAUTION: DO NOT pinch off fuel return line completely. DO NOT allow fuel pressure build-up to exceed specification, as damage to fuel pressure regulator may occur.
  6. If pressure is greater than 13 psi (.91 kg/cm 2 ), replace fuel pressure regulator (TBI 700) or fuel meter cover/pressure regulator (TBI 220). If pressure is less than 9 psi (.63 kg/cm 2 ), check for faulty fuel pump or incorrect part. Check fuel pump coupling hose and pump inlet filter in fuel tank. On models with dual fuel tanks, check for faulty fuel tank selector valve and meter switch.
  7. If fuel pump did not run in step 1), apply 12 volts to fuel pump test connector. For fuel pump test connector location, see underhood engine component views under COMPONENT LOCATIONS in «SYSTEM/COMPONENT TESTS»(ref-10256) article in this section. If fuel pump now runs, repair open in fuel pump relay drive or power circuit or repair faulty fuel pump relay. To test relay, see FUEL PUMP RELAY in «SYSTEM/COMPONENT TESTS»(ref-10256) article in this section.
  8. If fuel pump does not run with 12 volts applied to fuel pump test connector, repair open in fuel pump power or ground circuit or repair faulty fuel pump.

PFI (4.3L Turbo)

  1. Relieve fuel pressure. See FUEL PRESSURE RELIEF. Connect Fuel Pressure Gauge (J-34730-1) to fuel pressure connector on fuel rail. Turn ignition on. With ignition on and engine off, pressure should be 35-38 psi (2.46-2.67 kg/cm 2 ). If no fuel pressure is present, go to step 5).
  2. Start engine. Pressure should drop 3-10 psi (.21-.70 kg/cm 2 ). Turn ignition off. Pressure should hold. If pressure does not hold, check for leaking injectors or fittings. If injectors or fittings are not leaking, replace pressure regulator.
  3. If pressure is present but less than specification, check for restricted delivery line or fuel filter. Repair as necessary. If no restriction is evident, apply battery voltage to fuel pump test connector using a 10-amp fused jumper wire. For location of fuel pump test connector, see COMPONENT LOCATIONS in «SYSTEM/COMPONENT TESTS»(ref-10256) article in this section.
  4. Gradually pinch off fuel delivery line between gauge and fuel rail. If fuel pressure increases to within specifications, replace fuel pressure regulator. If fuel pressure does not increase with line pinched, check for faulty in-tank fuel pump or partial blocked fuel strainer.
  5. Use a 10-amp fused jumper wire to apply battery voltage to fuel pump test connector. For location of fuel pump test connector, see COMPONENT LOCATIONS in «SYSTEM/COMPONENT TESTS»(ref-10256) article in this section. Observe fuel pressure reading. If fuel pressure is still not present, check wiring between test connector and fuel pump. If wiring is okay, replace fuel pump.
  6. If fuel pressure is present with voltage applied to test connector, test fuel pump relay and voltage supply to relay. Refer to the «SYSTEM/COMPONENT TESTS»(ref-10256) article in this section.

FIELD SERVICE MODE CHECK

Note. O2 sensor may cool off while engine idles, causing system to enter open loop. To restore closed loop mode, run engine at part throttle several minutes and accelerate from idle to part throttle several times.

This test confirms proper fuel system operation and verifies closed loop operation. Clear codes and perform this test after any repair is completed. When performing this test, always engage parking brake and block DRIVE wheels. Parking brake on FWD models does NOT hold drive wheels.

  1. Start engine. With engine running, ground diagnostic test terminal "B" of ALDL. see scheme 15- see scheme 18. In closed loop mode, SERVICE ENGINE SOON light flashes once per second.
  2. In open loop, light flashes 2.5 times per second. A lean exhaust is indicated if light is usually off. A rich exhaust is indicated if light is usually on.

HEI-EST DISTRIBUTOR

Note. Only basic ignition timing is adjustable in HEI/EST ignition system.

Spark

  1. If factory tachometer is connected to ignition coil tachometer terminal, disconnect it before performing tests. When removing spark plug wire from spark plug, twist and pull on boot. DO NOT pull on wire.
  2. Using Spark Tester (ST-125), check for spark at coil wire (if applicable) and at each spark plug wire. Check spark plug wire resistance on suspect wires. Resistance should not be greater than 30,000 ohms.

Ignition Coil Power Source

  1. Turn ignition on. On models with remotely mounted ignition coil, use voltmeter to check voltage between terminal "B" of ignition coil and ground. If battery voltage does not exist, check for open circuit, blown ignition fuse or defective ignition switch.
  2. On models equipped with integrally mounted ignition coil, check voltage between BAT terminal and ground at distributor. If battery voltage does not exist, check for open circuit, blown ignition fuse or defective ignition switch.

Ignition Coil Resistance (Externally Mounted)

  1. Remove coil connectors and secondary coil wire. In test "A", use high ohmmeter scale. Resistance value should be very high (infinite). (Scheme 191) If reading is not infinite, replace coil.
  2. In test "B", use low ohmmeter scale. Reading should be very low (near zero ohms). If reading is not near zero ohms, replace coil. In test "C", use high ohmmeter scale. If no continuity exists, replace coil.

Scheme 191

Scheme 191
Scheme 191

Ignition Coil Resistance (Internally Mounted)

  1. Turn ignition off. Remove distributor cap and coil assembly. Invert cap. Set ohmmeter to low scale. Connect leads to coil BAT and TACH terminals. (Scheme 192) If resistance exceeds approximately zero, replace ignition coil.
  2. Set ohmmeter on high scale. Connect one lead to coil secondary terminal and other lead first to TACH terminal and then to ground terminal. If resistance reading in BOTH instances is infinite, replace ignition coil.

Scheme 192

Scheme 192
Scheme 192

Distributor Pick-Up Coil Short & Resistance Checks

  1. Disconnect pick-up coil leads from HEI-EST module terminals "N" and "P". Set ohmmeter to middle scale, and connect one lead to either pick-up coil lead and other lead to distributor housing. (Scheme 193) Flex pick-up coil leads by hand to check for intermittent shorts to ground. If reading is not always infinite, replace pick-up coil.
  2. Connect ohmmeter between both pick-up coil leads. Check for intermittent opens by flexing wires and connectors. If resistance is not 500-1500 ohms, replace pick-up coil.

Scheme 193

Scheme 193
Scheme 193

TACH Pulse (RPM) Signal

Use a Scan tester connected to ALDL connector of vehicle to check for a tach pulse signal. RPM should be indicated on tester when engine is being cranked or is running. Tach pulse (RPM reference) is indicated as a voltage signal when a DVOM (minimum 10-megohm input impedance) is touched to circuit No.430 ECM terminal. (Scheme 186)- (Scheme 190).

IDLE SPEED & IGNITION TIMING (GASOLINE)

Ensure idle speed and ignition timing are set to specifications. For adjustment procedures, see ADJUSTMENTS article in this section.

A-1, SERVICE ENGINE SOON LIGHT INOPERATIVE

  1. If SERVICE ENGINE SOON light does not illuminate with ignition on and engine off, attempt to start engine. If engine starts, go to step 3). If engine does not start, check fusible links at battery. Also check ECM fuse. If fusible links or ECM fuse are blown, repair short to ground.
  2. If fusible links and ECM fuse are okay, turn ignition on, and check power circuits to ECM, including keep-alive memory and ignition feed. See the «WIRING DIAGRAMS»(ref-10259) article in this section for power terminal identification. If power is not available to power terminals of ECM, check for opens in power circuits. If power is available to ECM power terminals, check for poor ECM ground circuits. If circuits are okay, replace faulty ECM.
  3. If engine starts and SERVICE ENGINE SOON light does not illuminate, turn ignition off. Disconnect ECM connectors. Turn ignition on, and jumper ECM SERVICE ENGINE SOON light driver terminal to ground using a test light. See the «WIRING DIAGRAMS»(ref-10259) article in this section for power terminal identification.
  4. If light is now on, repair light driver terminal connections at ECM or replace faulty ECM. If light stays off when test light is used to ground light driver terminal, check for blown instrument panel fuse, faulty bulb, open in light driver circuit between ECM and bulb, short in driver circuit to voltage or open in ignition feed to SERVICE ENGINE SOON light.

A-2, SERVICE ENGINE SOON LIGHT ALWAYS ON OR WON'T FLASH CODE 12

  1. Turn ignition off. Disconnect ECM connectors. Turn ignition on. If SERVICE ENGINE SOON light is on, check for short to ground in light driver circuit between light and ECM driver terminal. See the «WIRING DIAGRAMS»(ref-10259) article in this section for terminal identification.
  2. If light is off with ECM connectors disconnected, turn ignition off. Reconnect ECM connectors. Turn ignition on with engine off. Using a DVOM, check voltage at ALDL diagnostic "test" terminal "B". see scheme 15- see scheme 18. If voltage is greater than 9 volts, check for a short to voltage on ALDL terminal "B" wire between ECM and ALDL connector.
  3. If voltage is less than 5 volts, use DVOM to backprobe appropriate ECM terminal. See the «WIRING DIAGRAMS»(ref-10259) article in this section for terminal identification. If 5-6 volts is now present, repair open or short in wire between ECM and ALDL terminal "B". If voltage is 5-6 volts, go to step 4).
  4. If voltage at terminal "B" of ALDL connector is 5-6 volts, jumper wire terminal at ECM to ground. If SERVICE ENGINE SOON light now flashes a Code 12 and ALDL terminal "A" was used when previously grounding terminal "B", check terminal "A" for open circuit.
  5. If SERVICE ENGINE SOON light does not flash when ECM end of terminal "B" wire is jumpered to ground, check PROM/MEM-CAL for proper installation. If PROM/MEM-CAL is installed correctly, replace ECM. Install original PROM/MEM-CAL into replacement ECM. Repeat diagnostic circuit check. If Code 12 still does not flash, replace PROM/MEM-CAL. Replace PROM/MEM-CAL only after replacing ECM, as PROM/MEM-CAL is probably not at fault.

SUMMARY

If no faults were found while performing BASIC TESTING , proceed to article TESTS W/CODES - DIESEL (for Diesel engines) or article TESTS W/CODES - GASOLINE (for Gas engines) or refer to article in TESTS W/CODES - TRANSMISSION in this section (if transmission codes are displayed). If no hard codes are found in self-diagnostics, or vehicle does not have a self-diagnostic system, 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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