Engine Controls - Basic Testing: Other Chevrolet Sportvan G20

COMPRESSION

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

EXHAUST SYSTEM BACKPRESSURE

Before replacing any components, check exhaust system for restrictions. The exhaust system can be checked with a vacuum gauge or a low pressure (1-5 psi) pressure gauge.

If a vacuum gauge is used, connect it to intake manifold vacuum and start engine. Observe gauge and hold 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.
3. Diagnosis: 1) Start engine and bring to operating temperature. Increase engine speed to 2000-2500 RPM and note gauge. Reading should not exceed 1.25 psi (.09 kg/cm 2 ). Exhaust system is restricted if specification is exceeded. 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 required.

Definition

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

NO-START - ENGINE CRANKS OKAY (TBI WITH HEI)

Note. Check battery condition, engine cranking speed and fuel supply before performing the following tests.

IGNITION SYSTEM

1. Disconnect tachometer wire at distributor TACH terminal (if equipped). A shorted tachometer or tachometer circuit will not allow vehicle to start.
2. Check for battery voltage at BAT terminal at distributor with ignition on. Repair as necessary.
3. Connect ST-125 spark tester to one end of plug wire and crank engine. If spark is present, check fuel delivery.
4. If spark does not occur, disconnect 4-wire EST connector at distributor. If spark now occurs, replace pick-up coil in distributor.
5. If spark does not occur, reconnect EST connector and check voltage at TACH terminal at distributor with ignition on.
6. If voltage is less than one volt, repair faulty coil connection or replace faulty coil. If voltage is 1-10 volts, replace ignition module and recheck for spark. If spark still does not occur, replace ignition coil.
7. If voltage reading at TACH terminal of distributor is greater than 10 volts, remove and invert distributor cap with wires connected. Fabricate an HEI coil spark tester by trimming a spark plug boot and connecting it to ST-125 spark tester. (Scheme 19) Crank engine.
8. If spark occurs, check cap for cracks, water or other defects. Check pick-up coil connector and ignition coil lead wire colors. Ignition coil with Yellow and Red wires should be used with a Yellow pick-up coil connector. Ignition coil with White and Red wires should be used with a Clear or Black pick-up coil connector.
9. If spark does not occur, turn ignition off and disconnect pick-up coil leads from module. Turn ignition on. With voltmeter connected to distributor TACH terminal and fabricated coil spark tester connected, momentarily touch test light, connected to a remote voltage source (1.5-8.0 volts), to ignition module terminal "P".
10. If voltage at TACH terminal does not drop, check ignition module ground and for open in wires from ignition coil to module. If all is okay, replace ignition module.
11. If voltage at TACH terminal drops, check for spark at spark tester as test light is removed from terminal "P". If spark occurs, check pick-up coil connections and check for 500-1500 ohms resistance at pick-up coil leads. (Scheme 25) Repair as necessary.
12. If spark does not occur, test ignition module with module tester. If module tests okay, check ignition coil wire. If module tester is not available, replace ignition coil and again touch terminal "P". If spark occurs, system is okay. If spark does not occur, reinstall original ignition coil and replace ignition module.

Scheme 19

FUEL SYSTEM

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 appropriate BASIC FUEL SYSTEM CHECKS in this article.
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 of the injector terminals. 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 "noid" light) to injector harness. Crank engine and note light. If light flashes, check for stored ECM codes. See «DIAGNOSTIC CIRCUIT CHECK (GASOLINE)»(ref-10147-S20595235332001010200000) or «DIAGNOSTIC CIRCUIT CHECK (DIESEL)»(ref-10147-S36505731562001010200000) in this article. If no codes are present, refer to HARD START symptom in «TESTS W/O CODES»(ref-10150) article. 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, injector drive (ground) circuit, or 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, this could cause a no-start condition due to excessive fuel being delivered during cranking. Repair faulty injector or injector seal. If no spray or leakage occurs, refer to HARD START symptom in «TESTS W/O CODES»(ref-10150) article.

Scheme 20

Scheme 21

Scheme 22

MODEL 700 TBI UNIT

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 for 3 seconds to remove any fuel pressure left in fuel lines. Fuel lines are now safe for servicing.

MODEL 220 TBI UNIT

Fuel pressure is relieved and drops to zero when ignition is turned off.

FUEL PRESSURE RELIEF

Disconnect negative battery cable. Remove fuel filler cap. Since these TBI units contain an internal bleed-down feature, after a short time, system fuel pressure should dissipate.

FUEL PRESSURE CHECK

1. Relieve fuel pressure (if necessary). Remove air cleaner and plug thermal vacuum port on throttle body. When removing fuel line, always use 2 wrenches. Install Fuel Pressure Gauge (J-29658B or BT-8205) and Adapter (29658-85) in fuel line between steel line and flexible hose.
2. Turn ignition off. Apply battery voltage to fuel pump test connector using a 10 amp. fused jumper wire. Observe fuel pressure reading. Fuel pressure should read between 9-13 psi (.6-.9 kg/cm 2 ). If no fuel pressure exist, go to step 3). If fuel pressure is less than 9 psi (.63 kg/cm 2 ), go to step 6). If fuel pressure is greater than 13 psi (.91 kg/cm 2 ), go to step 7).
3. Listen for pump operation at fuel tank. If fuel pump is running, check for plugged in-line fuel filer, plugged pump inlet filter, restricted fuel line or leaking pump rubber coupling. If these are okay, replace in-tank fuel pump.
4. If pump was not operating, disconnect fuel pump relay. Using a 10 amp. fuse jumper wire, connect 12 volts to circuit 120 (Gray wire). If pump now operates, check for faulty connection at relay or faulty fuel pump relay. If pump will not operate, check for faulty in-tank pump or faulty pump ground.
5. If fuel pressure was less than 9 psi (.63 kg/cm 2 ), check for restricted fuel filter or restricted line between in-tank fuel pump and pressure gauge. If fuel filter and lines are okay, go to next step.
6. Disconnect injector connector. Apply battery voltage to fuel pump test connector. Gradually pinch fuel pressure gauge and note pressure. If pressure is greater than 13 psi (.91 kg/cm 2 ), replace fuel pressure regulator. If pressure is less than 9 psi (.63 kg/cm 2 ), check for faulty fuel pump or incorrect part. Check coupling hose and pump inlet filter.
7. Disconnect fused jumper wire from test connector. Disconnect fuel return line quick-connect fitting. Attach 5/16" hose to throttle body side of return line and insert other end of hose in a gasoline container.
8. Using a fused jumper wire, apply battery voltage to fuel pump test connector. If fuel pressure is greater than 13 psi (.91 kg/cm 2 ), check for restricted return line from throttle body. If line is okay, replace fuel pressure regulator. If pressure is between 9-13 psi (.63-.91 kg/cm 2 ), correct or replace fuel return line to fuel tank.

FUEL PUMP RELAY

See RELAYS, SOLENOIDS, MOTORS & MODULES in SYSTEM/COMP TESTS article.

FUEL PUMP RELAY BY-PASS PROCEDURE

See FUEL DELIVERY in SYSTEM/COMP TESTS article.

FIELD SERVICE MODE CHECK

Note. Oxygen sensor may cool off after while engine idles causing system to go into 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 check, 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 test terminal "B" of the ALDL. In closed loop mode, SERVICE ENGINE SOON light will flash at a rate of one flash per second.

2) In open loop, light will flash at a rate of 2.5 flashes per second. If light is off most of the time, a lean exhaust is indicated. If light is on most of the time, a rich exhaust is indicated.

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, NOT on wire.
2. Using Spark Tester (ST-125), check for spark at coil wire (if applicable) and at each spark plug wire using spark tester. Check spark plug wire resistance on suspect wires. Resistance should be no greater than 30,000 ohms.

IGNITION COIL POWER SOURCE

1. Turn ignition on. Using voltmeter, check voltage between terminal "B" of ignition coil and ground on models with remotely mounted ignition coil.
2. On models equipped with integrally mounted ignition coil, check voltage between BAT terminal and ground at distributor. Battery voltage should exist. If not, 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 23) If not, replace coil.
2. In test "B", use low ohmmeter scale. Reading should be very low or near zero ohms. If not, replace coil. In test "C", use high ohmmeter scale. (Scheme 23) If there is no continuity, replace coil.

Scheme 23

IGNITION COIL RESISTANCE (INTERNALLY MOUNTED)

1. Turn ignition off. Remove the distributor cap and coil assembly. Turn cap upside down. Set ohmmeter to low scale. Connect leads to coil BAT and TACH terminals. (Scheme 24) If resistance exceeds zero, or nearly zero, replace ignition coil.
2. Set ohmmeter on high scale. Connect one lead to coil secondary terminal and the other lead first to TACH terminal and then to ground terminal. If resistance reading in BOTH instances is infinite, replace ignition coil.

Scheme 24

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 the other lead to distributor housing. (Scheme 25) Flex pick-up coil leads by hand to check for intermittent shorts to ground. Reading should be infinity at all times. If not, replace pick-up coil.
2. Connect ohmmeter between both pick-up coil leads. Check for intermittent opens by flexing wires and connectors. Resistance should be 500-1500 ohms. If not, replace pick-up coil.

Scheme 25

TACH PULSE (RPM) SIGNAL

The easiest way to check for a tach pulse signal is using a Scan tester connected to the ALDL of the vehicle. RPM should be indicated on tester when engine is being cranked, or is running. Tach pulse (RPM reference will be indicated as a voltage signal when a DVOM (minimum 10-megohm input impedance) is touched to circuit No. 430 ECM terminal. For circuit and terminal reference, see appropriate schematic in NO-START - ENGINE CRANKS OKAY in this article.

IDLE SPEED & IGNITION TIMING (ALL ENGINES)

Ensure idle speed and ignition timing are set to specification. For adjustment procedures, see ADJUSTMENTS article.

CHART 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 appropriate diagram in WIRING DIAGRAMS article 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, or 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 appropriate diagram in WIRING DIAGRAMS article 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, driver circuit shorted to voltage, or an open in the ignition feed to the SERVICE ENGINE SOON light.

CHART A-2, SERVICE ENGINE SOON LIGHT 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 appropriate diagram in WIRING DIAGRAMS article 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 test terminal "B". 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, backprobe appropriate ECM terminal with DVOM. See appropriate diagram in WIRING DIAGRAMS article 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 that wire terminal at ECM to ground. If SERVICE ENGINE SOON light now flashes a Code 12, and ALDL terminal "A" was originally used when grounding terminal "B" at the beginning of the diagnostic circuit check, 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 installed correctly, replace ECM, using original PROM/MEM-CAL. Repeat diagnostic circuit check. If Code 12 still does not flash, replace ECM. Replace PROM/MEM-CAL only after replacing ECM, as PROM/MEM-CAL is not likely to be at fault.

Outlining Diagnostic Circuit Check Flow Chart. Scheme 26

Scheme 27

SUMMARY

If no problems were found while performing test procedures in this article, no trouble codes (or only intermittent ones) were found while performing either DIAGNOSTIC CIRCUIT CHECK and driveability problems exist, proceed to TESTS W/O CODES article for diagnosis by symptom (i.e., ROUGH IDLE, NO-START, etc.) or intermittent diagnosis procedures.