Engine Controls - Basic Testing: Diagnosis Chevrolet Sportvan G20

VISUAL INSPECTION

Perform a visual inspection of all electrical wiring, looking for chafed, stretched, cut or pinched wiring. Inspect electrical connectors and connections for tight fit and corrosion. Repair as necessary. Inspect all vacuum hoses for proper routing, cuts or pinches. If necessary, see the VACUUM DIAGRAMS article to verify routing and connections. Repair as necessary. Inspect air induction system for possible vacuum leaks.

NO-START DIAGNOSIS

Note. For diesel information, see TESTS W/CODES - DIESEL and TESTS W/O CODES articles.

GENERAL INSPECTION

1. Ensure proper starting procedure is being used.
2. Visually check vacuum hoses for splits, kinks and proper connections, as shown on underhood emission control information label. Check ignition wires for cracking, hardness and proper connections at both distributor cap and spark plugs.
3. Remove spark plugs. Check and replace as necessary.
4. Remove distributor cap and check for moisture, dust, cracks, burns and arcing to ground through coil mounting screws or rotor.
5. Try to turn distributor shaft by hand. Drive gear pin may be broken.
6. In very cold temperatures, ensure oil is proper viscosity and not contaminated with gasoline.

FUEL SYSTEM PRESSURE TEST

1. Turn engine off and relieve fuel pressure. See FUEL SYSTEM PRESSURE RELIEF. Remove air cleaner and plug air cleaner (THERMAC) vacuum port (if equipped). Disconnect fuel line between throttle body and fuel filter. Install Fuel Pressure Gauge (J-29658A) and Adapter (J-2968A-85) between throttle body and in-line fuel filter. Start engine and observe fuel pressure reading.
2. Fuel pressure should be 9-13 psi (.6-.9 kg/cm 2 ). Go to step 3) if fuel pressure is present but less than specification. If fuel pressure is greater than specification, go to step 4). If no fuel pressure is evident, check for power to fuel pump relay. If power is available to fuel pump relay, test fuel pump relay. See SYSTEM/COMP TESTS article.
3. If fuel pressure is present during cranking or with fuel pump relay by-passed, but is less than 9-13 psi (.6-.9 kg/cm 2 ), gradually pinch off fuel return line to fuel tank. If fuel pressure remains low, check for a plugged fuel filter or restriction in fuel delivery line. If filter is okay and no restrictions are present, replace fuel pump. If pressure increases to within specification when return line is pinched off, replace fuel pressure regulator. 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.
4. If fuel pressure is present, but is greater than 9-13 psi (.6-.9 kg/cm 2 ), check for restriction in return line to fuel tank. If restriction is not present, replace fuel pressure regulator.
5. After testing, relieve fuel pressure. Remove fuel pressure gauge and connect fuel line to fuel filter. Start vehicle and check for leaks. Remove plug from throttle body vacuum port (if equipped). Install air cleaner.

FUEL PUMP FLOW TEST

1. Disconnect fuel line at fuel filter inlet. Disconnect fuel injection pump electric shut-off solenoid wire (Pink).
2. Crank engine for 15 seconds. Fuel pump should supply approximately 1/2 pint of fuel in 15 seconds. If fuel delivery is not as specified, go to FUEL SYSTEM PRESSURE TEST.

1. Turn engine off. Disconnect fuel line at inlet to fuel filter assembly. Install low pressure fuel gauge to fuel line.
2. Crank or run engine for 10-15 seconds. Fuel pressure should be between 5.8-8.7 psi (.41-.61 kg/cm 2 ). If fuel pressure is not as specified, check fuel lines or fuel tank sending unit for restriction. If fuel lines or sending unit are okay, replace fuel pump.

DIAGNOSTIC CIRCUIT CHECK (GASOLINE)

After performing basic diagnostic procedures listed under PRELIMINARY INSPECTION & ADJUSTMENTS, BASIC FUEL SYSTEM CHECKS and BASIC IGNITION SYSTEM CHECKS, this is the starting point for utilizing the self-diagnostic system for determining computer related problems. The Diagnostic Circuit Check determines if the SERVICE ENGINE SOON light works, if the ECM is operating and can recognize a fault, and if any codes are stored. see scheme 8

After performing necessary tests as described in the diagnostic circuit check, if no codes are indicated and driveability problems still exist, see TESTS W/O CODES article and SCAN TESTER USAGE in TESTS W/CODES - TBI article.

1. Check SERVICE ENGINE SOON light operation. Turn ignition on with Scan tester not connected, ALDL test terminal not grounded and engine not running. SERVICE ENGINE SOON light should be on steady. If light illuminates and stays on steady, go to next step. If light does not illuminate, go to A-1, SERVICE ENGINE SOON LIGHT INOPERATIVE. If light flashes, go to step 3).
2. Grounding the ALDL test terminal at this time should cause SERVICE ENGINE SOON light to flash a Code 12, followed by any codes stored in ECM memory. If light goes from bright to dim, this is not considered a code. If this occurs, or light remains on and does not flash Code 12, see A-2, SERVICE ENGINE SOON LIGHT WON'T FLASH CODE 12.
3. If light begins to flash as soon as ignition is turned on, check for a short to ground on the diagnostic test terminal wire between ALDL terminal "B" and ECM terminal No. 5. If circuit is okay, replace ECM.

Circuit Description

This chart assumes that battery conditions and engine cranking speed are OK, and there is adequate fuel in tank. This chart should be used on engines with Model 700 throttle body.

Note. Numbers below refer to circled numbers on diagnostic chart.

Diagnostic Aids

If no trouble is found in fuel pump circuit or ignition system and the cause of a "Engine Cranks But Will Not Run" has not been found, check for the following

1. Fouled spark plugs.
2. EGR valve stuck open.
3. Low fuel pressure. See CHART A-6.
4. Water or foreign material in the fuel system.
5. A ground circuit 423 (EST) may cause a "No Start" or a "Start then Stall" condition.
6. Basic engine problem.

Scheme 7

Scheme 8

This chart assumes that battery conditions and engine cranking speed are OK, and there is adequate fuel in tank. This chart should be used on engines with Model 220 throttle body.

Note. Numbers below refer to circled numbers on diagnostic chart.

If no trouble is found in fuel pump circuit or ignition system and the cause of a "Engine Cranks But Will Not Run" has not been found, check for the following

1. Fouled spark plugs.
2. EGR valve stuck open.
3. Low fuel pressure. See CHART A-6.
4. Water or foreign material in the fuel system.
5. A ground circuit 423 (EST) may cause a "No Start" or a "Start then Stall" condition.
6. Basic engine problem.

Scheme 9

Scheme 10

Scheme 11

This chart should only be used if diagnosis in CHART A-3 indicated an injector circuit problem.

Note. Numbers below refer to circled numbers on diagnostic chart.

This chart should only be used if diagnosis in CHART A-3 indicated an injector circuit problem. If both injector circuits fail to blink when tested, diagnose one injector circuit at a time.

Note. Numbers below refer to circled numbers on diagnostic chart.

When ignition switch is turned "ON," the ECM will turn "ON" the in-tank fuel pump. It will remain "ON" as long as the engine is cranking or running, and the ECM is receiving distributor reference pulses. If there are no reference pulses, the ECM will shut "OFF" the fuel pump within two seconds after ignition "ON" or engine stops.

The pump will deliver fuel to the TBI unit where the system pressure is controlled to about 9-13 psi. Excess fuel is then returned to the fuel tank.

Note. Numbers below refer to circled numbers on diagnostic chart.

When ignition switch is turned "ON," the ECM will turn "ON" the in-tank fuel pump. It will remain "ON" as long as the engine is cranking or running, and the ECM is receiving distributor reference pulses. If there are no reference pulses, the ECM will shut "OFF" the fuel pump within two seconds after ignition "ON" or engine stops.

The pump will deliver fuel to the TBI unit where the system pressure is controlled to about 9-13 psi. Excess fuel is then returned to the fuel tank.

A fuel module is used on all 7.4L, G van with 5.7L, and all other 5.7L over 8500 GVW engines to correct a hot restart vapor (vapor lock) during a high ambient condition. It is designed to over-ride the ECM two second pump operation and will run the fuel pump for twenty seconds at initial ignition "ON".

Note. Numbers below refer to circled numbers on diagnostic chart.

An inoperative fuel module may be the cause of a hot stall/no start condition. Check for power and ground circuit to the fuel module and a complete circuit to the pump from terminal "A". If OK, and the pump does not run for the specified 20 seconds at initial ignition "ON," replace fuel module.

Note. Figs. (Scheme 12) -24 below are Courtesy of General Motors Corp.

Chart A-5 Schematic, Fuel Pump Ckt Diagnosis (C, K, R, V, G, P & GP Series) One Fuel Tank. Scheme 12

Flow Chart A-5, Fuel Pump Ckt Diagnosis (C, K, R, V, G, P & GP Series) One Fuel Tank. Scheme 13

Chart A-5 Schematic, Fuel Pump Ckt Diagnosis (R & V Series) Two Fuel Tanks. Scheme 14

Flow Chart A-5, Fuel Pump Ckt Diagnosis (R & V Series) Two Fuel Tanks. Scheme 15

When the fuel pump is running, fuel is delivered to the injector(s) and then to the regulator where the system pressure is controlled to about 9-13 psi. Excess fuel is then returned to the fuel tank.

Note. Numbers below refer to circled numbers on diagnostic chart.

1. If the vehicle is equipped with a fuel module, the module must be disconnected before performing the fuel system pressure test.
2. Fuel system is under pressure. To avoid fuel spillage, see FUEL SYSTEM PRESSURE RELIEF (GASOLINE) in «REMOVE/INSTALL/OVERHAUL»(ref-10152) article.
3. On V6 or V8 engines, fuel pressure drops to almost zero psi after pump shuts "OFF".

Scheme 16

Scheme 17

DIAGNOSTIC CIRCUIT CHECK (DIESEL)

Note. The 6.2L with light duty emissions is equipped with a self- diagnostic system. For information on the diagnostic circuit check and retrieving codes, see TESTS W/CODES - DIESEL article in this section.

PRELIMINARY INSPECTION

1. Check all vacuum hoses for correct routing, restrictions, cuts, or other damage. Inspect hoses see beneath the air cleaner assembly and other engine components.
2. Inspect all wires in engine compartment for correct and good connections. Also check wires for pinched or chaffed spots, as well as contact with sharp edges or exhaust manifolds.
3. The preliminary inspection is very important and should be performed CAREFULLY and THOROUGHLY as it can often lead to fixing a problem without further diagnosis.

DIAGNOSTIC PROCEDURE

1. Ensure all engine systems not related to Diesel Electronic Control (DEC) system are operating properly. DO NOT proceed with testing unless all other non-DEC system related problems have been repaired. This determines whether DEC system and ECM are working properly.
2. If trouble codes are displayed (other than Code 12), determine if codes are "hard" or "intermittent" trouble codes. A hard code is one which is present when working on vehicle and condition still exists. Hard codes will cause the SERVICE ENGINE SOON light to come on.
3. An intermittent code is one which does not reset itself and is not present while working on vehicle. An intermittent code is often caused by loose connections. The SERVICE ENGINE SOON light will go out 10 seconds after fault goes away.

Scheme 18