Engine Controls - System & Component Testing: Diagnosis Chevrolet TrailBlazer I

ALCOHOL/CONTAMINANTS IN FUEL DIAGNOSIS WITH SPECIAL TOOL

Note. For component locations, see COMPONENT LOCATIONS . For circuit reference, see WIRING DIAGRAMS .

Diagnostic Procedures

1. Turn on the Fuel Composition Tester (J-44175).
2. Verify the fuel composition tester is operational by measuring the AC frequency output with a DVOM. A frequency without a fuel sample in the test cell indicates that the tester is working correctly.
3. Install fuel pressure gauge.
4. Close bleed valve on fuel pressure gauge.
5. Place the bleed hose of the fuel pressure gauge into the 100 ml beaker.
6. Using scan tool, command fuel pump relay on.
7. Slowly open the bleed valve on the fuel pressure gauge, until an adequate fuel sample is obtained.
8. If water appears in the fuel sample, clean the fuel system and replace the fuel in the vehicle.
9. Pour fuel sample from the beaker into the tester until the level of the fuel is at the top of each fuel test port.
10. Observe the diagnostic LEDs on the fuel composition tester. If the Red fuel diagnostic LED is illuminated, a fuel contamination condition exists. Refer to «FUEL SYSTEM CLEANING»(ref-153168-S17372786832003032200000) .
11. Measure the output frequency of the fuel composition tester.
12. Subtract 50 from the reading on the DVOM to obtain the percentage of alcohol in the fuel sample.
13. If fuel sample contains more than 10 percent ethanol, add gasoline to the fuel tank or replace the fuel in the vehicle.

ALCOHOL/CONTAMINANTS IN FUEL DIAGNOSIS WITHOUT SPECIAL TOOL

Note. For component locations, see COMPONENT LOCATIONS . For circuit reference, see WIRING DIAGRAMS .

Alcohol In Fuel Testing Procedure

The fuel sample should be drawn from the bottom of the tank so that any water present in the tank will be detected. The sample should be bright and clear. If alcohol contamination is suspected then use the following procedures to test the fuel quality.

1. Using a 3.38 oz. (100 ml) graduated cylinder with 0.034 oz. (1 ml) graduation marks, fill the cylinder with fuel to the 3.04 oz. (90 ml) mark.
2. Add 0.34 oz. (10 ml) of water in order to bring the total fluid volume to 3.38 oz. (100 ml) and install a stopper.
3. Shake the cylinder vigorously for 10-15 seconds.
4. Carefully loosen the stopper in order to release the pressure.
5. Re-install the stopper and shake the cylinder vigorously again for 10-15 seconds.
6. Put the cylinder on a level surface for approximately 5 minutes in order to allow adequate liquid separation.

If alcohol is present in the fuel, the volume of the lower layer, which would now contain both alcohol and water, will be more than 0.34 oz. (10 ml). For example, if the volume of the lower layer is increased to 0.51 oz. (15 ml), this indicates at least 5 percent alcohol in the fuel. The actual amount of alcohol may be somewhat more because this procedure does not extract all of the alcohol from the fuel.

Particulate Contaminants In Fuel Testing Procedure

The fuel sample should be drawn from the bottom of the tank so that any water present in the tank will be detected. The sample should be bright and clear. If the sample appears cloudy, or contaminated with water, as indicated by a water layer at the bottom of the sample, use the following procedure to diagnose the fuel.

1. Using an approved fuel container, draw approximately 0.53 qt. (0.5 liter) of fuel.
2. Place container on a level surface for approximately 5 minutes in order to allow settling of the particulate contamination. Particulate contamination will show up in various shapes and colors. Sand will typically be identified by a White or light Brown crystals. Rubber will appear as Black and irregular particles.
3. Observe fuel sample. If any physical contaminants or water are present, clean fuel system. Refer to «FUEL SYSTEM CLEANING»(ref-153168-S17372786832003032200000) .

FUEL INJECTOR BALANCE TEST WITHOUT SPECIAL TOOL

Note. For component locations, see COMPONENT LOCATIONS . For circuit reference, see WIRING DIAGRAMS .

1. Perform Diagnostic System Check - Engine Controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-155898-S30062612662003062800000) under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT article. After performing Diagnostic System Check - Engine Controls, go to next step.
2. Perform «FUEL INJECTOR COIL TEST»(ref-153168-S35005026492003032100000). After performing Fuel Injector Coil Test, go to next step.
3. Observe ECT parameter with scan tool. Does scan tool indicate ECT parameter is less than 201°F (94°C)? If yes, go to next step. If no, allow engine to cool before proceeding.
4. Turn ignition off. Turn OFF all accessories. Install fuel pressure gauge. Turn ignition on, with engine off. Command fuel pump ON with scan tool. Observe fuel pressure gauge, with fuel pump commanded ON. Is fuel pressure within 48-54 psi (334-375 kPa)? If yes, go to next step. If no, go to «FUEL SYSTEM PRESSURE TEST»(ref-152692-S36160874242003021900000) under FUEL SYSTEMS in BASIC DIAGNOSTIC PROCEDURES - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT article.
5. Monitor fuel pressure gauge for one minute. Does fuel pressure decrease by more than 5 psi (34 kPa)? If yes, go to «FUEL SYSTEM PRESSURE TEST»(ref-152692-S36160874242003021900000) under FUEL SYSTEMS in BASIC DIAGNOSTIC PROCEDURES - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT article. If no, go to next step.
6. Disconnect multi-way harness connector of fuel injectors. Connect Fuel Injector Tester (J-39021) to ignition 1 voltage circuit terminal and appropriate fuel injector control circuit terminal using Connector Test Adapter Kit (J-35616-A). Set amperage supply selector switch on fuel injector tester to Balance Test.5-2.5-amp position. Command fuel pump ON and then OFF with scan tool. Record fuel pressure indicated by fuel pressure gauge after fuel pressure stabilizes. This is first pressure reading. Energize fuel injector by depressing "Push to Start Test" button on fuel injector tester. Record fuel pressure indicated by fuel pressure gauge after fuel injector has stopped pulsing. This is second pressure reading. Repeat this step for each fuel injector. Subtract second pressure reading from first pressure reading for one fuel injector. Result is pressure drop value. see scheme 18 Obtain a pressure drop value for each fuel injector. Add all individual pressure drop values. This is total pressure drop. Divide total pressure drop by number of fuel injectors. This is average pressure drop. Does any fuel injector have a pressure drop value more than average pressure drop or less than average pressure drop by 1.5 psi (10 kPa)? If yes, go to next step. If no, go to «SYMPTOMS»(ref-152936-S08695756652003030500000) in TROUBLE SHOOTING - NO CODES - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT article.
7. Replace affected fuel injectors. After repairs, go to next step.
8. Operate system in order to verify repair. Did you correct the condition? If yes, system is okay. If no, go to «SYMPTOMS»(ref-152936-S08695756652003030500000) in TROUBLE SHOOTING - NO CODES - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT article.

FUEL INJECTOR BALANCE TEST WITH TECH 2

Note. For component locations, see COMPONENT LOCATIONS . For circuit reference, see WIRING DIAGRAMS .

Circuit Description

Scan tool first energizes fuel pump and then injectors for a precise amount of time allowing a measured amount of fuel into manifold. This causes a drop in system fuel pressure that can be recorded and used to compare each injector.

1. Perform Diagnostic System Check - Engine Controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-155898-S30062612662003062800000) under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT article. After performing Diagnostic System Check - Engine Controls, go to next step.
2. Perform «FUEL INJECTOR COIL TEST»(ref-153168-S35005026492003032100000) . After performing Fuel Injector Coil Test, go to next step.
3. Observe ECT parameter with scan tool. Does scan tool indicate ECT parameter is less than 201°F (94°C)? If yes, go to next step. If no, allow engine to cool before proceeding.
4. Turn ignition off. Turn OFF all accessories. Install fuel pressure gauge. Turn ignition on, with engine off. Command fuel pump ON with scan tool. Observe fuel pressure gauge, with fuel pump commanded ON. Is fuel pressure within 48-54 psi (334-375 kPa)? If yes, go to next step. If no, go to «FUEL SYSTEM PRESSURE TEST»(ref-152692-S36160874242003021900000) under FUEL SYSTEMS in BASIC DIAGNOSTIC PROCEDURES - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT article.
5. Monitor fuel pressure gauge for one minute. Does fuel pressure decrease by more than 5 psi (34 kPa)? If yes, go to «FUEL SYSTEM PRESSURE TEST»(ref-152692-S36160874242003021900000) under FUEL SYSTEMS in BASIC DIAGNOSTIC PROCEDURES - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT article. If no, go to next step.
6. With scan tool, select Fuel Injector Balance Test function, within Special Functions menu. Select an injector to be tested. Press ENTER. This will prime fuel system. Record fuel pressure indicated by fuel pressure gauge after fuel pressure stabilizes. This is first pressure reading. Energize fuel injector by depressing PULSE INJECTOR button on scan tool. This will energize injector and decrease fuel pressure. Record fuel pressure indicated by fuel pressure gauge after fuel injector has stopped pulsing. This is second pressure reading. Press ENTER again to bring you back to Select Injector screen. Repeat this step for each fuel injector. Subtract second pressure reading from first pressure reading for one fuel injector. Result is pressure drop value. Obtain a pressure drop value for each fuel injector. Add all individual pressure drop values. This is total pressure drop. Divide total pressure drop by number of fuel injectors. This is average pressure drop. Does any fuel injector have a pressure drop value that is more than average pressure drop or less than average pressure drop by 1.5 psi (10 kPa)? If yes, go to next step. If no, go to «SYMPTOMS»(ref-152936-S08695756652003030500000) in TROUBLE SHOOTING - NO CODES - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT article.
7. Replace affected fuel injectors. After repairs, go to next step.
8. Operate system in order to verify repair. Did you correct the condition? If yes, system is okay. If no, go to «SYMPTOMS»(ref-152936-S08695756652003030500000) in TROUBLE SHOOTING - NO CODES - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT article.

FUEL INJECTOR CIRCUIT DIAGNOSIS

Note. For component locations, see COMPONENT LOCATIONS . For circuit reference, see WIRING DIAGRAMS .

PCM enables appropriate fuel injector on intake stroke for each cylinder. A voltage is supplied directly to fuel injectors. PCM controls each fuel injector by grounding control circuit via a solid state device called a driver.

1. Perform Diagnostic System Check - Engine Controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-155898-S30062612662003062800000) under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT article. After performing Diagnostic System Check - Engine Controls, go to next step.
2. Are any fuel injector DTCs set? If yes, diagnose affected DTC(s). See «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-155898-S19906497452003062800000) in SELF-DIAGNOSTICS - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT article. If no, go to next step.
3. Inspect the PCM 1 fuse. Is fuse open? If yes, go to next step. If no, go to step 6 .
4. Turn ignition off. Disconnect fuel injectors multi-way harness connector. Using a test light connected to battery voltage, probe the ignition 1 voltage circuit of the fuel injector, fuse side. Does test light illuminate? If yes, go to step 9 . If no, go to next step.
5. Turn ignition on, with engine off. Does test light illuminate? If yes, go to step 13 . If no, go to step 8 .
6. Turn ignition off. Disconnect fuel injectors multi-way harness connector. Turn ignition on, with engine off. Using test light connected to ground, probe the ignition 1 voltage circuit of the fuel injector, fuse side. Does the test light illuminate? If yes, go to next step. If no, go to step 10 .
7. Test for continuity between the ignition 1 voltage circuit terminal and a fuel injector terminal, at the multi-way harness connector, fuel injector side. Does the DVOM display OL? If yes, go to step 11 . If no, go to step 12 .
8. Test fuel injector ignition 1 voltage circuit between the multi-way harness connector and fuel injectors for a short to ground. Did you find and correct the condition? If yes, go to step 14 . If no, problem is intermittent.
9. Repair short to ground in the ignition 1 voltage circuit of the fuel injector. After repairs, go to step 14 .
10. Repair open in the ignition 1 voltage circuit between the PCM 1 fuse and multi-way connector. After repairs, go to step 14 .
11. Repair open or high resistance in the ignition 1 voltage circuit of the fuel injectors between the multi-way connector and splice. After repairs, go to step 14 .
12. Repair poor connection at the multi-way harness connector. After repairs, go to step 14 .
13. Repair short to a PCM ground in the ignition 1 voltage circuit of fuel injector. After repairs, go to next step.
14. Operate system in order to verify the repair. Did you correct the condition? If yes, system is okay. If no, go to step 3 .

Diagnostic Aids

1. Monitoring Fuel Injector circuit status with scan tool, while moving fuel injector harness, may help isolate an intermittent condition.
2. Performing Fuel Injector Coil test may help isolate an intermittent condition. See «FUEL INJECTOR COIL TEST»(ref-153168-S35005026492003032100000) .
3. For an intermittent condition, go to «INTERMITTENTS»(ref-152936-S40530116252003030500000) in TROUBLE SHOOTING - NO CODES - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT article.

FUEL INJECTOR COIL TEST

Note. For component locations, see COMPONENT LOCATIONS . For circuit reference, see WIRING DIAGRAMS .

Control module enables appropriate fuel injector on intake stroke for each cylinder. A voltage is supplied directly to fuel injectors. Control module controls each fuel injector by grounding Control circuit via a solid state device called a driver. A fuel injector coil winding resistance that is too high or too low, will affect engine driveability. A Fuel Injector Control circuit DTC may not set, but a misfire may be apparent. Fuel injector coil windings are affected by temperature. Resistance of fuel injector coil windings will increase as temperature of fuel injector increases.

1. Perform Diagnostic System Check - Engine Controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-155898-S30062612662003062800000) under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT article. After performing Diagnostic System Check - Engine Controls, go to next step.
2. Observe Engine Coolant Temperature (ECT) parameter with scan tool. Is ECT value within 50-90°F (10-32°C)? If yes, go to next step. If no, go to step 4 .
3. Disconnect fuel injector multi-way harness connector. With DVOM, measure resistance of each fuel injector between Ignition Feed circuit and Fuel Injector Control circuit, at multi-way connector. Do any fuel injectors display a resistance outside 11-14 ohms? If yes, go to step 6 . If no, go to «DIAGNOSTIC AIDS»(ref-153168-S17161573432003032100000) .
4. Disconnect fuel injector multi-way harness connector. With DVOM, measure resistance of each fuel injector between Ignition Feed circuit and Fuel Injector Control circuit, at multi-way connector. Record each fuel injector value. Subtract lowest resistance value from highest resistance value. Is difference equal to, or less than 3 ohms? If yes, go to «DIAGNOSTIC AIDS»(ref-153168-S17161573432003032100000) . If no, go to next step.
5. Add all fuel injector resistance values, to obtain a total resistance value. Divide total resistance value by number of fuel injectors to obtain an average resistance value. Subtract lowest and highest individual fuel injector resistance values from average resistance value. Replace fuel injector that displays greatest resistance difference, more or less than average. After repairs, go to step 7 .
6. Replace fuel injector(s) that are outside of 11-14 ohms range. After repairs, go to next step.
7. Operate system in order to verify repair. Did you correct the condition? If yes, system is okay. If no, go to step 2 .

1. Monitoring misfire current counters or misfire graph, may help isolate fuel injector that is causing condition.
2. Operating vehicle over a wide temperature range may help isolate fuel injector that is causing condition.
3. Perform fuel injector coil test within conditions of customer's concern. A fuel injector condition may only be apparent at a certain temperature, or under certain conditions.

Scheme 10

Scheme 11

The control module enables fuel pump relay when ignition switch is turned on. The control module will disable fuel pump relay within two seconds unless control module detects ignition reference pulses. The control module continues to enable fuel pump relay as long as ignition reference pulses are detected. The control module disables fuel pump relay within two seconds if ignition reference pulses cease to be detected and ignition remains on.

1. Perform diagnostic system check. See «DIAGNOSTIC SYSTEM CHECK -- ENGINE CONTROLS»(ref-155898-S30062612662003062800000) under SELF DIAGNOSTIC SYSTEM in SELF DIAGNOSTICS - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT article. After performing diagnostic system check, go to next step.
2. Turn ignition on, with engine off. Using scan tool, command fuel pump relay on and off. Repeat commands as necessary. Does fuel pump turn on and off? If yes, problem is intermittent. If yes, go to next step.
3. Using scan tool, command fuel pump relay on and off. Do you hear a click when you command fuel pump relay on and off? If yes, go to step 9. If no, go to next step.
4. Turn ignition off. Remove fuel pump relay, located in underhood fuse/relay block. (Scheme 10) Turn ignition on, with engine off. Probe control circuit of fuel pump relay with a test light that is connected to a good ground. (Scheme 11) With scan tool, command fuel pump on and off. Does test light turn on and off? If yes, go to next step. If no, go to step 6.
5. Connect a test light between control circuit of fuel pump relay and ground circuit of fuel pump relay. Command fuel pump relay on and off with a scan tool. Does test light turn on and off? If yes, go to step 19. If no, go to step 22.
6. Does test light remain illuminated with each command? If yes, go to next step. If no, go to step 8.
7. Test control circuit of fuel pump relay for a short to voltage. Did you find and correct condition? If yes, go to step 27. If no, go to step 26.
8. Test control circuit of fuel pump relay for a short to ground or an open. Did you find and correct condition? If yes, go to step 27. If no, go to step 20.
9. Turn ignition on, with engine off. Does fuel pump operate continuously? If yes, go to next step. If no, go to step 11.
10. Turn ignition off. Remove fuel pump relay. Turn ignition on, with engine off. Does fuel pump operate continuously? If yes, go to step 21. If no, go to step 25.
11. Inspect PCM "B" fuse. Is fuse open? If yes, go to next step. If no, go to step 14.
12. Test supply voltage circuit of fuel pump for a short to ground between PCM "B" fuse and fuel pump. Replace PCM "B" fuse if necessary. Did you find and correct condition? If yes, go to step 27. If no, go to next step.
13. Connect all disconnected electrical components. Install a new PCM "B" fuse. Command fuel pump relay on with a scan tool. Inspect PCM "B" fuse. Is PCM "B" fuse open? If yes, go to step 24. If no, problem is intermittent.
14. Turn ignition off. Remove fuel pump relay. Turn ignition on, with engine off. Probe ignition 1 voltage circuit of fuel pump relay, switch side, with a test light connected to a good ground. Does test light illuminate? If yes, go to next step. If no, go to step 23.
15. Connect a 20-amp fused jumper wire between ignition 1 voltage circuit of fuel pump relay and supply voltage circuit of fuel pump. Does fuel pump operate? If yes, go to step 19. If no, go to next step.
16. Test supply voltage circuit of fuel pump for an open or high resistance between fuel pump relay and fuel pump. Did you find and correct condition? If yes, go to step 27. If no, go to next step.
17. Test ground circuit of fuel pump for an open or high resistance. Did you find and correct condition? If yes, go to step 27. If no, go to next step.
18. Test for an intermittent and for a poor connection at fuel sender cover. Did you find and correct condition? If yes, go to step 27. If no, go to step 24.
19. Test for an intermittent and for a poor connection at fuel pump relay. Did you find and correct condition? If yes, go to step 27. If no, go to step 25.
20. Test for an intermittent and for a poor connection at PCM. Did you find and correct condition? If yes, go to step 27. If no, go to step 26.
21. Repair short to voltage in supply voltage circuit of fuel pump. After repairs, go to step 27.
22. Repair open or high resistance in ground circuit of fuel pump relay. After repairs, go to step 27.
23. Repair open in ignition 1 voltage circuit of fuel pump relay. After repairs, go to step 27.
24. Inspect for poor connections at fuel pump, within fuel tank, before replacing fuel pump. Replace fuel sender. Replace PCM "B" fuse, if necessary. After repairs, go to step 27.
25. Replace fuel pump. After replacing fuel pump, go to step 27.
26. Replace PCM. Perform PCM programing. See «REMOVAL & INSTALLATION - BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT»(ref-159085) article. After replacing PCM, go to next step.
27. Operate system in order to verify repair. Did you correct condition? If yes, system is okay. If no, go to step 2.

FUEL SYSTEM PRESSURE TEST

Note. For fuel system pressure testing, see FUEL SYSTEM PRESSURE TEST under FUEL SYSTEMS in BASIC DIAGNOSTIC PROCEDURES - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT article.

1. Perform Diagnostic System Check - Engine Controls. See «DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS»(ref-155898-S30062612662003062800000) under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT article.
2. Raise vehicle. Inspect fuel tank and fuel pipes for damage or external leaks. Did you find fuel leaking from fuel tank? If yes, go to step 6 . If no, go to next step.
3. Turn ignition on, with engine off. With scan tool, command fuel pump ON. Inspect for fuel leaking from fuel pipes. Did fuel leak from fuel pipes? If yes, go to step 7 . If no, go to next step.
4. Install Fuel Tank Cap Adapter (J-41415-40). Connect Evaporative Emissions System Tester (J-41413-200) to fuel tank cap adapter. With scan tool, seal EVAP system. DO NOT exceed 15 in. H2O. With EVAP pressure/purge diagnostic station, pressurize fuel tank. Did fuel leak from fuel tank? If yes, go to step 6 . If no, go to next step.
5. With EVAP pressure/purge diagnostic station, maintain pressure in fuel tank. With Ultrasonic Leak Detector (J-41416), test for leaks above fuel level in following locations: Fuel tank, fill limiter vent valve, pressure relief valve and rollover valves. Fuel sender housing and fuel sender seal. Fuel Tank Pressure (FTP) sensor seal. EVAP vapor pipes. Fuel fill pipe and hose. Did you locate a fuel leak? If yes, go to next step. If no, see «DIAGNOSTIC AIDS»(ref-153168-S19158592722003032100000) .
6. Replace fuel tank. After repairs, system is okay.
7. Replace leaking fuel pipe. After repairs, system is okay.
8. Operate the system in order to verify the repair. Did you correct the condition? If yes, system is okay. If no, go to step 2 .

1. Operate vehicle under condition of customer's concern. Under high temperature conditions fuel vapors may increase to point of EVAP canister vapor saturation. Fuel vapors would then be released into atmosphere. Once engine is running and EVAP purge is enabled, all fuel vapor release would be eliminated.
2. Test for fuel leaking in fuel tank, rear tank if equipped, and fuel feed and return pipes.
3. Test for fuel vapor leaks in fuel tank, fill limiter vent valve and rollover valves, fuel fill pipe, hose and fuel fill cap, fuel sender housing and seal, Fuel Tank Pressure (FTP) sensor seal and EVAP vapor pipe.
4. Movement of EVAP pipes or fuel pipes may help find an intermittent condition.

ELECTRONIC IGNITION SYSTEM DIAGNOSIS

Note. For component locations, see COMPONENT LOCATIONS . For circuit reference, see WIRING DIAGRAMS .

1. Perform Diagnostic System Check - Engine Controls. See «DIAGNOSTIC SYSTEM CHECK -- ENGINE CONTROLS»(ref-155898-S30062612662003062800000) under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT article. After performing Diagnostic System Check - Engine Controls, go to next step.
2. Are DTCs P0335 or P0336 set? If yes, go to «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-155898-S19906497452003062800000) in SELF-DIAGNOSTICS - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT article. If no, go to next step.
3. Attempt to start engine. Does engine start and run? If yes, go to next step. If no, go to step 15 .
4. Observe Misfire Current Counters parameter with scan tool. Does scan tool display any Misfire Current Counters incrementing? If yes, go to next step. If no, go to «DIAGNOSTIC AIDS»(ref-153168-S04570081702003032100000) .
5. Remove ignition coil for affected cylinder. Install Spark Tester (J-26792) to spark plug boot. Attach clamp end of (J-26792) to a good engine ground. Observe spark tester. Crank engine. Does spark jump tester gap? If yes, go to step 11 . If no, go to next step.
6. Turn ignition off. Disconnect ignition coil for misfiring cylinder. Turn ignition on, with engine off. Probe Ignition 1 voltage circuit of affected ignition coil with test light connected to good ground. Does test light illuminate? If yes, go to next step. If no, go to step 15 .
7. Probe Ignition 1 voltage circuit of affected ignition coil with test light connected to ignition coil ground circuit. Does test light illuminate? If yes, go to next step. If no, go to step 16 .
8. Start engine. Measure frequency from IC Control circuit of respective ignition coil to a good ground. Does frequency measure within 2-20 Hz? If yes, go to step 14 . If no, go to next step.
9. Turn ignition on, with engine off. Measure voltage from IC Control circuit of respective ignition coil to a good ground with DVOM. Does voltage measure more than one volt? If yes, go to step 17 . If no, go to next step.
10. Test IC Control circuit for an open, a short to ground or high resistance. Did you find and correct the condition? If yes, go to step 21 . If no, go to step 13 .
11. Exchange spark plug with a spark plug from a non-misfiring cylinder. Install ignition coil. Start engine. Does scan tool display that misfire followed that spark plug? If yes, go to step 20 . If no, go to next step.
12. Inspect insulating spark plug boot and conducting spring for contamination, deterioration or any arching. Did you find and correct the condition? If yes, go to step 21 . If no, check for base engine mechanical faults.
13. Test for an intermittent and for a poor connection at PCM. Did you find and correct the condition? If yes, go to step 21 . If no, go to step 19 .
14. Test for an intermittent and for a poor connection at ignition coil. Did you find and correct the condition? If yes, go to step 21 . If no, go to step 18 .
15. Repair open or short to ground in Ignition 1 voltage circuit. After repairs, go to step 21
16. Repair open in ignition coil ground circuit. After repairs, go to step 21
17. Repair short to voltage in IC Control circuit. After repairs, go to step 21
18. Replace ignition coil. After repairs, go to step 21
19. Replace PCM. Program replacement PCM. See «REMOVAL & INSTALLATION - BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT»(ref-159085) article. After repairs, go to step 21
20. Replace spark plug. After repairs, go to next step.
21. Attempt to start engine. Does engine start and run? If yes, go to next step. If no, go to step 2 .
22. Clear DTCs with scan tool. Turn ignition off for 30 seconds. Start engine. Let engine reach operating temperature. Are there any DTCs that have not been diagnosed? If yes, go to «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-155898-S19906497452003062800000) in SELF-DIAGNOSTICS - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT article.

1. Crankshaft Position (CKP) signal is not needed to start and run.
2. Many situations may lead to an intermittent condition. Perform each inspection or test as directed.
3. Inspect PCM and engine grounds for clean and secure connections.
4. For intermittent conditions, see «INTERMITTENTS»(ref-152936-S40530116252003030500000) in TROUBLE SHOOTING - NO CODES - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT article.

EVAP Control System Diagnosis

For EVAP control system diagnosis, see appropriate INSPECTION/MAINTENANCE PROCEDURES under DRIVE CYCLES in SELF-DIAGNOSTICS - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT article.

Inspection Procedure

Note. DO NOT perform this procedure unless instructed by EVAP diagnostic test. Use EVAP Pressure/Purge Diagnostic Station (J-41413) in order to provide a clean, dry, low pressure gas source. DO NOT substitute any other pressurized gas source. Damage may result to EVAP system.

1. Turn ignition off.
2. Remove EVAP canister purge valve.
3. Lightly tap EVAP canister purge valve on a hard surface.
4. Inspect for carbon particles exiting either vacuum port. If no carbon particles were detected, install original EVAP canister purge valve and continue with cleaning procedure. If carbon particles are found during inspection procedure, replace EVAP canister purge valve and continue with cleaning procedure. If you were instructed to replace EVAP canister purge valve and no carbon particles are found, return to EVAP diagnostic procedure that directed you here. DO NOT perform EVAP cleaning procedure.