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This guide will help the home mechanic test the EGR system on your 88-95 GMT400 truck, and does not require the use of a scan tool.
EGR issues are relatively simple to diagnose, if you have the proper tools.
You'll need:
Eyeballs
Vacuum pump
Multimeter
First things first. Visual/physical inspection.
Make sure that the vacuum lines are routed properly and are free from defect.
Inspect the wiring terminal at the EGR solenoid for good, corrosion free connection, and that the wires are in good condition - no fraying, cracking or looseness in the connector. Repair any deficiencies before further testing.
To check that the EGR pathways are clear, bring the engine up to operating temperature, then manually move the EGR diaphragm (be careful not to burn or pinch your fingers). If the engine stumbles and nearly stalls, the EGR pathways are clear. If the valve moves and the engine continues to idle normally, you'll need to remove the intake to fully clean the EGR passages. If you cannot move the EGR valve with hand pressure, it likely needs replacing, but a thorough cleaning *may* loosen it up.
If the EGR passed the visual/physical inspection and test, you'll next need a vacuum pump to continue your diagnosis.
With the engine off, disconnect the vacuum line at the EGR valve. Connect the vacuum pump directly to the valve, and apply 10" of vacuum. Verify that the valve opened. The diaphragm inside the valve should hold vacuum for a minimum of 20 seconds. If the valve will not build vacuum, or if the vacuum leaks to 0 within 20 seconds, the EGR valve needs to be replaced. If it held vacuum but the valve did not move, a thorough cleaning may return it to operation.
Your next test will determine if the valve reacts to exhaust back pressure properly.
With the engine off, apply 15" vacuum directly to the EGR diaphragm. Visually verify EGR valve movement. Have a partner start the engine while you monitor the vacuum reading. The vacuum should dissipate almost immediately, closing the EGR valve. If it does not, a thorough cleaning of the EGR valve and associated passages in the intake manifold should fix this.
If the EGR valve passed these tests, the problem is not in the valve itself, but may instead be the solenoid, vacuum lines, electrical connections, or the ECM.
To test the solenoid, you'll need both a vacuum pump and a multimeter.
First, disconnect the vacuum line from the intake side of the solenoid. Apply 10" vacuum to the solenoid. If it does build vacuum, it should hold vacuum for a minimum of 20 seconds. If the solenoid fails to build or hold vacuum, it has failed and needs to be replaced.
Next, disconnect the vacuum line from the EGR side of the solenoid. Attempt to apply vacuum to it. If you can build vacuum, the vent line (the second vacuum port that has no vacuum line) is plugged. Cleaning this *may* return operation. If not, replace the EGR solenoid.
To continue testing the EGR solenoid, you will need a multimeter.
Verify battery is fully charged. (key off, engine off, battery voltage should read ~12.6v)
With the key on, engine off, back probe the pink wire with the positive lead, and connect the negative lead to battery ground. Set the multimeter to DC Volts. You should read battery voltage (~12.6v) on the pink wire. If no voltage, or the voltage is lower than the reading at your battery by more than .5v, carefully inspect the grounds (paying close attention to the two thermostat housing grounds and the multiple grounds to and from the passenger cylinder head). If cleaning/repairing the grounds did not affect the voltage reading on the pink wire, find and repair the short in circuit #439 (This circuit originates at pin #A6 on the ECM)
If there are no issues with circuit 439, you'll next need to test the continuity (ohms) of circuit #435, which is the ECM provided ground circuit. This is the second wire connected to the EGR solenoid, and should be a light gray color. It originates at position A4 on the ECM.
If no shorts are found in circuits 435 or 439, suspect that the ECM may have failed internally. Other testing procedures will be required to verify.
This should give you enough to find and fix your EGR problem. Be advised that the GMT400 trucks are sensitive to which EGR valve is used, and if you need to replace it, get an original equipment part.
EGR issues are relatively simple to diagnose, if you have the proper tools.
You'll need:
Eyeballs
Vacuum pump
Multimeter
First things first. Visual/physical inspection.
Make sure that the vacuum lines are routed properly and are free from defect.
Inspect the wiring terminal at the EGR solenoid for good, corrosion free connection, and that the wires are in good condition - no fraying, cracking or looseness in the connector. Repair any deficiencies before further testing.
To check that the EGR pathways are clear, bring the engine up to operating temperature, then manually move the EGR diaphragm (be careful not to burn or pinch your fingers). If the engine stumbles and nearly stalls, the EGR pathways are clear. If the valve moves and the engine continues to idle normally, you'll need to remove the intake to fully clean the EGR passages. If you cannot move the EGR valve with hand pressure, it likely needs replacing, but a thorough cleaning *may* loosen it up.
If the EGR passed the visual/physical inspection and test, you'll next need a vacuum pump to continue your diagnosis.
With the engine off, disconnect the vacuum line at the EGR valve. Connect the vacuum pump directly to the valve, and apply 10" of vacuum. Verify that the valve opened. The diaphragm inside the valve should hold vacuum for a minimum of 20 seconds. If the valve will not build vacuum, or if the vacuum leaks to 0 within 20 seconds, the EGR valve needs to be replaced. If it held vacuum but the valve did not move, a thorough cleaning may return it to operation.
Your next test will determine if the valve reacts to exhaust back pressure properly.
With the engine off, apply 15" vacuum directly to the EGR diaphragm. Visually verify EGR valve movement. Have a partner start the engine while you monitor the vacuum reading. The vacuum should dissipate almost immediately, closing the EGR valve. If it does not, a thorough cleaning of the EGR valve and associated passages in the intake manifold should fix this.
If the EGR valve passed these tests, the problem is not in the valve itself, but may instead be the solenoid, vacuum lines, electrical connections, or the ECM.
To test the solenoid, you'll need both a vacuum pump and a multimeter.
First, disconnect the vacuum line from the intake side of the solenoid. Apply 10" vacuum to the solenoid. If it does build vacuum, it should hold vacuum for a minimum of 20 seconds. If the solenoid fails to build or hold vacuum, it has failed and needs to be replaced.
Next, disconnect the vacuum line from the EGR side of the solenoid. Attempt to apply vacuum to it. If you can build vacuum, the vent line (the second vacuum port that has no vacuum line) is plugged. Cleaning this *may* return operation. If not, replace the EGR solenoid.
To continue testing the EGR solenoid, you will need a multimeter.
Verify battery is fully charged. (key off, engine off, battery voltage should read ~12.6v)
With the key on, engine off, back probe the pink wire with the positive lead, and connect the negative lead to battery ground. Set the multimeter to DC Volts. You should read battery voltage (~12.6v) on the pink wire. If no voltage, or the voltage is lower than the reading at your battery by more than .5v, carefully inspect the grounds (paying close attention to the two thermostat housing grounds and the multiple grounds to and from the passenger cylinder head). If cleaning/repairing the grounds did not affect the voltage reading on the pink wire, find and repair the short in circuit #439 (This circuit originates at pin #A6 on the ECM)
If there are no issues with circuit 439, you'll next need to test the continuity (ohms) of circuit #435, which is the ECM provided ground circuit. This is the second wire connected to the EGR solenoid, and should be a light gray color. It originates at position A4 on the ECM.
If no shorts are found in circuits 435 or 439, suspect that the ECM may have failed internally. Other testing procedures will be required to verify.
This should give you enough to find and fix your EGR problem. Be advised that the GMT400 trucks are sensitive to which EGR valve is used, and if you need to replace it, get an original equipment part.
