Chart C-7 EGR System Check

Chart C-7:

EGR Solenoid Circuit:

CIRCUIT DESCRIPTION:
A properly operating EGR system will directly affect the air/fuel mixture requirements of the engine. Since the exhaust gas introduced into the air/fuel mixture cannot be used in combustion (contains very little oxygen), less fuel is required to maintain a correct air/fuel ratio. If the EGR system were to fail in a closed position, the exhaust gas would he replaced with air, and the air/fuel mixture would be leaner. The ECM would compensate for the lean condition by adding fuel, resulting in higher long term fuel trim values.

The fuel control on this engine is conducted within 4 fuel trim cells. Since EGR is not used at idle, the idle cell would not be affected by EGR system operation. The other fuel trim cells are affected by EGR operation, and, when the EGR system is operating properly, the long term fuel trim values in all cells should be close to the same. If the EGR system becomes inoperative, the long term fuel trim values in the open throttle cells would change to compensate for the resulting lean or rich mixtures, but the long term fuel trim value in the closed throttle cell would not change.

The difference in long term fuel trim values between the idle (closed throttle) cell and cell 2 is used to monitor EGR system performance. When the difference between the two long term fuel trim values is greater than 10, and the long term fuel trim value in cell 2 is greater than 135, Diagnostic Trouble Code (DTC) 32 is set. The system operates in fuel trim cell 2 during a cruise condition at approximately 55 mph.

TEST DESCRIPTION: Number(s) below refer to circled number(s) on the diagnostic chart.

1. DTC's should be diagnosed using appropriate chart before preparing a functional check. If Diagnostic Trouble Codes (DTCs) 14, 21, 23, 32, or 33 are set, use those charts first. Be sure shop exhaust hose is not connected during Steps 2 and 4.

2. The tail pipe ventilation hose must be removed for this test. The ventilation hose can sometimes cause a good EGR valve to fail this test.

3. Intake Passage: Shut "OFF" engine and remove the EGR valve from the manifold. Plug the exhaust side hole with a suitable stopper. Leaving the intake side hole open, attempt to start the engine. If the engine runs at a high idle (up to 3000 RPM is possible) or starts and stalls, the EGR intake passage is not restricted. If the engine starts and idles normally. the EGR intake passage is restricted.

4. Because the shop exhaust hose is not installed at this point, don't allow the engine to run longer than 15 seconds.

Exhaust passage: With EGR valve still removed, plug the intake side hole with a suitable stopper. With the exhaust side hole open, check for the presence of exhaust gas. If no exhaust gas is present, the EGR exhaust side passage is restricted.

DIAGNOSTIC AIDS:
This chart is a functional check of the EGR system. If the EGR system works properly, check other items that result in high long term fuel trim values in fuel trim cell 2, but not in the closed throttle cell.

Incorrect fuel pressure or lean/rich fuel injectors can also cause incorrect long term fuel trim values. Related Tests, Information and Procedures If fuel pressure checks out OK, Chart C-2A Injector Coil Test Procedure It may be necessary to monitor fuel pressure while driving the vehicle at various road speeds and/or loads.