Part 2 of 3

EGR Testing Part 2 Of 3:

EGR Wiring Diagram:

CIRCUIT DESCRIPTION:

The ECM operates a vacuum solenoid to control the EGR valve. The solenoid is normally closed. The ECM provides a ground path to energize the solenoid. This allows vacuum to flow to the EGR valve.

The ECM control of the EGR valve is based on the following inputs;

^ Engine coolant temperature above 25°F
^ TPS off idle
^ MAP
^ If code 24 is stored use that chart first


TEST DESCRIPTION: The numbers below refer to circled numbers on the diagnostic chart.


6. The remaining tests check the ability of the EGR to interact with the exhaust system. This system uses a POSITIVE BACKPRESSURE EGR VALVE which will not hold vacuum until sufficient exhaust backpressure is at the base of the valve.

7. The EGR valve diaphragm should move when sufficient exhaust backpressure is present at the base of the EGR valve and when vacuum is applied to the valve. Rapidly snapping the throttle from idle should provide sufficient exhaust back pressure to the base of the EGR valve, which will close an internal vacuum bleed.

With the EGR valve's internal vacuum bleed closed, the jumpered vacuum supply can now lift the valve off its seat.

8. Excessive exhaust back pressure from bent or restricted exhaust system components, could provide enough back pressure to the base of the EGR valve to close the EGR valve's internal vacuum bleed and allow undesired EGR valve operation at idle.

9. A Plugged EGR exhaust passage can stop exhaust back pressure from reaching the base of the EGR valve. If this takes place, the EGR valve's internal vacuum bleed may remain open, and prevent vacuum from operating the EGR valve.


DIAGNOSTIC AIDS:

^ Suction from shop exhaust hoses can alter engine back pressure, and may affect the functional test of the EGR valve.