Positive Back Pressure (EGR) System

INCORRECT EGR OPERATION:

1. If too much EGR is flowing at idle, cruise, or cold operation,
any of the following conditions could exist:
a. The engine dies after cold start.
b. The engine stalls on deceleration.
c. The vehicle surges during cruise mode.
d. Rough idle.

2. If too little or no exhaust gases flow, the combustion chamber temperature may increase under acceleration or heavy load conditions. These conditions could cause:
a. Detonation (Spark Knock).
b. The engine to overheat.
c. The emissions test to fail.

EGR Wiring Diagram:

EGR SYSTEM CHECK - POSITIVE BACKPRESSURE

CIRCUIT DESCRIPTION:

The ECM operates a solenoid to control the EGR valve. The solenoid is energized and de-energized by the ECM, and is normally closed. The ECM provides a ground path to energize the solenoid, which allows vacuum to flow to the EGR valve. The ECM controls the operation of the EGR valve based on all of the following engine sensor input:

1. Engine coolant temperature above 77°F (25°C).
2. Throttle Position Sensor (TPS) off base idle.
3. Manifold Absolute Pressure (MAP).

EGR Diagnostic Chart - 1 of 2:

EGR Circuit (2 of 2):

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

1. Determine if the EGR intake or exhaust passages are plugged. Once the EGR valve is removed, plug each hole one at a time, and start the engine. If either hole is plugged, see EGR INTAKE & EXHAUST CLEANING under MAINTENANCE PROCEDURES.
2. Grounding the diagnostic test terminal should allow the EGR solenoid to energize, and vacuum should flow to the gauge. The vacuum at the gauge may or may not slowly bleed off.
3. When the diagnostic test terminal is not grounded, the vacuum gauge should bleed off through a vent in the EGR solenoid. The vacuum gauge may or may not bleed off, but this does not indicate a problem.
4. This test determines if the electrical control portion of the system is at fault, or if the EGR solenoid and/or connector is at fault.
5. See EXHAUST GAS RECIRCULATION (EGR) SYSTEM, "EGR IDENTIFICATION," under COMPONENT OPERATION to assist in determining the type of EGR valve used.
6. The remaining tests check the ability of the EGR valve to interact with the exhaust system.
7. When the engine is running, exhaust back pressure at the base of the EGR valve, and vacuum present at the EGR valve should allow the EGR valve to open. A rapid snap of the throttle should provide sufficient exhaust back pressure to the base of the EGR valve, which will close the internal vacuum bleed valve. With the internal vacuum bleed closed, the vacuum source should lift the valve off its seat. Note: Pintle movement may be slight.
8. Excessive exhaust back pressure caused by a bent or restricted exhaust component, could provide enough back pressure at the base of the EGR valve to close the EGR valve's internal vacuum bleed. If this happens, undesired EGR valve operation may take place at base idle speed.
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 source vacuum from operating the EGR valve.

NOTE: The EGR valve will not hold vacuum, from an external vacuum source, with the engine not running. Once the test sequence is completed, clear codes and verify NO "SERVICE ENGINE SOON" light exist.




DIAGNOSTIC AIDS

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