Exhaust Gas Recirculation: Testing and Inspection

EGR Circuit:

EGR System Check:

Circuit Description
A properly operating Exhaust Gas Recirculation (EGR) system will directly affect the air/fuel requirements of the engine. Since the exhaust gas introduced into the engine is inert gas (contains very little or no oxygen), less fuel is required to maintain a correct air/fuel ration. If the EGR system were to become inoperative the the inert gas would be replaced with air and the fuel mixture would be leaner. The Engine Control Module (ECM) would compensate for the lean condition by adding fuel, resulting in higher block learn values.

The engine control system operates within two block learn cells, a closed throttle cell and open throttle cell. Since EGR is not used at idle, the closed throttle cell would not be affected by EGR operation. The open throttle cell is affected by EGR operation and when the EGR system is inoperative, the block learn value in the open throttle cell would be changed (higher) to compensate for the resulting lean system, but the block learn value in the closed throttle cell would not change.

Diagnostic Aids
The EGR valve chart is a check of the EGR system. If the EGR system works properly, check other items that could result in high block learn values in the open throttle cell, but not in the closed throttle cell.

Check
^ EGR Passages (restricted or blocked)
^ Manifold absolute pressure (MAP) sensor
A MAP sensor may shift in calibration enough to affect fuel delivery. Use CHART C-1D, MAP output check.