GM Celebrity/Century/Ciera/6000 1982-1996 Repair Guide

Exhaust Gas Recirculation System (EGR) - Gasoline Engines



Not all vehicles are equipped with an EGR system.

The EGR system is used to reduce oxides of nitrogen (NOx) emission levels caused by high combustion chamber temperatures. This is accomplished by the use of an EGR valve which opens, under specific engine operating conditions, to admit a small amount of exhaust gas into the intake manifold, below the throttle plate. The exhaust gas mixes with the incoming air charge and displaces a portion of the oxygen in the air/fuel mixture entering the combustion chamber. The exhaust gas does not support combustion of the air/fuel mixture but it takes up volume, the net effect of which is to lower the temperature of the combustion process. This lower temperature also helps control detonation.

The EGR valve is a mounted on the intake manifold and has an opening into the exhaust manifold. On some vehicles, the EGR valve is opened by manifold vacuum to permit exhaust gas to flow into the intake manifold. On others, the EGR valve is purely electrical and uses solenoid valves to open the flow passage. If too much exhaust gas enters, combustion will not occur. Because of this, very little exhaust gas is allowed to pass through the valve. The EGR system will be activated once the engine reaches normal operating temperature and the EGR valve will open when engine operating conditions are above idle speed and below Wide Open Throttle (WOT). On California vehicles equipped with a Vehicle Speed Sensor (VSS), the EGR valve opens when the VSS signal is greater than 2 mph. The EGR system is deactivated on vehicles equipped with a Torque Converter Clutch (TCC) when the TCC is engaged. There are three basic types of systems as described below, differing in the way EGR flow is modulated.

Positive Backpressure EGR Valve

See Figure 1

An air bleed valve, located inside the EGR valve assembly acts as a vacuum regulator. The bleed valve controls the amount of vacuum in the vacuum chamber by bleeding vacuum to outside air during the open phase of the cycle. When the EGR valve receives enough backpressure through the hollow shaft, it closes the valve. At this point, maximum available vacuum is applied to the diaphragm and the EGR valve opens. If there is a small amount of vacuum or no vacuum in the vacuum chamber such as wide open throttle or at idle, the EGR valve will not open. The positive backpressure EGR valve also will not open if vacuum is applied to the valve with the engine stopped or idling.

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Fig. Fig. 1: Positive backpressure EGR valve

Negative Backpressure EGR Valve

See Figure 2

The negative backpressure EGR valve is similar to the positive backpressure EGR valve except that the bleed valve spring is moved from above the diaphragm to below and the bleed valve is normally closed. The negative backpressure EGR valve varies the amount of exhaust gas flow into the intake manifold depending on manifold vacuum and variations in exhaust backpressure. The diaphragm on the valve has an internal air bleed hole which is held closed by a small spring when there is no exhaust backpressure. Engine vacuum opens the EGR valve against the pressure of a spring. When manifold vacuum combines with negative exhaust backpressure, the vacuum bleed hole opens and the EGR valve closes. This valve will open if vacuum is applied with the engine not running.

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Fig. Fig. 2: Negative backpressure EGR valve

Digital EGR Valve

See Figure 3

The digital EGR valve, used on most of the newer engines such as the 3.1L (VIN T) and 3.1L (VIN M), it is designed to control the flow of EGR, independent of intake manifold vacuum. The valve controls EGR flow through 3 solenoid-opened orifices, which increase in size, to produce 7 possible combinations. When a solenoid is energized, the armature with attached shaft and swivel pintle, is lifted, opening the orifice.

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Fig. Fig. 3: Digital EGR valve

The digital EGR valve is opened by the ECM/PCM, grounding each solenoid circuit individually. The flow of EGR is regulated by the ECM/PCM which uses information from the Engine Coolant Temperature Sensor (ECT), Throttle Position Sensor (TPS), Manifold Absolute Pressure (MAP) sensor and Mass Air Flow Sensor (MAF) to determine the appropriate rate of flow for a particular engine operating condition.

Incorrect EGR Operation

Too much EGR flow at idle, cruise, or during cold operation may result in the engine stalling after cold start, the engine stalling at idle after deceleration, vehicle surge during cruise and rough idle. If the EGR valve is always open, the vehicle may not idle. Too little or no EGR flow allows combustion temperatures to get too high which could result in spark knock (detonation), engine overheating and/or emission test failure.

EGR Valve Identification

Positive backpressure EGR valves will have a "P" stamped on the top side of the valve below the date built.
Negative backpressure EGR valves will have a "N" stamped on the top side of the valve below the date built.
Port EGR valves have no identification stamped below the date built.


See Figures 4 and 5

Backpressure EGR valves
  1. Disconnect the negative battery cable.
  3. Remove the air cleaner assembly.
  5. Tag and disconnect the necessary hoses and wiring to gain access to the EGR valve.
  7. Remove the EGR valve retaining bolts.

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Fig. Fig. 4: Remove the EGR valve retaining bolts with a suitable wrench

  1. Remove the EGR valve. Discard the gasket.

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Fig. Fig. 5: When removing the EGR valve, be sure to also remove all old gasket material and clean off the engine's valve mounting surface

  1. Buff the exhaust deposits from the mounting surface and around the valve using a wire wheel.
  3. Remove deposits from the valve outlet.
  5. Clean the mounting surfaces of the intake manifold and valve assembly.

To install:
  1. Install a new EGR gasket.
  3. Install the EGR valve to the manifold.
  5. Install the retaining bolts and torque to 11-22 ft. lbs. (15-30 Nm).
  7. Connect the wiring and hoses.
  9. Install the air cleaner assembly.