See Figures 1, 2 and 3
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.
The EGR valve is usually mounted on the intake manifold and has an opening into the exhaust manifold. Except for the fully electronic linear valve, the EGR valve is opened by manifold vacuum to permit exhaust gas to flow into the intake manifold. 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, in some applications, below Wide Open Throttle (WOT). The EGR system is deactivated on vehicles equipped with a Transmission Converter Clutch (TCC) when the TCC is engaged. There are 3 basic types of systems:Negative Backpressure EGR Valve
The negative backpressure EGR valve is used on most of the earlier model vehicles covered by this guide and by later model vehicle engines such as the 2.5L engine. The negative backpressure 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 valve is normally closed. The negative backpressure 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. Negative backpressure EGR valves will have an "N" stamped on the top side of the valve below the part number and after the date built.
The ported EGR valve is used on many of the later model engines covered by this guide including versions of the 4.3L (VIN Z) engine, the 4.3L MFI-Turbo and some 2.8L TBI engines. This valve is controlled by a flexible diaphragm. It is spring-loaded in order to hold the valve closed. When ported vacuum is applied to the top side of the diaphragm, spring pressure is overcome and the valve in the exhaust gas port is opened. This allows the exhaust gas to be pulled into the intake manifold and enter the cylinders with the air/fuel mixture. Port EGR valves have no identification stamped below the part number. On most applications covered by this guide, the ported EGR valve vacuum source will be controlled electronically through an Electronic Vacuum Regulator Valve (EVRV) or EGR vacuum solenoid.Linear EGR Valve
Some late model versions of the 4.3L TBI (California emissions) and the all 4.3L CMFI (VIN W) engines utilize a linear EGR valve. This is a fully electronic EGR valve which is constantly monitored and controlled by the engine (powertrain or vehicle) computer control module. The control module will monitor input from the TP, MAP, ECT and pintle positions sensors, then will send an output signal to the EGR valve indicating the proper amount of exhaust gas recirculation necessary to properly lower combustion temperatures. This electronic metering is about 10 times faster than vacuum-operated models and offers improved diagnostic capabilities.
ELECTRONIC EGR CONTROL
See Figures 4, 5, 8 and 9
On certain vehicle applications (most late model and California emission vehicles), EGR flow is regulated by the computer control module (linear valves) or by an computer control module-regulated Electronic Vacuum Regulator Valve (EVRV)/EGR solenoid (ported or negative backpressure valves). On vacuum controlled valves, the vacuum supply is controlled by a solenoid using pulse width modulation. This means that the ECM turns the solenoid on and off many times a second and varies the amount of ON time (pulse width) which in turn varies the amount of vacuum and exhaust gas recirculation.
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.
TESTING (EXCEPT GASOLINE FUEL INJECTED ENGINES)
- Check hose routing (refer to the Vehicle Emission Control Information (VECI) label).
- Check the EGR valve signal tube orifice for obstructions.
- Connect a vacuum gauge between EGR valve and carburetor, then check the vacuum; the engine must be at operating temperature of 195°F (90°C). With the engine running at approximately 3000 rpm there should be at least 5 in. Hg (16 kPa).
- Check the EGR solenoid for correct operation.
To check the valve, perform the following procedures:
- Depress the valve diaphragm.
- With the diaphragm still depressed hold a finger over the source tube and release the diaphragm.
- Check the diaphragm and seat for movement. The valve is good if it takes over 20 seconds for the diaphragm to move to the seated position (valve closed). Make sure the source tube is completely covered.
- Replace the EGR valve if it takes less than 20 seconds to move to the seated position.
TESTING (GASOLINE FUEL INJECTED ENGINES)
Use the trouble tree charts to check the EGR systems on fuel injected vehicles
EGR VALVE CLEANING
DO NOT wash the valve assembly in solvents or degreaser - permanent damage to valve diaphragm may result. Also, sand blasting of the valve is not recommended since this can affect valve operation.
- Remove the EGR valve from the engine.
- With a wire brush, remove the exhaust deposits from the mounting surface and around the valve.
- On vacuum valves, depress the valve diaphragm and look at the valve seating area through the valve outlet for cleanliness. If the valve and/or seat are not completely clean, repeat Step 2.
- Look for exhaust deposits in the valve outlet. Remove the deposit build-up with a small scraper.
- Clean the mounting surfaces of the intake manifold and the valve assembly, then install the valve using a new gasket.
REMOVAL & INSTALLATION
See Figures 12, 13, 14, 15, 16, 17, 18, 19 and 20
- If necessary for access, remove the air cleaner or duct, as applicable.
- For vacuum actuated valves, detach the vacuum hose or line from the EGR valve.
- For linear valves, disconnect the negative battery cable, then disengage the valve electrical connector.
- If equipped on the 2.8L engine, disconnect the temperature switch from the EGR valve.
- Remove the EGR valve flange retainers, then remove the valve from the manifold.
- Install the valve to the intake using a new gasket, then secure using the nuts or bolts, as applicable.
- If equipped on the 2.8L engine, connect the temperature switch to the EGR valve.
- For linear valves, engage the valve electrical connector, then connect the negative battery cable.
- For vacuum actuated valves, reconnect the vacuum hose or line to the valve.
- If removed for access, install the air cleaner or duct.
See Figure 21
- Disconnect the negative battery cable.
- If necessary for access, remove the air cleaner assembly or duct.
- Disengage the electrical connector, then tag and disconnect the vacuum hoses from the solenoid.
- Remove the retainer(s), then rem