Most engines covered by this guide are equipped with an exhaust gas recirculation (EGR) system. The EGR system typically consists of a metering valve, a vacuum line to the carburetor and cast-in exhaust gas passages located inside the intake manifold. The EGR valve is controlled by carburetor vacuum, and accordingly opens and closes to admit exhaust gases into the fuel/air mixture. The exhaust gases lower the combustion temperature, thereby reducing the amount of nitrogen oxides (NO X ) produced. The valve is normally closed at idle and open between the two extreme throttle positions.
In most early model installations, vacuum to the EGR valve is controlled by a thermal vacuum switch (TVS). The switch, which is installed into an engine coolant passage such as the thermostat housing, shuts off vacuum to the EGR valve until the engine is properly warmed. This prevents the stalling and lumpy idle which would result if EGR occurred when the engine was cold.
As the car accelerates, the carburetor throttle plate uncovers the vacuum port for the EGR valve. At 3-5 in. Hg, the EGR valve opens and some of the exhaust gases are allowed to flow into the air/fuel mixture in order to lower the combustion temperature. At full throttle the valve closes again.
Some California engines are equipped with dual diaphragm EGR valve. This valve further limits the exhaust gas opening (compared to the single diaphragm EGR valve) during high intake manifold vacuum periods, such as high speed cruising, and provides more exhaust gas recirculation during acceleration when manifold vacuum is low. In addition to the hose running to the thermal vacuum switch, a second hose is connected directly to the intake manifold.
Most 1979-81 vehicles are equipped with backpressure EGR valves. The EGR valve receives exhaust backpressure through its hollow shaft. This exerts a force on the bottom of the control valve diaphragm, opposed by a light spring. Under low exhaust pressure (low engine load and partial throttle), the EGR signal is reduced by an air bleed. Under conditions of high exhaust pressure (high engine load and large throttle opening), the air bleed is closed and the EGR valve responds to an unmodified vacuum signal. At wide open throttle, the EGR flow is reduced in proportion to the amount of vacuum signal available.
All models have a ported signal vacuum EGR valve. The valve opening is controlled by the amount of vacuum obtained from a ported vacuum source on the carburetor and the amount of backpressure in the exhaust system.
EGR VACUUM CONTROL SOLENOID
For better control over the EGR system flow on most 1981 and later models, a solenoid is used in the vacuum line. The solenoid is controlled by the Electronic Control Module (ECM), which uses information from the coolant temperature, throttle position, and manifold pressure sensors to regulate the vacuum solenoid. On most models, the solenoid is turned on or off for periods of time in order to cut or allow vacuum to the EGR valve. For example on vehicles utilizing a normally open solenoid valve, when the engine is cold, a signal from the ECM energizes the EGR solenoid, thus blocking vacuum to the EGR valve.
On some later models, the ECM uses "Pulse Width Modulation" during cold engine operation (meaning it turns the solenoid on and off many times a second) to achieve an even more exact control over the amounts of exhaust gas that are recirculated.
The solenoid controlled systems on most carbureted vehicles and fuel injected vehicles through 1987 utilize a normally open solenoid valve. The solenoid is energized during cranking and wide-open throttle in order to block vacuum and prevent gas recirculation. When the engine warms up, the EGR solenoid is turned off by the ECM, and the EGR valve operates according to normal ported vacuum and exhaust backpressure signals.
For most 1988-89 vehicles, a Normally Closed (N/C) solenoid is utilized. The N/C solenoid operates on the opposite signals from its normally open predecessor. The ECM will energize the N/C solenoid in order to open it and provide a vacuum signal for exhaust gas recirculation. The solenoid is typically energized only when the ECM detects a coolant temperature above 77°F (25°C), the Throttle Position Sensor (TPS) signal is off idle and the Manifold Absolute Pressure (MAP) sensor signal is within a certain specification.
FAULTY EGR VALVE SYMPTOMS
An EGR valve that stays open when it should be closed causes weak combustion, resulting in a rough running engine and/or frequent stalling. Too much EGR flow at idle, cruise, or when cold can cause any of the following:
An EGR valve which is stuck closed and allows little or no EGR flow causes extreme combustion temperatures (too hot) during acceleration. Spark knock (detonation or pinging), engine overheating and excess engine emissions can all be a result, as well as engine damage.
REMOVAL & INSTALLATION
- Detach the vacuum line(s) from the EGR valve. If there is more than one line, be sure to label them to assure proper installation.
- Unfasten the 2 bolts or the bolt and clamp which attach the valve to the manifold, then remove the valve.
- Make sure the old valve gasket is removed from the manifold.
- Thoroughly clean the gasket mating surfaces of the valve and manifold.
- Install the valve to the manifold. Always use a new gasket between the valve and the manifold.
- Install and tighten the valve retainers.
- Attach the vacuum line(s). On dual diaphragm valves, attach the carburetor vacuum line to the tube at the top of the valve, and the manifold vacuum line to the tube at the center of the valve.
- Drain the engine cooling system to a level below the TVS switch.
- Disconnect the vacuum lines from the switch noting their locations for assembly purposes.
- Carefully unthread the switch from the bore.
- Apply sealer to the threaded portion of the switch.
- Install the switch to the bore and tighten it to 15 ft. lbs. (20 Nm).
- If possible, rotate the head of the switch to a position that will permit easy hookup of vacuum hoses.
- Connect the vacuum hoses to the proper connectors as noted during removal.
EGR VALVE CLEANING
Valves That Protrude from Mounting Face
- Remove the EGR valve from the intake manifold.
- Holding the valve assembly in hand, use a small plastic hammer to tap the valve lightly and remove exhaust deposits from the valve seat. Shake out any loose particles. DO NOT put the valve in a vise.
- Carefully remove any exhaust deposits from the mounting surface of the valve with a wire wheel or putty knife. Be careful not to score or damage the mounting surface.
- Depress the valve diaphragm and inspect the valve seating areas through the valve outlet for cleanliness. If the valve and/or seat are not completely clean, attempt to remove the remaining deposits using the plastic hammer.
- Look for exhaust deposits in the valve outlet, and remove any deposits with an old screwdriver or small prytool.
- Clean the mounting surfaces of the intake manifold and valve assembly, then install the valve to the intake.
- Remove the valve from the intake manifold.
- Clean the base of the valve with a wire brush or wheel to remove exhaust deposits from the mounting surface.
- Clean the valve seat and valve in an abrasive-type spark plug cleaning machine or sand-blaster. Most machine shops provide this service. Make sure the valve portion is cleaned (blasted) for about 30 seconds, and that the valve is also cleaned with the diaphragm spring fully compressed (valve unseated). The cleaning should be repeated until all deposits are removed.
- The valve must be blown out with compressed air thoroughly to ensure all abrasive material is removed from the valve.
- Clean the mounting surface of the intake manifold and valve assembly, then install the valve.