How EGR works

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Sep 18, 2009
Queens, NY
This is a pretty thorough explanation. How the EGR valve works-

How the EGR Valve works:

The EGR valve is designed specifically to recirculate the exhaust gas into the air/fuel mixture, thereby diluting the air/fuel mixture enough to keep the NOx compounds within breathable limits. This is done by allowing a specific amount of inert gas to pass from the exhaust manifold into the intake manifold through the EGR valve.
It was discovered that short peak combustion temperatures create NOx. By blending an inert gas with the air/fuel mixture, scientists discovered that the rate of combustion slowed down, the high temperatures were reduced and the NOx compounds were kept within limits.

Modern engines are equipped with oxidation/reduction catalysts and fuel injection systems that keep the NOx compounds to a minimum. But even with these newer, more efficient systems, the EGR system is still necessary to reduce the excess emissions.

Early EGR systems are made up of a vacuum-operated valve( EGR valve ) that admits exhaust gas into the intake manifold, a hose that is connected to a carburator port above the throttle plate and a thermostatic vacuum switch ( TVS ) spliced into a pipe that is threaded into the coolant passage near the thermostat. The TVS detects the operating temperature of the engine.
At idle, the throttle plate blocks the vacuum port so no vacuum reaches the EGR valve and it remains closed. As you accelerate, the throttle uncovers the port in the carburator or the throttle body, the vacuum signal reaches the EGR valve and slowly opens it, allowing exhaust gases to circulate into the intake manifold.
Since the exhaust gas causes a rough idle and stalling when the engine is cold, the TVS only allows vacuum to the EGR valve when the engine is at normal operating temperature.
Also, when the pedal is pushed all the way to the floor under acceleration, there is very little vacuum available, resulting in very little mixture dilution that would interfere with power output.

The EGR valve on early carburated engines without computer controls acts solely in response to the temperature and venturi vacuum characteristics of the engine.

The EGR valve on engines with electronic fuel injection systems is controlled by the engine control computer ( ECM ) . EGR valves on computerized vehicles normally have a computer controlled solenoid in line between the valve and the vacuum source. They also often have an EGR position sensor that informs the computer what position the EGR valve is in.

There are 2 common types of EGR valves: Ported vacuum EGR valves and backpressure EGR valves.
The valve we described earlier is the ported EGR valve, besides this type, there are basically 2 types of backpressure EGR valves; The most common type is the positive backpressure valve, the other one is the negative backpressure valve.
It is important to know the difference between positive and negative backpressure valves because they work differently and they are tested differently also.

Positive backpressure EGR valve:
This type of valve is used largely on domestic models. It uses exhaust pressure to regulate the EGR flow through a vacuum control valve. The stem of the EGR valve is hollow and allows backpressure to enter at the bottom of the diaphragm. When sufficient exhaist backpressure is present, the diaphragm moves up and closes off the control valve, allowing the full vacumm signal to be applied to the upper portion of the EGR diaphragm. This opens the valve and allows recirculation to occur during heavy loads.
Be careful not to incorrectly diagnose this type of EGR valve. Because backpressure must be present to close the bleed hole, it is not possible to operate the EGR valve with a vacuum pump at idle or with the engine off. The valve is acting correctly when it refuses to move when vacuum is applied or it refuses to hold vacuum. Remember that anything that changes the pressure in the exhaust stream will disturb the calibration of the backpressure system including aftaermarket exhaust systems, headers and even clogged catalytic converters.
To distinguish this valve, turn it upside down and note the pattern of the diaphragm plate. Positive backpressure valves have slightly raised X-shaped rib. Negative backpressure EGR valves are raised considerably higher. On some GM EGR valves, the only way to distinguish each type is by a letter next to the date code and part number. N means negative and P means positive.

Negative backpressure EGR valve:
In this system, the bleed hole is normally closed when exhaust backpressure drops, the bleed valve opens and reduces the vacuum above the diaphragm, cutting the vacuum to the EGR valve. The negative backpressure EGR valve is similar to the positive backpressure EGR valve but operates in the opposite way. This type of valve is typically used on engines that have less than normal backpressure such as high performance vehicles with free flowing mufflers and large diameter exhaust systems.

Other types of EGR valves:

Ford air pressure EGR valve:
Most commonly installed on 1978 and 1979 Ford EEC-I systems, this type of EGR valve is operated by the thermactor air pump pressure instead of vacuum. Pump output is routed to the underside of the diaphragm , some models are equipped with an EGR position sensor.

Dual diaphragm EGR valve:
This EGR valve receives ported vacuum to the upper portion of the vacuum diaphragm while the lower portion receives manifold vacuum. The simultaneous response characteristics control both throttle position and engine load. The dual diaphragm system is easily recognized by the 2 vacuum lines attached to the EGR valve.

Ford electronic control EGR valve:
This EGR valve resembles the air pressure type but it is dependent on the ECM and the EGR position sensor to detect the correct conditions and regulate the EGR valve angle.

Chrysler/Mitsubishi dual EGR valve:
Most commonly equipped on the 2.6 L silent shaft engine, this type of EGR valve uses both a primary and a secondary EGR valve mounted at right angles to each other. This system allows for accurate measurement of the exhaust gases.

Computer controlled EGR valves:
On the newer type computerized EGR systems, the EGR valve is regulated by the use of different sensors, transducers or vacuum solenoids directly linked to the EGR valve, some of these items are:

1- Remote backpressure transducer: This device is not mounted inside the EGR valve, instead it is found in the vacuum line leading to the EGR valve. At idle or light loads, the transducer bleeds off the signal to prevent recirculation to the EGR valve.

2- Electronic pressure sensor: This capacitive sensor converts exhaust system backpressure into analog voltage signal that is sent directly to the computer. This type of pressure sensor is commonly found on newer EEC-IV Ford systems.

3- Wide open throttle valve: This device is located in-line between the EGR valve and the vacuum source. Controlled by a signal from the carburator venturi, the wide open throttle valve bleeds off the signal to the EGR valve at wide open throttle to eliminate any mixture dilution.

4- Solenoid vacuum valve: This valve works directly with the computer to control the vacuum signal. It is found most commonly on the GM systems and it is referred to as " pulse width modulation"

5- Electronic vacuum regulator: Instead of the on/off function of a solenoid vacuum valve, the electronic vacuum regulator adjusts vacuum to the EGR valve through the pressure sensor and the ECM. This device is most commonly found on Ford EEC-III and EEC-IV systems

6- Delay timer: This valve interrupts the vacuum to the EGR valve to prevent stalling when the engine is cold. The actual delay time can be anywhere from 30 to 90 seconds after the engine is started. The delay timer works in conjunction with a solenoid vacuum valve.

7- Charge temperature switch: This switch senses the temperature of the intake system, but it acts strictly as ON/OFF switch to prevent current from reaching the delay timer when the temperature is below 60 degrees F. This prevents any EGR mixture and consequently rough idle or stalling when the engine is cold. This system is commonly found on Chrysler emissions systems.

8- EGR valve position sensor: The EGR valve position sensor ( EVP ) detect the exact position of the EGR valve and send the information to the ECM, from this data, the computer can calculate the optimum EGR flow for the lowest NOx emissions and the best driveability, then control the EGR valve to alter the EGR flow through the EGR solenoid.
The EVP sensor is a linear potentiometer that operates very much like a TPS, its electrical resistance changes in direct proportion to the movement of the EGR valve stem. when the EGR valve is closed, the EVP sensor registers maximum resistance; as the valve opens, resistance decreases until it finally reaches a minimum value when the EGR valve is fully open.

Electronic EGR valve:
Some of the most recent designs ( primarely on GM vehicles ) employ an EGR valve that is not operated by vacuum at all. An electronic solenoid in the valve is operated electrically by the ECM. Diagnosing this type of EGR valve requires a special scan tool.


G-Body Guru
Mar 14, 2007
Margate, UK>
Good post.

I believe recirculating exhaust gas was first used by Volkswagen, not so much for emission control but to keep peak temperature spikes down, important for an aluminium (or aluminum for you stateside guys :) ) air cooled engine.

It was almost unheard of over here until our emissions regulations came into line with North America, maybe because we routinely used higher octane gas, or possibly because we had much smaller engines as a rule - 2 liters or larger was considered a big engine over here.

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