The EGR does not operate when cold, nor when below 1500 RPM's. If it is stuck open, you will have a problem when cold or below 1500 rpm. Replacing the EGR may solve your problem if it was stuck open. Read the following to better how to understand how your EGR system works.
How the EGR valve works
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 carburetor 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 carburetor 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 carbureted 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 o 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 exhaust backpressure is present, the diaphragm moves up and closes off the control valve, allowing the full vacuum 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 after market 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.