The EGR vacuum modulator is a component found on some EGR systems. It controls the flow of the EGR system by using a vacuum operated diaphragm to increase or decrease EGR flow. When the vacuum is high, combustion temperatures and airflow are naturally lower so the diaphragm will close and restrict flow, when vacuum is low airflow and combustion temperatures are higher, so EGR flow must be increased.

For vehicles equipped with them, the EGR vacuum modulator is one of the most important components of the EGR system. Any issues with it can cause the system to malfunction, which can lead to increased emissions and other problems. Usually a bad or failing EGR vacuum modulator will produce a few symptoms that can alert the driver of a potential problem that should be serviced.

1. Engine performance issues

One of the first problems of a potential issue with the EGR vacuum modulator is engine performance issues. If the EGR vacuum modulator diaphragm tears or leaks it will disable the EGR system and may cause a vacuum leak. An excessively large vacuum leak can result in engine performance issues such as a decrease in power, acceleration, and fuel economy.

2. Check Engine Light comes on

Another symptom of a potential problem with the EGR vacuum modulator is an illuminated Check Engine Light. If the computer detects a problem with the EGR vacuum modulator signal or circuit it will set off the Check Engine Light to alert the driver of an issue. The Check Engine Light can also be activated for a variety of other reasons, so having the vehicle scanned for trouble codes is highly recommended.

3. Failed emissions test

Another symptom of a potential problem with the EGR vacuum modulator is a failed emissions test. Sometimes the EGR vacuum modulator can fail, and not set off the Check Engine Light. This will result in increased NOx emissions, which can cause the vehicle to fail an emissions test if the levels climb too high. This is particularly important for states that require emissions testing for all of their registered vehicles.

While the EGR systems on newer vehicles are being controlled by a wide variety of other methods, EGR vacuum modulators are still commonly found on many road going cars and trucks. They serve an important purpose in the overall functionality of the EGR system and when they fail can cause an increase in emissions. If you suspect that your vehicle’s EGR vacuum modulator may be having an issue, have the vehicle inspected by a professional technician, such as one from YourMechanic to determine if the component should be replaced.

This article originally appeared on as Symptoms of a Bad or Failing EGR Vacuum Modulator and was authored by Ed Ruelas.

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