THE ABCs of EGR Part II

In part one of this feature on the EGR system, we covered both the development and the basic theory behind its operation. Now we'll get into some important information you'll need to know in order to identify and service your car's vacuum-operated EGR system. (As noted previously, electronic systems are another matter better left to a service pro.)

Variations
There are a number of variations to the vacuum-operated theme. They all try to accomplish accurate metering of exhaust gas, regardless of their configuration. Here's the breakdown:


Non-Backpressure Controlled: Early systems used this method where the amount the valve opened was controlled solely on the strength of the vacuum signal applied to it. A few manufacturers improved on it by hyper-accurately controlling the vacuum signal to the valve, or adding an additional throttle-actuated mechanism in the intake passage.


Backpressure Controlled/Externally Modulated: In this system, a modulator controls the strength of the vacuum signal to the valve. The modulator has a port connected to the exhaust system and, depending on the amount of backpressure present in the system (more during acceleration), the appropriate amount of vacuum is applied to the valve.


Backpressure Controlled/Internally Modulated: In this system, instead of exhaust backpressure being used to control the vacuum signal to the valve, it is used to provide additional lift to the valve. This is accomplished via an internal exhaust passage and special valve design.