So now you know how the EGR works and how it can be shut off without throwing a code. You also know that the Epsilonian device reduces a low voltage DC current by 0.7 volts when a vacuum is applied to the switch. It's up to you to figure out why you'd want to reduce a DC current by 0.7 volts. It's part of an Epsilonian religious ceremony, but you may have other plans.
Unfortunately, we cannot explain it directly, but let me make a few statements and perhaps you can draw your own conclusions.
Your car is equipped with an EGR system.
The purpose of the EGR system is to reduce NOx emissions during light to moderate load conditions (i.e. most of the time).
A side effect of the EGR system is that your intake manifold will now have a portion of exhaust going through it, which may eventually lead to a loss of performance or other problems due to gradual clogging-up with soot.
A further side effect of the EGR system is that since your cylinder are now compressing exhaust in addition to air, the crankcase blowby will contain an increased amount of soot, which will end up in the oil.
The EGR system operates as follows. The ECU knows how much air is supposed to go into the engine under given load conditions. If the MAF (air flow meter) measures more air than desired, the EGR valve is opened. If the MAF measures less air than desired, the EGR valve is closed.
If the ECU cannot reduce the intake air volume sufficiently, it will throw a "check engine" code.
At idle, the voltage reduction in MAF signal corresponding to "EGR on" versus "EGR off" is 0.7 volts.
The Epsilonian device consists of a vacuum switch and two diodes. When a vacuum signal is present, it reduces the voltage passing through it by 0.7 volts.
Be aware that under extremely cold weather conditions, the EGR system also serves to raise the temperature of the intake air, thus assisting with engine warm-up. In addition, diesel engines depend on heat in the cylinders to ignite the fuel, and under some load conditions during extremely cold weather, the EGR system can make the difference between having a smooth-running engine and having a rough, misfiring engine...