that via the PRVR solenoid (on driver side of car). However someone may have deleted that system along the way to simplify the engine bay. This explains it: https://z32.wikispaces.com/PRVR+Solenoid
You'll need to inspect further...if still on the car the valve may have gone bad or one of the hoses coonected to it could be cracked?
Many have reported no issues in removing but there could be certain circumstances where an issue is seen? Others here may have other suggestions.