Positive crankcase ventilation

EGR usually is sucked SHUT when engine vacuum is high (idle) and will open against a spring at anything off idle. then at WOT when vacuum is very low, it will shut again via another spring so you wont even notice its working. Trouble is our cammed up motors generate less idle vacuum than a stock camshaft so it fools the EGR into opening at idle and casuing a diluted crappy idle. Adjustable units will have the ability to set the idle vaccum leves as its baseline and then work off that. EGR is similar that it opens off idle. when they go bad (EGR valves) they tend to either open at idle (vacuum leak symptoms) or not open at all causing high NOx levels.