the ecu will open or close a vacuum valve that goes to that egr valve, and the way to test a diesel egr is to squash or remove that vacuum pipe at idle and the maf reading will roughly double (grams/sec or whatever) because diesels run about 50% egr at idle and low load
petrol egr,s however run just 20% egr and only at higher loads
petrol egr,s however run just 20% egr and only at higher loads