I am a new owner of a 1986 Buick GN. I have a question about the EGR system. My service engine light comes on. After a scan it shows the egr system is at fault. My egr does test fine with vacuum or movement. Should I have vacuum at this valve at idle, it seems to right now? It runs rough when the vacuum line is hooked up to it. Once the hose is disconnected it idles smooth. What should I check next?