You can try cleaning the EGR valve. This will fix about 70% of them. One problem on the Northstar is there is a metal fuel line tab that shares one of the mounting studs. This turns a 15 minute job of removing the EGR into about an hour and a half. If your EGR is sharing the stud go ahead and obtain the large throttle body o-ring gasket. You will find the throttle body has to be removed in order to access the fuel line. The original o-rings swell up and are imposible to reinstal.A leaking EGR will cause a rough idle.