I have been chasing a P0401 code on my 1995 Riviera for 9 months. I have replaced the MAF sensor, EGR, EGR connector, PCV, Vacuum Harness, and Smoke-Checked for vacuum leaks several times. I have plenty of vacuum at the EGR mount, so I'm certain the EGR path is not blocked. (I have not removed the Throttle Body to clean the ports.) Nonetheless, the code returns every 80 to 200 miles, with no accompanying codes. Using premium gas seems to get me longer periods between codes. I have read everything on the internet about the problem. Anyway, I would appreciate any thoughts on what to try next. Regards.