Okay, I thought I'd resolved this recently, but had an old problem crop back up, so I'm hoping someone here might have some ideas... I have a 1996 Buick Century Custom 3.1L V6 with about 93k miles. As background, I've done quite a bit of work to the car in the last month and a half - replaced the engine with a rebuilt, replaced the catalytic converter (that was damaged when the old engine misfired badly), replaced my ignition control module and two coils, and replaced my fuel injectors as a couple weren't firing well. In addition, I replaced my EGR valve to resolve a P1406 (EGR Valve Pintle Position Sensor) code that came up last month (the original EGR valve looked to have quite a bit of carbon buildup, and was likely original to the car from 1996). That replacement was done about 9/25, and my check engine light came on again this week. Now it's showing a P1406 (EGR Valve Pintle Position Sensor) code, and occasionally (maybe every 2-3 starts) will also have a P0300 random intermittent misfire and P1406 code shown as pending. In terms of running/performance, if I leave the car in park and give it gas once I get the pedal to about 1/2 down, around about 1800RPM, it sounds almost like it's coughing, and that's also the case if I leave the car running and go listen/feel at the tailpipe, the exhaust seems to pulse rather than being just a steady stream. Performance wise on the road I haven't noticed any major problems - the car seems to run alright, I haven't noticed any serious misfires or performance problems, seems to have good acceleration, etc.
The one thing I will mention is when the catalytic converter was replaced the shop mentioned that my O2 sensors may need to be replaced as well down the road, as it's possible that they were damaged when the old catalytic converter was. When I checked the live data on my scan tool I do see what seem to be good values from both O2 sensors (
I'd love to hear any ideas on the next steps for troubleshooting or thoughts of what this could be. I'm not an expert DIY'er, but do my own oil changes and brake jobs, and did manage to replace the EGR valve and one of my coils myself, and I also have a fairly good scan tool (that shows live and freeze frame data) so I've got some knowledge/experience. I'd greatly appreciate anybody's thoughts if you have ideas.