Reoccurring P0304 Misfire
My 99 SSEI is popping up this code when driving under load after warmed up. About a month ago the light came on, and it had 2 misfire codes and an EGR code. I found a loose plug wire (different cyl), replaced all the spark plugs and did nothing to the EGR. I read that the EGR can cause the misfire code, so I wanted to see if it would come back. It worked well for a few weeks around town, but once warmed up on the highway at speed I can get the light to come on again. It'* only the misfire code though, everytime.
Only happens when going uphill, or passing someone, feels like the car is surging, or lockup torque converter is trying to unlock. Just overall lack of power. If you step it down, you can feel the supercharger kick in, the boost comes up, but the car is fighting it the entire time. Feels like a cylinder dropping, or getting flooded out the entire time you're on it. About now is when the CEL starts flashing and sets the code. Let off, and everything is fine again. The car has no problems when cold and is a joy to drive.
I Don't think the EGR has been touched, the car has 200k on it, and I try to keep up on the routine maintenance. The wires aren't due to be replaced, but nearing the end of their life. Delco wires with NGK copper plugs, and new fuel filter earlier this year. This is my third set of NGK through the car and first misfire issue, so I don't think it'* that. Anyone have a post on cleaning/inspecting an EGR? Seems odd that the EGR could be the culprit without triggering a code though.