Hi, I have a 2004 Optima 4-cyl with 160k on it. When we got it, it was idling rough, with arcing on one of the ignition coils, but no CEL. Once we replaced both coils, it smoothed out a little, but was still pretty rough, to the point where the car was shaking and surging, and the idle would fluctuate a few hundred rpm.
The CEL then came on, and showed a P0401 “Insufficient EGR Flow”. We went and replaced the EGR valve immediately, then reset the CEL and reset the ECU.
Afterwards, the car runs much smoother, almost bearable, the idle is stable, no fluctuation, but it is still idling rough, and the car shakes a little, compared to before it is much better.
Any ideas on how to make the car idle smoothly again?
Thanks in advance!