I recently had a tuneup that served as a tunedown: spark plugs + wires, fuel filter, pcv valve all replaced. It started running a little rough during breaking and idle. In addition, there was a faint fuel (oil?) Smell which entered through the passenger ventilation after idling for a while.
So I took it back. It had a code 301 - misfiring cylinder 1. The diagnostics reported an occasional (< 1%) misfire on some of the other cylinders, but there were no other codes. The repair shop replaced the plugs and wires. The faulty cylinder 1 seemed fixed but the rare misfires were still there.
But I have noticed that the fuel smell is still present. What is the most likely cause?