After replacing pcv valve and related hoses, I experienced a rough idle and eventually a p0171 code. I have a 2000 wagon with zetec engine and read many posts by others on this problem. I finally stumbled across an unused vacuum port on the back of the throttle body above the pcv port. It was sealed with a plastic plug which had broken due to age, heat, etc. I plugged the leak and all is ok again. Hope someone else can find this post helpful.