2004 Ford Taurus Question: What is the most effective process to find the cause of a P0171 fault?

I bought this car used about 2 weeks ago from a private party. After driving it for I while I have noticed that occasionally it finds the P0171 code when starting. I estimate it finds the code in about 10 to 20% of cold starts. It never finds the p0174 code or any other code. Occasionally while cranking during a cold start there is a knock or a popping sound. There is never a knock or rough running at any other time. There may be a slight lag in acceleration from slow speeds. Fuel consumption seems to be elevated at about 19 mpg of mostly highway driving. The car has about 115k miles on it. I am interested in what is the most effective set of steps to resolve this problem. I think if the problem was the MAF sensor or a vacuum leak, etc, it would find both 171 and 174, at least some of the time. My OBD-II scanner can not read real time engine data (is it worth investing in a more capable scanner or should I take the car to a mechanic?). It can only read and erase stored codes. Will driving the car without attending to this problem cause further damage? Where should I start in troubleshooting?