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

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?

by in Ashford, CT on December 27, 2011
1 answer 2 comments
ANSWER by on December 27, 2011
171 is just bank 1. i agree, maf would be both 171,174. if fuel pressure was low or pcv, it would throw both. i think you have a vacuum leak linked to bank 1. you need a smoke test to verify the leak. a shop will monitor long term trim to make sure it is only bank 1. you have narrowed enough at this point. now take all this good info to a shop and it will help with the diag process. Roy
COMMENT by on December 29, 2011
Now it has 174 as pending. What is you estimate of the cost and time have a shop diagnose the cause of this trouble (not cost to repair)? Could this be caused by a loose gas cap some how letting too much air into the fuel system or does that have its own code (seems unlikely since one would expect the pump to always be submerged unless it is also designed to take fumes from the tank into the injection system.)
COMMENT by on January 01, 2012
1.2 is for the diag only, no repairs. these will be additional. Roy

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