update 2000 windstar stalling

thanks for suggestions, heres what i did: replaced the iac valve and the fuel pump relay and went through the relearning process and still the same problem and the engine seems to be surging or studdering when stepping on the gas so i went to auto zone and had the trouble codes read and they are po172 and po175 and talking to the guy who took them this seems to be a genralized codes which still leaves me searching for answers