1999 Ford Mustang Engine idles rough! Codes P0171 and P0174

Fuel trim codes in both banks with a rough idle indicates you have a bad vacuum leak. The easiest way to find the leak is by smoking the intake system. But from my little experiance I have, I would be looking for a collapsed PCV Hose. Look for the PCV Valve on your vehicle. Looking at the emissions sticker under the hood will tell you where the location is. Once you find the PCV Valve, inspect the hose and feel it. While the vehicle is idling wiggle the vacuum hose and you should hear the hiss get loader or go away. If you do you have found the problem. Replace the hose with the valve. This will be most likely a dealer item. Good luck and let me know what you find with the hose.