04 CR 5.9 injectors possibly going bad need help
 

this is my dads DD and he has been detecting a slight miss and has had a serious mpg loss over the past few days. He feels it is an injector leak but i know from the first gen forum that you guys are a great help. I hooked a modis up to it and disabled the injectors one at a time and the engine responded the same way to each of them, which leads me to believe the culprit hides elsewhere. Sorry for the long post but we need to fix it asap. We are replacing the air and fuel filter, is there anything to replacing the filter or is it just pop the cover off and replace it? Any suggestions or help is greatly appreciated! thanks in advance