I have a 2013 Hyundai Accent with 250K miles. In the past year the MPG dropped from 38 to 35. I see that the STFT at idle is about 8-9 and at highway speed stays around 0. This indicates a vacuum leak. I can't find one but while looking for one I pulled my PCV hose off and noticed the STFT went to and stayed around zero. This is the opposite that I would expect. Is this because the ECM noticed the massive vacuum leak and stopped trying to compensate? I know the mileage isn't bad but if there's a way to get more I'll be happy to fix it.