Mitsubishi Repair Problems? Ask a Mechanic for Answers ASAP
With regard to the MAF sensor, was it a new Mitsubishi (from a dealer) sensor you replaced it with, or an aftermarket (this includes ones bought from places like OReilly claiming it is genuine)?
These vehicles are particularly sensitive about aftermarket sensors and we may have worsened the condition by putting that on there. There is no factory replacement available outside of a Mitsubishi dealer, Mitsubishi will not ship these parts to other retailers, no exception.
Is the old sensor still available to reinstall?
What are your long term and short term fuel trims looking like when idling and while driving?
Well my concern was more that the new MAF may have introduced an issue as we see that a lot. Having had the condition change briefly when you had it replaced, and then it got worse again... that is a strong indicator that the replacement had a problem.I'm not sold on the MAF being the issue, though we do see them fail a good bit. Seeing what the fuel trims say may help steer us toward or away from that though.
If you can return the NAPA one, I would go ahead and get the right one. If you are stuck with the NAPA one, lets see the trims before we make any decision... though if history is any indicator, we will very likely still need to get a stock one back in there.