2007 Ford focus base model sedan, and I mean the BASE, base model. No cruise or power anything. Just so you know which kind it is lol.

TL;DR: Bad gas mileage, no CEL, can’t change “lifetime air filter”, new plugs, fluids, and filters, and still getting MAYBE 15 mpg combined.

Anyways, I got the car about 2k miles ago or so (135k now) . Put a lot of work into it already. I noticed I wasn’t getting the 25-35 mpg it is reputed for, so I started doing the routine maintenance I was told it had been having it’s whole life. changed the black as night oil, dropped the transmission pan and changed the fluid, swapped the FACTORY “Motorcraft” plugs that were there since 2007. Discovered a spark plug tube halfway full of oil in number 4, so I changed the gaskets. This has a lifetime air filter, so I can’t change it without a several hundred dollar assembly if that’s the problem. No check engine light, I used a scan tool anyways to go through all the specs the car could tell me and it says everything’s normal. But again, it’s the very, very basic base model. The spark plug well filled with oil I could tell was causing a misfire and it still didn’t throw a CEL. I’m now only getting about 180-200 miles a tank. I don’t drive it hard, I don’t drive often, and it’s about 50 percent highway and 50 percent city. What can I do about this? Anyone have any ideas? Could it be MAF, O2, or some other sensor even without a CEL? I just don’t want to throw unnecessary money at it.