Could you guys give your opinion on my (probably wrong) theory? So the car has never ever run like it should. I haven’t driven a Buick Century in good condition before, but this one has to be too slow and it also idles roughly and occasionally misfires. It has trouble keeping highway speeds, too, emits gas smells and is bad on gas mileage. It also has a check engine light that my dad hasn’t checked before, but it can’t be good (it’s a very old and weathered car so I guess that is the reasoning). So something’s wrong with the combustion, and the fuel isn’t being totally consumed, right? So what if the air filter is clogged and preventing adequate oxygen from reaching the combustion chambers and burning up the fuel? I have read online that the engine computer should lower the amount of fuel reaching the combustion chambers to compensate for the lack of oxygen, but I don’t know why this car doesn’t do that. Maybe it’s too old to have that technology? But the car did have an additional source of oxygen, which is the vacuum leak, maybe enough to keep it from totally stalling out from unburned fuel. But when I repaired the vacuum leak, it only had the air intake’s supply of oxygen, and stalled out because it is clogged and needs replacement so it isn’t letting as much air in as it should. And when I accidentally worsened the vacuum leak before by fiddling with the old cracked hose and breaking a seal, the car seemed to have more power because it was sucking in more oxygen from the vacuum leak, possibly. It idled extremely roughly because it’s uncontrolled, right? But it seemed to have more acceleration power. So it seemed maybe the cracked PCV hose was its main source of oxygen. Could you guys let me know if this is possible? There are a million possible problems for these symptoms, but considering that the car started doing this after fixing a vacuum leak, maybe it narrows the problem list down. I could try taking out the PCV valve and blocking only portions of the hose with my thumb, as crazy as that sounds, and maybe modulate the amount of oxygen coming in through the vacuum leak to see if it smooths out the idle, but replacing the air filter is something I should do anyway. My grandpa’s mechanic dropped a pencil in the engine bay and just left it there, so maybe he wasn’t the best mechanic. 
But I’d better take it to a real mechanic to see if he could figure it out.