I'm not an idiot, but I know enough to know I need a mechanic to do a lot of stuff. I'll try to be complete here, but feel free to ask me dumb questions. I won't be offended.
A few months ago we had some accel issues, and took it to a mechanic. Great guy, came well recommended. Plugged in the code scanner for free, found it was "random misfire" and replaced the spark plugs. I do IT work, and their notebook needed a little help, so it was a labor swap. The car ran great after that but started to seem to have a similar but not the same issue with acceleration. This was in late March.
The issue now is that if I gun it, the RPMs will eventually jump up there and it'll pick up while before that wouldn't help (bad plug and all), but between about 1900 and 2500 it's sluggish, and you can hear the engine groaning. The fuel economy is shit now, while if I have it up at high way speeds it'll go back up to 25-30, yet at street speeds it's under 20, between 14 and 19. I had a car-full of people thursday night, and there was barely enough power to move at all in that RPM range.
The kicker is, there are no diagnostic codes being tossed out, they put the code reader on, nada. The mechanics were a bit confused by that too. One suggestion was to try a trans fluid/filter change, but they admitted that might not fix this issue. By the maintenance schedule it could use one soon, but I'd like to hold off for a few months if I can on that, and focus on this issue.
The car is an 08, 4cyl (and previously peppy as heck for a 4cyl), 60k miles, my girlfriend is the only owner, oil change every single time it needs one by the oil life meter. They'd check the air filter and while at the most recent oil change the dealer said the air filter was still good, it was closing in on replacement time, this was in February. I replaced it in March before we diagnosed the spark plug problem thinking maybe the loss of power was the filter being clogged, (but it was the plugs then).
My amateur thoughts were maybe the MAF is malfunctioning in a way that the computer isn't seeing as a failure mode yet? Or maybe with the bad plug in there for a month, fuel-rich exhaust has fouled the catalytic converter? I've seen people with similar sounding problems put in a tank of good gas and a can of Seafoam and it's helped, but I'd rather hear someone say I should try that before I try random things.
So, what info did I leave out that might help, and I appreciate any feedback anyone might have.