2009 1.8L automatic pontiac vibe starting + shifting issues

[Ginko]

Hi Y'all, I have a 2009 Pontiac vibe with about 177K miles that I've been trying to get back into working order for over a month now and I'm stumped.

Here's the breakdown: about a year ago I first noticed some issues, rough idle, and poor acceleration uphill when I had a heavy load, even had what felt like the transmission slipping once. But the rough idle would sometimes go away, and the poor acceleration would go in and out. A month and a half ago the alternator died, so I replaced it, and then the rough idle issue got worse. I did a long drive soon after replacing the alternator (~500 miles) and during it there was some issues starting where if I didn't give it gas the engine would die. Once I made it back, the damn thing wouldn't start. I could hear the starter cranking, but the engine wouldn't catch.

I've tried a whole bunch of things and I still have some significant issues. First I replaced the fuel pump, then the spark plugs, and then the starter. After replacing the starter I could get it going if I gave it some gas when turning, but the engine will rumble and die if I shift it into drive or reverse without giving it some gas. The car isn't throwing any codes, and I have no idea what's going on, I'm just hoping I don't need to replace the transmission or anything else expensive.

Anyone have any idea what's going on?

[Capt.Vibe]

Probably a bit late relying but if you disconnected the battery the ECM has 'forgotten' it's settings and has reverted back to factory settings. As carbon builds up in your throttle body the ECM makes adjustments to compensate. If you disconnect the battery such as when installing the alternator, those adjustments are forgotten and it reverts back to the specs when it left the factory with a new throttle body and no carbon deposits.

I would clean the throttle body/plate with Toyota Throttle Plate Cleaner. Eventually though the computer will relearn but this can take over 1-2 weeks. I went through this a few years ago when replacing my battery. Ran rough for days and it needed some help with the throttle to not stall on cold start. Eventually it stopped but took a while.