Background: I inherited my granddad's 1994 Buick Lesabre Custom, found that the battery was totally dead after sitting in the yard for a few months (changed it, more on that in a bit). He had a special mechanic, who I'm not sure I trust and is about a county-and-a-half away anyway. I'm also unemployed, so it's on me to learn how to fix it myself. If I begin to mention selling or trading it, it will be traded in with my mom's (working) van for a new van for her, for some reason. Changed the battery, which only introduced more problems...

Problem: When I start it, it revs in a pulsing, rhythmic matter, which I'm pretty sure it didn't do before. It also revs when the brake is applied. The service engine light not only turns on, it flickers, along with a few other notification lights. I also noticed some steam and/or smoke coming from around maybe the transmission. I have no idea if this is just dust shaking/burning off, or what.

It has a nearly full tank of gas, though the tank was filled a few months ago.

The AC was not running, because it has not worked for years.

I think the battery is no longer the issue and that I installed it correctly, but on the off chance it is the cause of my troubles: The battery post is not a part of the battery, but threads through the solid terminal (looks like a washer) and screws into the battery. On the positive end, there's two terminals (yes, two, it's not the negative) and two washers (one thin, one thick). When uninstalling the battery, I made a mistake and forgot to make a note of the order things go in on the positive battery. As far as I can tell, it's the first terminal connecting to the battery, the two washers in either order, the second terminal, and then thread the post through into the battery. I know which terminal is the "first" and which is the "second," because they won't work in any other order. I've tried other configurations, and battary-terminal1-washers-terminal2-post seems to be the only one that works.