Hey guys, I thought I'd finally join here after lurking for a while.

I have been having a couple of huge problems. I'm no mechanic but I can replace some stuff (alternator, etc) and I have a few questions.

My biggest one is a problem with my car dying. It used to only happen sometimes when the engine was cold and I'd start it and back out without giving much gas (it would die as soon as I started turning the wheel). But this time it happened when I pulled away from a red light and turned. I coasted into the closest parking lot and started it again. The car would start fine, and then whenever I put it in drive and turned the wheel it would die, but only when I turned the wheel. At first I thought it was something with the power steering, but there are a few variables:

I had just been on like a 30ish mile highway trip. The car had three people, a guitar and an amp in it. It was the first really hot day in a while.

The thing is, we went back and put some gas in it and got it home and it hasn't done it since. It had been sitting for a day and I wish I would've tried to recreate it before I put more gas in because it had like 1/4 of a tank according to the gauge when it was dying. What do you guys think about this? Is it something to do with the power steering pump, the idle air control sensor, or something else? Could being low on gas cause something like this to happen? (my gas gauge floats so who knows)

My second biggest problem is LAUGHABLE gas mileage. I have the 3.1L V6 and I usually get about 12mpg average. I know that I should be getting way more than this. The other thing is that my gas mileage is MUCH worse on the highway. I don't use the AC or anything, seeing as it'd probably drop me down to like 10mpg. What are some things that could be the cause of this? I've recently replaced all the tires, the alternator/serpentine belt, the fuel pump, the brakes, and all of the ignition coils. The air filter is brand new, I pulled a spark plug and it looked brand new. This was my grandparent's car before mine and my grandpa keeps his cars in good shape. Could this be the O2 sensor or a combination of a lot of things? It really makes me mad.

Thanks in advance for your help.