So my car has been sitting for a few months. I tried to crank it and it wouldn't fire up. The engine turns and the starter is working but it won't fire up. I got it running once and then as soon as I revved it a little (to try to keep it running) it shut off. I've added a few gallons of gas to it. I've swapped batteries. I've added coolant and engine oil. What else could it be?