Hi I currently own a 1997 Chevy Lumina and have had it for about a year. It has about 116k on it and I have had numerous problems with the cooling system ever since I got it. I already had the radiator and thermostat replaced, which stopped the car from overheating which is good but it has had a rough idle now for a while. I am wondering what it could be but it seems that it is the dreaded lower intake gasket. There seems to be blackish yellow stains going all around the lower intake manifold so I am assuming that is from the gasket failure. So I think Im gonna buy the Felpro replacement set and get a quote from my mechanic for replacement.
I think this may help fix the rough idle, and I also may need to replace the spark plugs and wires because I am not sure if the previous owner had them done at 100k and have no way of knowing. I posted a Youtube video for anyone who could maybe tell if there is a vacuum leak from the sound:


Also, the car seems to get terrible MPG (like 15-1, any idea what that could be? It is also curious because the owner'* book claims the fuel tank can hold 16.1 gal but everytime I fill it up from near empty it only gets about 10 gal in, what happened to the other 6 gallons?

For the fuel tank, do you no if the car has an anti-siphon ball? If so it may be stuck. I have a similar problem on some of my previous cars.

I'm not sure if it has that actually! Where would I look to see if it has one?

Generally inside the filler neck or where the filler neck meets the fuel tank.