I'm a rookie. I'd like to how to check the fuel injector, valve, etc to improve my MPG by myself. ( now it is 15mpg)

Anyone can help?

 

change fuel filter,air filter, check plugs, and if neccesary, use some injector cleaner in the gas tank but be careful, if the system is really dirty expect some driveability problems for a couple tanks of gas

 

Other than poor gas mileage, do you have other problems? bucking/stalling/lag, poor idle, smoke, etc?

if you haven't replaced your oxygen sensor yet, it's a very good chance it's dead. they usually "only" last about 100k miles, and most 3rd gens are over that by now.

Also, engine temp sensor could be dead. I'm gonna investigate this on mine when I get it out of the body shop (hopefully tomorrow! woohooo1!!!!!)
there are actually 2 on your engine.. one controls the temp gauge on your dash, the other sends info to the ECU for engine operations. So if the gauge on your dash still works, this is why.