I have a 98 kia sportage. It will crank but soon as I press the gas it will cut off but soon as I let off the gas it'll stay idled? The fuel filter has been changed. I'm hoping its not the timing belt because I know that can get expensive, Could it be the fuel pump or something else? I have also changed the air filter and it was pretty dirty and that helped it idle better. Any sugestions?

Try replacing the spark plugs with the same ones that came with the vehicle. After that, check for codes--the Camshaft Position Sensor and/or Crankshaft Position Sensor may be failing.

Check fuel pressure with gauge and compare to factory spec for a start

Excellent advice above as usual, I would also check your Exhaust and see if it is dented/damaged restricting flow.
New Air filter was a good thing.

Okay thanks for the advice... I will try all these things:)