i have a 98 silverado 2wd 4.3 vortec auto and its not running

it seems when i turn the key sometimes i hear the fuel pump come on and sometimes i dont.

i put a new fuel pump relay and it still the same.

but just to test it i can spray some starting fluid in it and it fires right up then dies right after.

can it be the fuel pump even though i hear it kick on sometimes?

 

Yes. You can have someone turn the key to attempt to start the vehicle and while they are cranking vehicle you can bang on gas tank with mallet or dead blow hammer. If this causes your truck to fire up I'd suspect you have a fuel pump problem.