starting problem

engine turns over and starts fine but only runs for a few seconds then dies. fuel pump or filter? it is a '90 mitsubishi mighty max

could it be an anti-theft device preventing the car from running without the magnetic or whatever keys?

what else is it doing?

sounds like a fuel pump to me, can you hear the fuel pump (arm) whine when you turn the key before it starts??

What you said doesn't explain anything. What about sounds? How does it run (rough or smooth like usual)? I doubt a 90's car has a decent anti-theft system that shuts off the car anyways. You might want to make sure that your vacuum hoses aren't off or aren't torn. If it keeps running and turning off, I would doubt it's a fuel problem. A weak pump would probably give you a hard time starting the second time around.