I have a 1990 GT 5.0 I replaced FP relay under driver seat and in the engine compartment. cleaned MAS and KN filter. Dont see any gas leaks. It sounds like an electrical problem and I have been working on the alarm and car stereo wiring so i dont kno if that has anything to do with it. The car starts and it runs great for about 15 seconds then lights come on and off on dash, idle is erratic, fuel pump comes on and stays on (u can hear it), and I hear electrical noise in engine compartment, then shortly after that it dies. Do i need a new fuel pump or is it an electrical problem?

Thank you

 

Sounds like a voltage problem. Check your voltage at the battery and at the alternator, both running and off.

12.5~ while off
14.4~ running

Check the cables for corrosion and battery acid.