1990 Toyota Celica my car wont stay on for than a few secon

i have that 1990 Celica i just replaced the idle air control valve and still the same i replaced the spark plug it ran longer that a few puff like it was backfiring or something then turned off. now i turn it on and immediatly turns off again. a few days ago i turned it on it was fine the only time it gave me prob was when the engine was cold but when its on and warm the engine runs great. please help me