Pontiac Repair: gas problem, grand prix gtp, 97 grand prix gtp

Question
Hello I bought a 97 grand prix gtp 3.8 supercharged about a month ago. I only had it for 5 hours then it broke. A mech said a seal broke and it needed to be replaced,we did that then he said that a piston a a hole in it and the motor was dead. So we bought a used 2003 motor it worked. Now the car turns on but turns off 3 secounds later. The mechanic said we now need a new fuel pump. My dad said the fuel pump is working. I'm confused I dont know what to do. Another mechanic said it could be the "fly gasket". I do know that fuel is getting to the motor.

Answer
It sound like a problem in the theft deterant system. It will start and run for about 3 seconds and then the computer will shut down the fuel pump and/or the fuel injectors. That is how the system works to stop someone from stealing your car. You need to tow it to the dealer and have it checked out by someone who knows what they are doing.
Also I have no idea what a fly gasket is.