Will start but only run for 5 sec. Then shuts off

I have a 93 bonneville, it has 163,000 miles on it.
its got a 3.8L in it.
I went out today and I was going to replace the muffler with a glass pack.
So I cut the muffler off, then I started the car and ran it just to hear what it sounded like.
I should off the car and dis connected the battery positive and negative. I placed the glass pack and welded in place ( note I had the ground to the welder about 4 inches away from where I was welding.)
I re-connected the battery and went to start it starts up fine but won't stay running.
I checked fuel pressure and the pump is running, I get spark to all the plugs.
I also took off the intake and sprayed starting fluid while it was running for the 5 sec. It still shut off.
I checked all the fuses thats not it.
So I must be loosing spark to the plugs, but I don't know what would stop sending spark.
And if its the computer is there a way I could bypass it?