01 5.7L cranks and fires but wont run.

I have changed the fuel filter and pump. It will crank and fire. It will run for about two seconds then dies. Doesn't spit and sputter just dies. It has an aftermarket security system on it. I think it is something electrical maybe something in that security system but not sure. And if you touch the throttle while it fires it dies faster that if you don't. Any ideas?

b simpson 6, it could be the security system malfunction. Check engine light on? Could be a sensor but they usually set a code when bad. Have you checked fuel pressure or for spark after it dies?

My brother in law put it on the computer and there were no codes. He could keep it running for a little bit with carb cleaner in throttle body. So it still has fire. He said fuel pump.

Put a fuel pressure gauge on it just to confirm you have pressure.

What kind of pump did you put in?

Ive got an update. It was the Passlock sensor. However the aftermarket security system wouldn't allow the computer to read the code, that's what I was told by the chevy dealer. They removed that security system and it showed the code. They replaced the sensor and all is good. Thanks to everyone for the suggestions.