I have a 2001 impala with the 3.8. I've recently been experiencing a problem I can't figure out and after researching on the Internet, troubleshooting with a Haynes manual and asking multiple mechanics their opinion I am clueless. I've tried jus about everything short of replacing the PCM. The problem is I go out and start it and everything works jus fine. After about 20 seconds it starts surging really bad for about 30 seconds. Then it runs fine again for about 30 seconds and then it dies. If I try to start it after that I have to keep my foot on the gas to keep it running. So far I put new spark plugs, spark plug wires, fuel filter, fuel pressure regulator, MAP sensor, MAF sensor, IAC valve, PCV valve, throttle position sensor, I checked the fuel pressure, tested the fuel injectors, replaced the coolant temp sensor, hollowed out the catalytic converter, checked for vacuum leaks, checked all the fuses, checked all my grounds, tested the fuel pump, ran some seafoam through it. I have no clue what else it could be. Any thoughts?

It seems like its starving for fuel. Open the gas cap next time it does it and see what happens.

you check for vacuum leaks with carb cleaner? clean iac tb seat?

Also, it'* rare, but often the fuel system builds the right amount of physical pressure (psi), but doesn't supply enough volume (gal per min). I'm not sure how to check this other than to energize the pump for exactly a minute, pumping fuel from the tank into a jug and measuring how much gas actually flows.

But I concur with Mike, it sounds like it could be a venting problem with the tank. I'd make sure the vapor recovery line isn't plugged, it'* possible that while driving your tank gets under negative pressure because air isn't replacing the fuel removed from the tank. The working theory is a loosened gas cap, which will probably set your SES light, will allow the tank to equalize.

As always, keep us posted! And good luck!