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First!!!!
Carbon buildup in the EGR system WONT WONT WONT cause a driveability problem.. well, assuming you mean carbon buildup as in a clog.
A clogged EGR will only throw a code/ cause high NOx emissions.
To understand how this works, know how the EGR works. When you cruise the EGR lets in exhaust gasses, which ARENT burnable. These gasses replace oxygen, which in effect richens the mixture. This causes lower combustion chamber temps, which lowers your NOx emissions. EGR only opens at low load cruise situations... if it's stuck open at idle all that exhaust gas will flow in and the car won't get enough oxygen to run.
Next, the reason it worked when he hit the wrench is because it was stuck open, and when he hit it it freed up so it could close again.. and then probably worked for a little while before it got jammed again.
GM's had LOOOOOOADS of trouble with this EGR design... it's crap.
You can look up the wiring diagram, backprobe the position sensor signal wire (I think middle one on the 5wire harness)... should see .5V at idle, and rev it and see it go up at some point when the EGR opens.
Chances are you'll see like 2.5V or something.. that's be around half open. If it's much over .6 - .7 it's stuck too far.
If you don't like testing electronics do this... take off the EGR, plug the hole that goes to the intake, and start the car up. Does it run now? If it does the EGR is your problem.
If it indeed is stuck.. take it off... you can TRY cleaning it... but would probably be best to just replace it.
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