Pontiac Repair: 1999 Pontiac Grand Prix SE, throttle position sensor, intake manifold gasket

Question
I have turned to you as a last out.  I have had troubles with my Pontiac Grand Prix SE 3.1 for about a year now. It has been in the garage over 6 times and now I have a new problem.  For starters, the car has new factory wires, plugs, and filters.  The biggest thing here lately is that when I go and add gas at the gas station, and try to start my car it always dies, and then when it starts, it will rev all the way up to 5 and 6k RPMs.  If it doesnt rev up, then it dies with a jump and the TRAC OFF light comes on.  Noone is able to give me an idea what this is.  In addition, it is idling like a harley (with a real rumble).  This is only one of several problems but this one really worries me.  Any suggestions.  I had the dealership check and see if the injectors were working on the computer and they said yes, but noone seems to know the problem.  Thanks

Answer
Hi Karen:

I think you may have 2 issues here. First off the stall after filling up the tank ??? It could have a bad pressure sensor in the tank and it is affecting other sensors that are tied into that signal feed to the tank sensor... Has this car has any major engine work done like an Intake manifold gasket repait? If so the hoses for the evap system may be swapped in the control valve for the evap system and when car is filled up the vapors from filling the tank age backing up into the engine. That vavle could be sticking up partially open as well. This could also cause a wierd idle as well. You could have a bad throttle position sensor on this car where for some reason the Idle position "voltage" is changing every time the gas pedal is released and the computer sees an idle input in % that would give you a wierd idle or a high idle. Has any service engine soon light come on? If so it would give a place to start. Good luck :}