My Girl has a 99 Grand Am GT 3.4. It was sputtering a little bit on the freeway for about a month or two, but it wasn't a really big problem. It would always seem to idle funny at start up as well. The idle would be normal, then dip down, then normal, then down, etc.

Recently the problem got worse. The idle is surging. Ranging from around 300 rpm to about 1000 rpm. It surges bad and usually stalls. The lights inside the car dim a little with the low idle and get bright as it revs as well. She can hit the gas and it will run and she has to hurry up and shift it into gear while on the gas so it doesn't stall. I have replaced the plugs, air filter, and fuel filter about 2 weeks ago. None of that seemed to make a difference. The car isn't displaying any problem codes at all. No check engine light on or anything! I wish it was cause that would make it easier to figure out the problem. Is it possible to take it to autozone or something and have a problem come up with no check engine light on?

Any suggestions?

Check the IAC valve for carbon and or fuel deposits. The IAC valve is small stepper motor that adjusts air at idle and part throttle. Take if off and clean the tip as well as the bore it rides in on the throttle body. If that doesn't fix it, check the EGR valve, as it sounds like something is stuck open when it should be closed or vise/versa messing up your air/fuel ratio.