Two weeks ago my car started to have a bit of a rough idle right when I start the car up, but after a few minutes it'd be perfectly fine. I searched online and found that it's quite common, not a big deal. Then a few days later, as I was parking, the car started to shudder and it almost stalled. I put the car in neutral, the the RPMs started dropping to the point where it was going to stall had I not revved the engine a little to try to stabilize it. I could feel the vibration throughout the steering wheel and seat, it was very pronounced. The CEL light eventually came on. I had planned on taking the car into the mechanics to get it fixed over the weekend. That same night, I had to go somewhere so I figured I'll just have to deal with the CEL light and hope for the best. I filled up my tank and suddenly everything was better (it still idles a little rough after sitting for a while). No more almost stalling and the CEL is gone. I think the acceleration isn't as fast as before, but it might just be in my head.

I of course searched again to find answers and it seems like it's probably just a cause of bad gas. But my question is why did the symptoms start happening when my tank was half full? Wouldn't I start having problems the second I filled up that bad "batch" of gas? And do you think it's worth having a mechanic check it out? Thanks!

 

i had something similar which turned out to be one or many of my ignition coils going bad. My mechanic said he's had quite a few 3gs with this issue lately.

 

first thing you should always do is find out what code it threw.
after you know what code, its easier to diagnose and fix the problem.

 

Could be vac leak.. find that TDC and report back. If it was bad gas a plugged primary cat could be probable.

 

Can I still find it if the CEL is no longer lit? I was planning to get it taken care of on the weekend since I'm so busy with work during the weekdays, but the CEL disappeared after refilling my thank.

 

Could be an EVAP code then since it disappeared after filling up, not related to stall issue. Usually the CEL code is stored so it's possible to look it up even though it's not registered.

 

Yes, it can be EVAP. I had the same problem like this. CEL came on, then off, then on again, then off again. Already fixed under warranty.