Based on a post I read here a few days ago about the cost/benefits of switching to premium gas now that prices are higher (for me, only a 1.6 mpg improvement is need to justify the cost), I filled up my latest tank of gas using premium fuel.

The car initially ran no differently that I could tell (still some regular gas to use), but now that I've gotten down to the last 1/4 or so, which I presume is now mostly premium gas, it seems that the car has been idling low. It's an automatic transmission, and I've noticed the problem whenever the car goes into or out of gear--from park to reverse, and reverse to drive. The car actually did stall one afternoon, when I was getting ready to back out of our driveway.

Could the premium gas have anything to do with my car's low idle? I was thinking that maybe, with richer fuel, less gas is being injected into the engine, so little that the car almost stalls. Previously and while on regular gas, I had had problems with a stumbling idle due to a dirty throttle body. Once that was clean, the car ran much better, but it was then idling a little bit higher than before. Over the past year the idle has crept down, but now it seems to be almost too low. I haven't adjusted anything or messed with the throttle body, but my definitely is definitely at a lower idle while switching gears than it was this time last month.

My only other guess as to what could be causing this behavior is that it's been quite humid around here lately. Maybe less oxygen is getting into the engine and so the fuel mixture is being cut back as well?

I'm not going to worry myself too much because the car drives fine otherwise, but I was wondering if anyone else who had switched to premium gas had noticed a change like mine.
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'89 244 GL -- 115,625 miles (see profile for info on car)