engine idle

Toyota as typical was very late to the game with fuel injection on the Corolla. This generation car tended to have a carbuertor. I am assuming you have the standard 1.8L engine with 138 hp that was carbuerated. If that is the case, it might be as simple as the air filter needs to be replaced. I am betting it has to do with the carbuerator. My first car was a used 1986 Toyota Corolla that was also carbuerator, and it was much worse as in cold weather the choke did not work well and it wanted to stall and hesitated badly until it warmed up. In that case, I had to rebuild the carbuerator and it was fine then.

With the minor conditions you are talking about (I am assuming this occurs nearly all the time) the air filter cleaning probabily will do it. If not, then it is something else regarding the carbuertor almost guaranteed. Good luck.

The Corolla was carburated until 1992. My neighbor has a 2005 Corolla and he works on it all the time for fun. I hear his A/C compressor turn on and off making the engine rev. This happened in my wife's Odyssey for several years. The A/C still worked well well. It stopped doing it about 5 years ago. There would be a ticking noise every second and the engine would rev about 200 RPMs. It wasn't a big deal. It is hard to figure out when the last car with a carburetor was sold, but I think it was the 1994 Isuzu Pickup according to Autotrader.

Thanks LectroFuel, I was wrong about this as I got mixed up on the year - the sedan was carburated until 1998 - so I was completely off.

Could be a vacuum leak, faulty idle air control valve, or faulty O2 sensor. No easy way to tell without using diagnostic equipment. Can get expensive to randomly replace the idle valve and O2 sensors without knowing the cause.

With my 2009 Corolla, I find identifiable engine idle speed variations occur because of A/C cycling (as LectroFuel said), electrical load, and battery charge state. The car's computer controls idle for many variables beside these, including pollution control, so the variation in idle speed is likely normal. (I assume the engine malfunction warning light is not on, of course!)