I've had this problem for a few months but seems to be worse now. If the car sits at idle for a few minutes, it will start spewing white smoke for a couple of seconds if I rev the engine (fuel not being burned completely). I've also notice lately that the car now idles at 500 rpms when before it idled at 700 rpms. Gas milage has decreased but not significantly. I've had no SES lights for this (yet). I'm looking for some insight on why this is happening. Could it be the O2 sensors or a bad cat? Other than this the car seems to run fine. Mileage is just over 79k.

I just recently had my car in the shop to fix secondary air valve failure and replace timing and valve cover gaskets.

 

I would guess you're probably looking in the right area (cats, o2 sensors). I removed both of my catalytic converters for slightly better sound/performance and encountered the same thing a month or two later- white smoke pouring out from the exhaust after idling for about 5-7 minutes (the car was already warm before I started idling the car for 5-7 minutes). Only happened once for me, and I turned the car off immediately to have it checked out. In my case it was a "faulty" o2 sensor or simulator.

 

your probably burnin oil, leaking coolant into the chambers or possibly the throttle body needs to be replaced. thats what the "stealership" told me. 1800$ fix. wheres is that secondary air valve located?

 

mine had a rough idle at a cold start...then spewed out thick white smoke....turned out to be the ccv