Mine did the same thing. It was the throttlebody leaking from a crack that I couldn't see. I found out by filling the radiator...then running the car hot (full AC) and then watching for leaks...if you don't see it while the engine is hot, check as soon as you turn it off. Mine used to puddle a bit right under the throttlebody (telltale white residue). If it turns out to be the throttlebody--DO NOT have Nissan fix it unless you are under warranty--they will charge around $700. I got a used throttlebody on EBay for $150 and installed it myself (takes 30 minutes to do the job). If you want to email me you can.