i believe i have the same problem as u, my car tends to emit grey smoke, but when driving it is completely fine.
Since it is an old engine, there may be carbon build-up in the engine and may take a while for the engine to break it down? i might be wrong. i'm not an expert. just giving my suggestion.
I only believe this because when i took my car to get smog checked recently, it failed two times before it passed the third time, and only because the guy drove it around a while, to "break up the carbon in the engine" as what the guy said. After he did that, it passed by a reasonable margin.