I live just down the road from a bus garage (South Croydon) and catch a
bus pretty much every day. I quite often see (and sometimes get on)
buses which have just come from the garage that are giving off a pretty
large amount of blue smokey exhaust. There is usually enough smoke that
the air behind the bus is turned hazy for a short while afterwards, and
that the smoke also gets into the passenger compartment. These are
double-decker buses that are no more than a couple of years old. Afraid
I haven't paid any attention to what model of bus they are.

What I was wondering was whether these buses have some sort of engine
damage, or whether they need to warm up properly. I suspect the latter
because the times it's been worst, the bus has been cold so presumably
making first trip of the day, and also because it seems to have got
better by the end of my journey (around 10 mins later).

Can anyone confirm my suspicion, and if so, are the bus companies
"supposed" to let the buses warm up before sending them out?