I took off the heat shield from the exhaust manifold on my 2003 Nissan Primera while looking for an exhaust leak, and I noticed this yellowish poweder / dust on it:
Any ideas what it is?
EDIT Sept. 19th 2017
So I wiped a section of the yellow powder with my finger and this was the result:
There seems to be another layer of something under the powder which got wiped away at the same time, and the yellow powder itself seems grainy.
I also verified ( as well as I could ) that there is only one exhaust leak which is from the base of the O2 sensor. I stuck a funnel in a length of hose and had my wife block the exhaust with a rag and this was the only place I heard anything, and the change in sound was quite pronounced when she blocked the exhaust.