Boeing strongly advises against water or detergent washing of the engine gaspath as this can cause accumulation of foreign material in the engine cooling flow passages.
From
http://www.ifalpa.org/downloads/Leve...h%20Advice.pdf
referenced here:
http://www.pprune.org/5684618-post2753.html
[edit]
Looks like dry motoring is the starting point, but not a lot of otherwise helpful info out there, other than vacuuming up ash wherever it is found.