You say the chief mechanic refuses to remove the engine? This sounds very strange to me. I would think it's a simple as following the published procedures in the engine and/or airframe maintenance manuals. It's an If-Then type logic. If it fails, perform engine wash and try again. If it still fails, remove for servicing. Plain and simple.
Based on our engineering discussions with Turbomeca, their engines are NOT certified for below spec power operations. As a matter of fact, only just recently were they asked it be added to an engine TC (Arriel 2S2). They told us that currently the DGAC will ONLY approve below spec operations on a case-by-case basis, and then ONLY so aircraft could be ferried to a maintenance facility.
As an engineer, I would want written approval from the engine AND airframe manufacturer clearly approving a flight (WAT) envelope. Including all limitations. (Good luck getting this!)
As a pilot, I would insist on some sort of paper from the cognizant local regulatory agency clearly allowing this operation. A iron clad CYA, if you will. (And even MORE good luck on getting a gov't employee to risk his neck on this one!)
Ask this Maintenance person for the approval paper trail. After all, it's your rear in the seat!
HOSS-1