Used to do a water wash on a Twin Otter every evening after the aircraft flew around the Channel Islands all day. Used De-mineralised water, blanked the P3 line and motored the engine for 30 secs. with the water fed through a spray nozzle in the intake.
I remember doing a series of detergent washes to try and recover a PT6 performance. After about 6 washes there was no improvement so I interchanged some instruments between the engines and low and behold the fault transferred to the other engine. I think it was Ng and Tq that both had errors