C182 Inspection Authorized Technician reported had a loss of engine power during a test flight. They discovered the coupler from the inter-cooler outlet joint they repaired prior to flight had slid apart.

Narrative

I was accomplishing a pre-buy inspection on an aircraft at ZZZ; the aircraft inspection panels and cowl were opened up to insect aircraft for purchase and the engine washed to accomplish inspection. The aircraft was in good condition with the exception of minor discrepancies. One of the discrepancies was the coupler from the intercooler outlet to intake piping joint with silicone tubing was slightly loose and covered in oil which seemed to be a factor. I disassembled the clamps and tube; cleaned all the parts with mineral spirits and acetone to remove the oil accumulation and reassembled the tubing connection then retorqued the clamps to specifications. After work was accomplished the joint seemed much more secure and I made a logbook entry for work performed. After work was performed I was also providing instruction and ferry with the aircraft owner to get him proficient in flying the new aircraft. While enroute to ZZZ1 the engine experienced a loss of power; [priority handling was requested] and the aircraft landed safely at ZZZ2. Upon inspection on the ground; it was discovered that the intercooler outlet joint had slid apart causing the loss of normalized air to the engine. The joint was again cleaned and reassembled with a new Cessna clamp and safetied; witness marks are made to easily see if the joint is slipping. The engine was test run successfully then the aircraft was test flown successfully with subsequent flights and reinspections of the joint accomplished. Contributing factors included oil inside the charge piping; cleaning of the turbocharger oil outlet check valve was accomplished and seemed to remedy the oil in the subsequent engine runs and flights after incident. also; the air inlet charge piping was not designed with any mechanical connection to the silicone tube and clamp; the connections were smooth round aluminum pipe. If the manufacturer had designed a bead/barb to the ends of the pipe to provide a positive mechanical connection with the clamps the pipe connection would be made 'failsafe' and the situation would not have occurred.