914 Smoke from Exhaust

Hi, On a first flight after a prolonged period of no use, A Europa Aircraft fitted with a 914 turbo was undertaking a test flight when half way through the take off run smoke started pouring from the exhaust. The pilot was not aware of this and saw no anomalous indications in the Cockpit. after getting airborne ATC notified the pilot who immediately returned to the airfield and landed safely. The fault was assumed to be oil being burnned in the Turbo due to a restriction in the Turbo Oil return system. The System was stripped and cleaned, although the Turbo sump could not be fully removed as the oil return pipe was ceased on to the sump and could not be removed without potential damage to the oil pipe and/or sump. Compressed air was used to ensure the pipes were clean and the sump was cleaned as best as possible with clean oil and compressed air. The sump in the old style without the filter. 

The System was rebuild and two engine ground runs to full boost power were carried with no sign of any smoke. A further test flight was arranged and the aircraft again ran up to full boost power prior to the flight with no sign of any smoke. However as the aircraft approached the runway Smoke once again re-appeared and the flight was aborted. The engine had been running on the ground for 10-15 minutes without any issues and there were no anomalous engine indications either prior to or after the appearance of smoke. 

Does anyone have any ideas?