Flight/Ground Idle Issues - CF6-80C2B6

This is another "bad from stock" adventure so far... The installed MEC was causing intermittent EEC EICAS messages...  Our Reliability folks wanted it changed due to the MEC having a history of this same fault in its previous life. The first replacement MEC was installed. During high-power runs, the MEC would not make takeoff power and the flight idle was erratic, but the ground idle was stable. The second replacement MEC was installed... This round of high-power runs, the MEC would make takeoff power fine, but again the flight idle is hunting. We adjusted the flight idle screw, no change at all. We rang out all aircraft wiring from breakers to the right engine flight/ground idle pins twice on two shifts, no issues. We decided to take the aircraft out of the equation by depinning the wires for the flight idle solenoid at the MEC connector, then running jumper wires from the MEC to a 28 volt DC power supply outside the engine, then ran the engine. The flight idle still hunts erratically, with ground idle normal with no fluctuations. And now, a second MEC change required. Further updates as the plot thickens...

Photo of the setup:
 


 

To eliminate aircraft wiring as a issue, we ran 28VDC directly to the Flight/Ground Solenoid from a DC Power Supply.

When the engine was running, I was able to change RPM's by either appling or removing power to the solenoid.
  
     

Using an external power supply for troubleshooting is mentioned in AMM 20-80-24.