Okay, in a custom modpack I connected a creative (RF) energy cell from TE4 to a buildcraft quarry using rednet energy cable (REC) and it powers the quarry, no problems there. The bug comes from when you break the REC and the energy cell the quarry now has infinite power and won't change that power status until you break the quarry. Ran across this unintentionally and thankfully it was in a creative test world just trying to find out the cheapest method of transmutating RF into BC MJ power.

Buildcraft no longer uses MJ. It uses RF now, but that aside - have you tried with other energy cells / RF energy sources?

bug in ((MFR)) RC4, which is why there's an RC5