Investigating a very unbalanced idle, I checked compression on a newly rebuilt motor, then started ringing out injector and ignition coil wires. The real culprit was that I found the #5 ignition harness was connected to the narrowband O2 sensor and vice versa ( I had a colleague helping on connecting these). When pulling the plug I noticed it was varnished over, indicating some problem there. I swapped the connectors over and reinstalled the plug and it runs MUCH better (it still misses a little at idle but that's probably from the fouled plug). The question is, could I have damaged the O2 sensor or the PTU from this mistake?
The real strange part here is that cylinder 5 WAS firing before, as in a power balance test from pulling the injector wire lowered the idle.
