Have the main loom plugs been sorted (under passenger side bonnet vent). I had something similar on my old EsCos prior to having the wires sorted - basically moving the wiring slightly changed the temp' value sensed and altered fuelling.

Was diangosed using a Pectel monitor, although the corrosion was plain to see in the loom plugs, once taken apart.

Not too sure on the L6, but ECU possibly reverts to a safe value when a sensor is unplugged (I know P8 has a limited operation strategy LOS when a sensor goes down)