SURGING FINALLY SOLVED!!!!!
After getting to the point of having very few remaining options, and convincing myself it was an ECU issue - I went back through the listing of ECU inputs that I found in the schematic below (I included the full extract in a previous post).
Using Techstream I re-checked that each of these items was providing stable inputs and reasonably relevant measurements. Except I couldn’t test crank angle or cylinder recognition - largely assumed these to be fairly major inputs that would definitely throw multiple codes and would cause more significant symptoms.
View attachment 322941
All the readings seemed spot on. And I was really only left with the intake airflow. Readings from the sensor have been hard to monitor properly as there is constant fluctuations. I’d previously discounted this sensor because it gets cleaned frequently and I guess the logic had been that if the sensor was faulty, then the car would have problems running across all rev ranges.
So as a last resort I tracked a second hand MAF sensor down from the wreckers ($50) and plugged it in.
BLOODY STOKED!! Car now runs perfectly.
TONYC11 - from the uk forum nailed it with his inputs. Bloody genius who gave me the push I needed to go back over these inputs and to not discount the MAF sensor.
@Fry1988 - really glad you got the solve on this one too. Does somewhat show that the MAF can still largely function, albeit with dead bands that affect specific rpm ranges. Mine was cleaned frequently, but just no longer functioning in specific ranges.
And - a really big thanks to everyone else on the site who chimed in in this thread to help - particularly
@Tractionman @karl2000 , to name a few.