Okay so we swapped over both internal springs from the old valve to the new one - it has made no difference.

Also adjusted the TPS up to read 0.34v closed, 4.5v wide open. Again, no difference.

Still an erratic idle, still hunts and still revs itself with the idle control valve plugged in.

Something bothering me is why there is virtually no difference with the idle bypass screw almost for the entire length of travel until the very extreme ends. Why did the previous owner of the engine have the idle screw siliconed up?

Beyond replacing the idle bypass screw and a whole set of new engine sensors (all of which where new when I built the engine up anyway) I've got no idea where to go from now.