I see now that you're in AUS.
Is your truck OBD1 or OBD2?
What year?
The EGR (if equipped) is on the intake manifold back by the firewall. The wiring harness comes down from the firewall, right next to it, down to the injectors.
Yes, we need pictures and videos.
Never know what the blokes...
So you don't know for sure if you have a broken valve spring or a valve spring out of spec? Did you do a leak down test to determine if the valves were leaking after the HG replacement?
I'd go back and check the spark plug wires, cap, rotor, and order of wires.
Did you use Toyota wires or...