1HZ to 1HD-T swap on 105 (6 Viewers)

[rngkll]

I recently acquired a 1999 HZJ105 that now has a 1HD‑T engine taken from an 80‑series truck. The previous owner owned both vehicles and performed the swap.

I have a few questions:

1. Is this a true 1‑to‑1 replacement?
   • Are there any mounting, wiring, or ECU differences I need to be aware of?
2. What specific checks should I run after the swap?
   • Any particular bolts, hoses, or sensor alignments that are commonly missed?
3. Diagnostics:
   • Should I be able to use the diagnostic port with an adapter from toyota 22 pin to OBD-II 16 pin?
   • What baseline tests should I perform to verify the swap is sound?
4. Idle‑speed problem:
   • The engine idles at ~650 rpm when cold, but after about an hour of driving the idle drops to ~500 rpm.
   • I’ve tried adjusting the idle speed using the procedures from the Toyota service manual (see HOME | LC100 Factory Service Manual - https://lc100e.github.io/).
   • Any ideas on what could be causing the drop—e.g., idle air control valve, throttle position sensor, vacuum leaks, or ECU mapping?

Any advice, checklists, or diagnostic steps would be greatly appreciated. Thanks in advance!