Hello,
I have a 2003 rx3000 and just did a JDM engine replacement.
Car is having trouble idling after it has been sitting a few hours.
Misfires,, mostly on bank 2.
Low rpms, bounces around, engine shakes.
Runs perfect after it warms up.

I cleaned the idle air control valve and reinstalled, did not fix.
Took off the IAC electrical controller, and started the car while manually turning the IAC control rod. Ran beautiful when held in a specific position.
Cleared ECU memory.
Installed new, non-lexus IAC with new gasket.(it's Sunday)
Problem still persists.

TB cleaned, VVTI solenoids cleaned, new plugs(checked gap), fuel and fuel pump and regulator checked, all new timing system(checked),

Any ideas? could really use some help.

 

The passages for air need to be clear along with proper operation of IACV. By itself it is just a smart valve, but the pipes need to be clear too.

There are other things like injectors working properly.

Most likely the computer mapping has been reset with the battery removed and that should have set the ECU in the middle and then it takes couple of days to settle out.

From your description, I would concentrate more on 'misfire'.

Salim