So I while back I started a JDM B20A Swap into a 1989 Si. I got as far as swapping the motor but I was not able to finish everything because I left to college. Well now I'm back home for the summer and I'm picking up where I left off.

The problem I'm having is extremely erratic idling.

I'll give you a break down of what the car has right now

Break down:

So, I have a 1989 Honda Prelude SI 4WS, I have swapped into it a JDM B20A. It has:
- JDM B20A motor
- USDM Transmission
- USDM Intake Manifold (ITM)
- USDM Exhaust manifold (EM) [with all o2 sensors, which connect to the (EM)]
- USDM Speedometer cable since I am running the USDM transmission

Problem:

I have started the car successfully and it drives fine, however when it comes to idling in the driveway or at a red light it is horrible!!! The idle is very erratic.

YouTube - Erratic Idling



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Which ECU?

 

The USDM ECU, but I don't think that is the problem.

 

And you've already checked for vacuum leaks/proper vacuum hose routing, and that the idle control is clean?

 

Throttle Position Sensor.


i had erratic idling for 18 months and i just could not figure it out. eventually i thought to change the tp sensor, and it worked.


try unplugging the tp sensor while its idling erratically, when i did this what happend for me was the car held idle (no longer erratic) but it was holding it high around 2k rpm.

so i bought a new one and voila!