1G ISC motor issue, idle is completely random

[Josh E]

Hey,

I'm Josh, and I'm new to the forums, at least posting here. (Been reading posts for a couple years)
I think I'm posting in the right area...

Vehicle - 1990 Talon Tsi
Engine - 4g63 from a 93
ECU - 1991+ into a 1990 chassis
MAF - 2g

Coil Pack, PTU, CAS, TB, ect have all been swapped to the 1991+ electronics

Alright, so let's begin. Simple as it may be, I'm out of answers. My ISC is failing to do it's job at keeping the engine idling. The only time the engine will have idle surge is if I purposely make a vacuum leak. It will "idle" anywhere from 500-1600 rpms for about 15 seconds and then eventually stalling.

What I know/Tried

The TPS is correctly adjusted and verified with a multi-meter.

The Idle Switch passes a continuity test.

The ISC tests at 40 ohms, I've read that's what new ISCs read instead of the 30 ohms.
I have also tried a known working ISC from my friend and tried mine on his Eclipse and mine worked fine on his.
I figured this would be wiring harness related, I pulled a donor ISC harness and 4 ECU pins and ran the ISC directly to the ECU plug with no change.
The stepper moves very slightly from its resting place when the ignition is turned on. However, if I bump the starter with the ISC out, the stepper moves in and out generously before going back to its resting place.

Out of curiosity I connected the MAF to the throttle body to be 100% sure it wasn't a vacuum leak in the inter cooler piping, no difference.

Also, normal driving and WOT are unaffected with this issue I'm having, and AFRs are where they need to be.


What I don't know

Is the ISC dependent on other parameters? I'm guessing the obvious would be the Idle Switch, but is there anything else needed to make the ISC work?

The ECU worked flawlessly in my Laser, but I have not tested another 1991+ ECU in the chassis yet.

I promise I have searched, haha, maybe I'm just completely missing something.

I have a data logging cable coming in on Saturday, but until then I figured I'd go ahead and see if anyone can point me at something to look it.

 

[AWD-Tony]

Have you removed the throttle plate recently? I was having issues getting ISC to correct position. Found my throttle plate was off slightly when I reinstalled it and you won't see from BLT unless there's a gauge on it.

 

[Josh E]

Update

As of now I got the engine to idle a solid 750. What I did is listed below.

Stock cams
Stock injectors
Good known 1990 ECU
Isc wired directly to ECU
Boost leak was fine
Throttle body was pulled off and I correctly adjusted the tps and idle stop position

Thanks for everyone's input, also evoscan has the idle steps at 35. So that seems good enough to me. Now to slowly re-molest the engine back from a stock setup.