SR20 won’t return to idle. Any help appreciated

Hey guys. I’m not Mr.SmartestGuyEver on like anything. I do what I can, use what common sense I have, and try to drive around old cars. As you can imagine, this comes with its problems. I’ve got myself in a situation where I’m in over my head and don’t know where to turn. My issue isn’t performance oriented enough for the high performance guys, regular mechanic oriented enough for regular mechanics in my area, and too Japanese for anyone else I’ve talked to. I have a few friends that have old Nissans and none of them have solved it either. I’m here now because I watched a video where the guys from HPacademy were diagnosing a ground issue on an s14. Thought maybe someone here may have the questions or answers I seek.

Have a 1992 240SX (USDM S13) with red top SR20DET

I bought this car with a no return to idle issue. Cranks fine. Idles fine. Drove fine last time I drove it. Just can’t let off throttle completely after revving and engine return to idle. I’ve been at this for two years. In my free time granted, but still.

I have done

New engine harness - Wiring Specialties OEM Spec

Repaired chassis harness to oem specs so new wiring specialties harness would work (the brown plug where engine harness attaches to chassis harness was previously cut off and modified. Dash is still out of the car)

New fuel pump and wiring (still controlled by ecm using factory wire as relay trigger) Warlboro 255

Changed ecm - both old and new are Nissan (One does have a Mine’s flash on it but it’s stock enough for testing and results are the same)

Changed maf with known good unit - oem

Changed cas with known good unit - oem

Changed coil packs with known good units - oem

Changed igniter with known good unit - oem

Changed tps with known good unit - oem

Changed 02 with new Bosch unit

Installed five point grounding setup - Wiring Specialties

Resealed intake and replaced IACV with NEW OE Nissan.

Purchased obd to usb cable from Australia and had it shipped to US - ECUTalk

Downloaded four different Nissan scan softwares.

Watched every reading those softwares offer that look relevant.

Set idle at throttle body.

Set tps

Set iacv

Set timing

I’ve switched up the order of this hoping for different result

I’ve set throttle body in several different positions and matched tps and iacv thinking throttle body stop position could be the issue

Boost leak tested turbo intercooler and piping

Changed bov to recirculating type

Removed bov entirely

Changed oe maf to intake old taped up rubber pipe with metal isr one

Changed air filter

Changed injectors with known good Nissan units

Sent injectiors to have them professionally cleaned, flow tested, and rebuilt - deatchworks

Adjusted throttle cable

Changed spark plugs - bkr7e, I did set them, don’t remember spec but I did whatever I thought was right at the time.

Checked cam timing

Checked compression

Checked for leak down

Checked vacuum system for leaks

Replaced fuel pressure regulator

Replaced fuel filter

Rev engine and let off, engine will slow until it stops. Sometimes it’s difficult to restart and smells like gas when it starts, like it’s flooded. All Nissan. Everything that is available OEM is there.

I have a copy of an S14 Silvia FSM that I’ve been using for sr20 information. My car and engine is S13 sr20. In the US, sr20det was not available. We have no manual. The S14 one is all the local community has and we just do our best with similar information. If you are AUS based and S13 sr20det OEM information is available in English, I would gladly pay you for a copy of it as well. Thank you to anyone interested in helping!

So when you're saying that there's no return to idle you mean it stalls out?

In your Nissan diagnostics, can you see that the TPS idle becomes active before it stalls?

I would start with a thorough smoke test of the intake manifold if you have not already done so. After that check idle TPS voltage with a meter and ensure the throttle blade isn't sticking open.