ParkerVann

Hi Guys, Im hoping to get some help on an issue with my SC. This is my first post and I hope I am posting this in the right place.
So I bought the car in August, in late October my problems started. By mid-November I stopped driving it, and it has been in my driveway since. I really like the car, and it's supposed to be my daily, so I just want to drive it again.
It is a 1992 SC400 with TRAC and federal emissions. 152k on odometer, timing belt service at 91k.
The car stalls at stops and when coming to a stop. It would usually restart after stalling, and run normally, but randomly it would stall and not restart. It would run but only if I kept my foot on the gas. It also seems to have a low idle compared to before, ~400-600 rpm when warm and ~1000 rpm when cold. I will attach pictures of the tach (CEL is from unplugging MAF I believe in the second pic). The car doesn't really idle rough, just low. I can also prevent the stalling by giving the car gas.
One time after restarting it, there was black smoke and it smelled very rich, but that hasn't happened since that one time. It also seems that the rpms drop to idle/stall very quickly when taking my foot off the gas.
I am unsure if this is related, but it also has an occasional misfire and occasional rough start.
I have read many threads about stalling, but none of the fixes seem to help me. I have tried or inspected the following:
1. The fuel pump ECU was already bypassed by a previous owner, and I redid the work to ensure that nothing was loose or wrong.
2. I have also installed another engine ECU and the problem persists.
3. I have tested the resistance of the MAF, and it is good. The car sorta runs better without it (higher idle) but will still stall.
4. I inspected and cleaned the IACV, and it looked okay. Bearings were smooth and the screw thing was clean.
5. Ignition coils both work, and the car will run on either half of the ignition.
6. Throttle body seems very clean, and the TRAC throttle plate is not closed or anything unusual.
7. No apparent vacuum or intake leaks.
8. I pulled the covers off for the spark plug wires to check the timing belt, and all the marks seem to be correct at 0 degrees. I am unfamiliar with timing belts and how to check if it is correct though, so I may be incorrect.

I probably forgot some info or steps that I have taken, but will post more if needed. I will take any help or suggestions you guys may have to help me, so thank you in advance.


Also wanted to mention that this first occurred after punching the throttle on the interstate to pass, and then exiting to an off ramp where I stalled.

ParkerVann

Just wanted to post an update to help anyone who may have similar issues. I ended up putting in another IACV from the junkyard, and swapped driver's side ignition coil. After this the car has yet to stall (knock on wood). Even though my readings appeared to be within spec, I assume that the IACV was my culprit. The idle is still unusual though, as it will idle at ~850 to ~500 when hot, with no apparent pattern. Anybody know what I should check now?

Tallyhoe

Maybe replace the fuel injector relay. The round one nearest to the engine. I had similar issues, replaced it and it appeared it was resolved... Atm I am cooking my cats so I am following partially what you've done with ignition coils.