JDM 3rd Gen 3SGTE Closed Loop Lean Misfire / Poor Idle, Engine Goes to Closed Loop EFI, it Coughs, Stumbles, and Misfires Options

[J3rkyBoy]

About 6-8 months ago, I installed an EBC solenoid for the already-wired in Greddy ProFec B Spec II in my ST205 front clip conversion (JDM 3SGTE, 3rd Gen, stock ECU) 1996 GT. During the week I had it in, it developed a misfire at idle (and a sputtering cutout between 2800-3100 RPM). The misfire appears to be as soon as it switches into closed-loop EFI mode and is as dramatic as someone flipping a switch: one second it is fine, and the next: cough, sputter, choke and AFR usually around 16-18 from 14.7. It is usually worse the hotter the engine is. It seems to be intermittent and can go a day or two without issue. As soon as I step on the gas and get it north of 3k it is typically fine and I'm good.

Please advise.

Aaron

[slavie]

Vacuum leak? That's one thing that gets better at higher RPM, as the engine is gulping more air and the leak has less affect on the engine. The AFR is really lean, too, which means you've got more air and not enough fuel, naturally. There are several backyard methods to test for this, depending on how well equipped you are with tools (everything from a spray bottle to a smoke machine).
Or, bad O2 sensor reporting incorrect readings. What's the history of the O2 sensor? Do you have a scanner capable of reading the O2 output? I know things are a bit weird with JDM ECU's using a different protocol, but never had one so don't know.
My hunch is on the O2 here. When the ECU switches into CL, it starts running off O2 output rather than pre-set maps, and it sounds like too much for it to be just a vac leak. Could be both though.