Preface: This is my sister's car, so I was not there at the moments leading up to it misfiring and stalling, but I'm the one who gets to fix it.
It's a '99 Lexus RX300, and about a month ago I replaced all of the plugs, PCV valve, cleaned the intake up to the plenum, and cleaned the IAC valve. Prior to that it was misfiring on cylinder 6 due to a damaged coil, so that was replaced as well. She also had a cracked coil body on cylinder 4, so I replaced that recently.
When the vehicle stalled, it registered P0300 (Random Misfire), P0301 (Cylinder 1), P0302 (Cylinder 2) and P0305 (Cylinder 5). It will barely run when cold (consistently misfiring inconsistently), but will not stay running once it hits closed loop without holding the throttle open at least a little. It appears to misfire at any given RPM. Toyota numbers their cylinders 1,3,5 in the rear bank and 2,4,6 in the front bank, so it sounds like the two cylinders furthest from the fuel rail supply may both be missing.
I won't be able to touch the car for the next week - too busy - but I want to make sure my line of diagnostics is making sense and solicit help for anything I might be missing.
I started looking for a vacuum leak. No obvious cracks on any hoses or anything like that, so I started spraying carb cleaner around the various intake component gaskets to see if it changed the behavior of the engine. It had a remarkably delayed reaction. About 30-60 seconds after I sprayed some cleaner around the gasket in the intake side of the throttle plate, the misfiring smoothed out somewhat for about the next 30 seconds. It was not an instant improvement, and the improvement was not momentary. I've only read about the carb cleaner method, but never tried it before, however I expected a much more direct effect, so I continued to look around...
I tried starting it with the MAF disconnected so I would revert to the TPS map. It did not idle any better, and just ran a little worse as I provided throttle, so this again makes me question if it is a vacuum leak, because I would've expected something more significant.
The two banks were reporting significantly different O2 voltages, but I have no idea if that's normal. TPS reported 11% at idle to 75% at WOT with no 0 spots noticed - I'm assuming that's a ranging issue with my bluetooth OBDII scanner, so please let me know if I should suspect that.
I don't feel like I'm anywhere closer to a solution, but I'm starting to suspect fuel delivery, so my next plan is to hook up a fuel pressure gauge to see what it tells me. Does this sound like a good next diagnostic? Does it sound like I'm missing something? I'm really scratching my head on this one, so any advice or suggestions on what to test/inspect next would be very helpful.