WELL!
While there was no lip on the plenum gasket and the manifold gasket was rock hard and came off in pieces...
My issue persists.
And yes, I reset the ECU. It was unplugged for the duration of the repair.
I'll re-check the torque on all the bolts tomorrow but... unless I torqued them incorrectly, this was NOT my issue and we're back to confirmed no vac leak.
Injectors all looked a bit coked up around the edges but the nozzles were clear. I don't have means to test them out of the truck but I no longer suspect them at this point. I gave the seating surfaces a good cleaning with carb cleaner, greased the o-rings with silicone dielectric, and they went in effortlessly and with no leaks.
Between all of the above, the steady vacuum, the 3% compression variance, the EGR solenoid being new and tested to be good, the IAC being new and tested to be good, coils, plugs, and wires being new and tested to be good, EGR valve being cleaned and tested to be good, MAF being cleaned and tested to be good, IAT sensor being cleaned and tested to be good, crank position sensor being tested to be good, upstream O2 sensor tested to be good...
That really just leaves the DPFE sensor and the downstream O2 sensor.
Can anyone confirm the likelihood of the downstream O2 causing this? My understanding is the DPFE would only cause this once the EGR system became active, and that the EGR system is not active at idle, so I'm leaning toward it being something other than the DPFE -- even though I know I *should* replace that thing anyway given that it's the older aluminum bodied part that is known to fail often.
Anyone got anything else I should maybe look at?
EDIT: A point I keep forgetting is that EGR doesn't affect (or barely affects) fuel trims on MAF-controlled engines. If I were dealing with a MAP sensor I would be right to look at the DPFE sensor. Given that I'm looking at a MAF-controlled engine, I'll disregard the DPFE and the rest of the EGR system for now. Which leaves the downstream O2 sensor... Any input?
EDIT 2: I forgot to mention, the truck idled butter smooth when I first started it (well, after it cleared out all the carb cleaner). Like, smoother than it's ever been. As I drove it and fuel trims adjusted (just STFT at this point but it's in the high-teens range) it got a little rougher. Just back to what it was before, which isn't really rough but you can feel the vibration. That tells me it's unnecessarily adjusting fuel trims, which points to a sensor. I'm going to replace the downstream O2 since it's not behaving as I expect it to, and see where that goes. I found the OE part for $32 at a parts store nearby; they also just offered me a job today, to start next week, so I doubt they'd screw me on the part, that's just a really good price. The upstream O2 is $31 from a number of local suppliers, in case the downstream doesn't do the trick, but the upstream seems fine -- the truck goes closed loop within 2 minutes of a cold start after sitting overnight.
