chasing down P1124 and P1126 culprit

[S50Sinner]

As part of the diagnosis process for my 1998 2.5 5MT LTFT lean codes, I unplugged my MAF and drove a cycle to see if it would run any better with the base map as a means to rule out a bad MAF. I did this at the recommendation of an old thread I found during my search for solutions.

The car struggled to idle and got awful fuel economy, which to me indicates that the MAF is not the culprit. At the end of the cycle, I had codes P0102, P0112, P1316, P1317, P1318 and P1319. The first 2 have to do wit the MAF itself, which was expected, but the remaining 4 were misfires. All of those codes were clearly caused by me unplugging the MAF, so I plugged it back in confident that it's not the source of my problem, as its absence only made it worse.

But now I'm wondering if the misfire codes could tell me anything more than that. Do they offer any conclusive evidence either way, or have I only ruled out the MAF? It would be nice if I could just go ahead and order parts, but I suspect I'll have to keep testing for vacuum leaks until I cave in and take it to a shop.

[NKNE986]

I am working on a similar issue that had this 1126 as passive, primary was a 0420. The previous owner had a hack job of an exhaust system on there, first off the aftermarket cats were welded in after both o2 sensors and there is a crack in the header, pre sensor. In the trunk there was a box of coils, maf sensor, ok plugs and various filters. Mine is also a 98 2.5. I will pull the current header off and see what I can mend. Since fixing the o2 sensor placement issue the car runs great and smooth as silk, I just hear this tinny buzz and know of the crack at the merger, passenger side (US). The cel is very random and has cleared itself.
When I did plugs for some reason I had misfires, never made it off the driveway, I used electrical spray cleaner on the connectors for the coils and harness contacts then made sure they were pressed on good, was fine after that. Only had car couple months the shakedown was good, just odds and ends, did 2 spark plug sleeves then also.

[S50Sinner]

Updates! I haven't gotten around to the smoke test yet, but new codes and behaviors have appeared that point towards the secondary air injection system as the culprit.

On startup, I've been getting lurching and uneven engine response for about a minute, and trying to drive under those conditions has triggered twelte (!) new check engine lights, which would normally suck, but in this case it's really exciting, as I now have more information. The car drives perfectly normal after what I assume is the end of the SAI start-up cycle.

Besides the pre-existing 1124 and 1126, I got:

0410 and 1411 this morning, along with a loud exhaust pop, after which the car started driving normal. Both codes are normally attributed to the SAI system.

0300, 0306, 0305, 0304, 1316, 1317, 1318, 1319, aka misfires on the entirety of bank 2. This seems like a side-effect of whatever is happening with the SAI, as the car drives perfectly any other time.

The only on-going symptom is my idle, which seems to fluctuate by a bit less than 100rpm at all times. This also sounds very much like an SAI thing, as that loop controls the idle, as far as I'm aware. I'll start letting my car warm up longer before rolling away to avoid stressing the engine under poor mix conditions, and perform the smoke test as soon as possible.