Problem Solved - Post Op Analysis (thankfully not a postmortem; additional details available higher in the thread). I think the initial problem that kicked this mess off was a failed EGR diaphragm. Theory is the EGR diaphragm failed, the control solenoid worked overtime to try and compensate, it failed and the shorted coil fried part of the PCM. I first replaced the spark plug wires thinking I had a ground (from a previous experience). Then MAF and TPS based on symptoms; chased vacuum leaks (found the EGR valve problem). Evap purge valve threw a code and I replaced it. Then the P0174 code popped – new O2 sensor went in. Still problems. Further testing revealed no vacuum to the EGR, exposing the lack of signal from the PCM to the EGR Control Solenoid. PCM was replaced... problems continued. Found a loose spark plug wire. Lean bank codes persist - start swapping out coils, no luck. Gave it to the mechanic for a week (he put in the new PCM), he put in a new fuel filter then threw up his hands thinking it was clogged fuel injectors (at 220K mi, its possible). So I break it down, replace the injectors and all the connectors, intake manifold gaskets, spark plugs (copper plugs were at life expectancy, put in Bosch double plats), and wires (one of which was bad [lesson learned: Ohm out your wires before you put them in ~1k Ohm/ft, (rookie mistake)] – problem persists. New crank and cam position sensors go in, no change. I get the Shop Manual DVD (
PRO TIP – Get the Shop Manual DVD!! Available on Ebay, $45 cheap) and find a bona fide test procedure, buy a PCM safe test light, tests revealed bad wiring between the coil and the PCM. Identified the bad wire and replaced the drivers side coil connector. Still missing. Cleaned all the grounds. Problem persists, additional testing from the procedure then isolates to the PCM being bad (AutoZone replaced under warranty). A defective replacement PCM, now who would expect that! Between, winter weather, work, ordering parts, testing, and life in general, the whole ordeal took six months to work out. Now its running well, though the climate control had failed to DEF – no vacuum leak detected from the engine compartment, so THAT is a work in progress.
@allmyEXes I posted the ignition test procedure, might help with your rig, or anyone else's for that matter!
Thanks to everyone for the analysis and suggestions, this one was a real challenge! As mentioned earlier, didn't mind replacing engine sensors as most of them were original, but a no $#!+ test approach won out at the end of the day.