Hi all - trying to troubleshoot a rough start/idle problem. Thanks in advance for tolerating this wall of text, I personally prefer too much over not enough when troubleshooting...

TL;DR: when cold: hard starts, lumpy idle, rough revs, worse in colder weather. Smooths out when warm. Good electrical and diagnostic checks on injection equipment. See google link at bottom for audio and spark plug pictures.

Symptoms:
Cold starts are generally hard, and result in a lumpy, unstable, high idle (usually up to 1500 RPM). Sometimes the engine will not turn over on the first try. Trying to rev the engine results in sputtering. Audio of representative starting, idling, and revving are available at the google link at the bottom. Spark plugs appear to be fouling consistent with running rich (pics at google link).

Being very gentle on the throttle will make it drivable enough to get going, then it will smooth out and run well after a couple minutes (colder weather = takes longer to smooth out). It seems to me that it gets better when the O2 sensor warms up and the ECU goes into closed loop mode, but I haven't tested to confirm that. Even then, it will sometimes idle above 1k RPM.

Just want to reiterate - when the engine has warmed up, it runs well and will restart on the first couple cranks with no bad symptoms. The above only happens on cold starts.

Diagnostics:
I've replaced the fuel pressure regulator, which had no effect. Injectors were replaced about 20k miles/2 years ago. A casual check for vacuum leaks turned up nothing (pulled intake boot and MAF, visual check on some under-hood hoses). The throttle position sensor, coolant temp sensor, and air mass meter are all good per Bentley specs. The diagnostic control functions 2 (inputs) and 3 (outputs) tests all pass. I have yet to perform electrical checks from the ECU connector.

Guesses:
Bentley shows a cold start injector or cold start valve for LH3.1 (not very clear) - my car has neither. My exhaust situation is pretty janky at the moment, but the system is 100% stock to the end of the cat and has no apparent leaks (was running ipd sport 2.5", switched to a 2.5" Flowmaster when that rusted out). This problem did not appear concurrent to the exhaust change. I haven't 100% ruled out a vacuum leak, though the spark plugs and high idle make me think it's running rich, not lean (though introducing a vacuum leak when idling rough doesn't seem to help).

Anecdote:
Was at the shop for brake work a couple months ago. They were pulling it in for work - according to them, they cold started it to pull it in front of the garage, then stopped it to open the door. It would not start again; they pulled plugs and found them wet and fouled. They installed new plugs and it got somewhat better, but still not perfect. The plug pics below are of the new plugs today.

Audio and pictures: https://drive.google.com/drive/folders/1q9ZPZJ6nH5lKDdfDQLW_rcmgN-Vh0VoN

Thanks for any help!
~Nick
--
My Volvo has lasted longer than some of my friendships...