I reset the DTCs this morning and did my best to monitor fuel trims during my commute. STFTs of course fluctuated quite a bit, but from what I saw, they seemed to stay near or below +10% while the car was in motion. At idle, STFTs would typically climb to approximately 15%; STFTs went to 0.00% whenever I was completely off the throttle while the car was in motion (transmission in gear, clutch engaged) OR if I revved the engine while the car was stopped (clutch disengaged or transmission in neutral). LTFTs fluctuated more than I was expecting; the main thing I noticed was that LTFTs seemed to go to a "default" value of 19.53% anytime I was completely off the throttle (regardless of whether the car was moving or stopped). LTFTs dropped fairly quickly down to around 10% as I increased throttle. I never saw any negative values for either STFTs or LTFTs.

I wasn't able to monitor fuel pressure as closely, but from what I saw, it stayed pretty steady at around 55 PSI. As you said, this should be plenty high (in fact, it's quite a bit higher than I was expecting). I'll try to monitor fuel pressure more closely on my afternoon commute, just to watch for any unexpected changes.

Incidentally, the CEL did not come back on during my commute this morning. As far as I can tell, this all still seems to be pointing squarely at an undetected vacuum leak, but please chime in if you have any other ideas!

 

Pull the gas filter off and pour the slug of water you will find in it out.

Then add a full 16 ounce can of Seafoam into a 1/2 full fuel tank to dry the remaining fuel out.

And run only Premium Grade non-ethanol gas in the future!

 

Alright, I finally had to bite the bullet and take it to a Jag dealer. They determined that both IMT seals were leaking (I'm still a bit unclear as to whether that is something the mechanic who worked on the car previously had already replaced); after replacing those, they found that the MAF sensor (which definitely had been replaced earlier) was still reporting abnormal values, so they replaced it again (at no charge). Everything is now back to normal (almost…but I'll save that for another thread).

This is not the first time I've had trouble with an aftermarket (CarQuest, specifically) sensor not behaving correctly—an ABS sensor that was replaced in the 2003 Ford Escape that I used to own failed within about a week.

Thanks to everybody for your suggestions.