Getting too lean codes (P0171, P0174) on bank 1 and 2 on my 2006 XC90 V8… | SwedeSpeed

I recently jumped off the cliff and embarked on the timing cover re-seal job on my 2006 XC90 V8, which has 197k and had never had it done… I’ve had the car for 12 years, with the common progressive oil leaks from all the different places, getting gradually worse over the last few years. Aside from that, though, the car has been entirely reliable! Oil consumption even considering the leaks is pretty minimal…

I picked up the timing cover re-seal kit from FCP (seals, PCV hoses, etc), and chipped away at it over the last week. I finally got it put back together last night. Along the way I also replaced the idler pulleys, tensioner, serp belt, fuel pressure sensor, spark plugs, and water pump. I also rebuilt the injectors using the kit you can buy off of Amazon (baskets, spacers, o-rings), and cleaned them out with carb cleaner…

Long story, but wanted to provide the list of stuff I did in case it’s useful…

I drove it around a bit today and it generally drives fine, but my scanner (OBDII, not Vida) reported a transient P0101 (MAF sensor, only showed up once) and P0171 + P0174 (banks 1 and 2 too lean). It idles a bit roughly and low, and had a CEL show up once, but did not recur yet. The scanner confirms that fuel trim is high, with short term trim at ~25% at idle. During the week I was working on it I had the battery disconnected, so it would have lost any pre-existing trims, I think?

According to what I’ve read, this can be indicative of a vacuum leak. I’ve done the propane leak detection test around the PCV, vacuum tank, and in a few random other places around the intake, without any change in idle… I did not replace the PCV valve, as most folks seemed to think the V8 doesn’t generally need that, and mine was relatively clean. I noticed that when I pulled the dipstick with the engine at idle there is pretty significant suction, not sure if that’s normal? I can’t seem to find anything conclusive for the V8.

I cleaned the MAF with CRC MAF cleaner, but it was already pretty much pristine… The original P0101 never returned, only the lean codes have been recurring.

Fuel pressure is nominal at ~68 PSI, and out of paranoia I put the old FPS back in just in case… No difference. It wasn’t faulty, either, I just replaced it since I was in there.

I’ve not driven it significant distance yet (just around my neighborhood a bit), as I’m trying to take it slowly and work through these issues. A few questions:

1. Is it possible I just need to drive it for a bit to let the ECU re-learn everything? Higher speeds, etc? I’ve not gone over 25 mph yet, probably 2 miles in total, lol. (I’m a relative novice at wrenching, so this was a biiiig project for me and I’m trying to be careful.)
2. Is there anywhere else I should be looking for a vacuum leak? I don’t hear anything whistling, etc, and it seems like possible locations are really only around the vacuum tank and the throttle body / PCV, at least as far as bits I’ve messed with when taking it apart…
3. Is it normal to have significant negative pressure when I pull the dipstick? The car also idles much rougher when I do that.
4. I’m hesitant to put a new MAF on it, as that doesn’t seem to have helped with most similar reports I’ve found, all of which seem to be vacuum leaks. However, that transient P0101 I saw is giving me pause.. Is it worth considering this without a code?

Next steps I’m considering:

1. New PCV valve?
2. New MAF?
3. Drive it some more and see if it sorts itself out?
4. Hand it to my local import shop and see if they can figure it out?
   1. Since I can’t do a proper smoke test, if I have an actual vacuum issue this seems likely where I’ll end up if it the prior doesn’t fix it.
5. Other brilliant idea from one of you fine folks?

Thanks! This forum has been a tremendous help in this journey… I’m grateful for any tips / insight you can offer!