3RZ P0172 code popped up and hopefully I'll resolve it

My 1999 4Runner with a 3RZ and automatic transmission has been having a stalling and stumbling issue, and I've started to more seriously try to resolve it. I think it started with the idle fluctuating drastically while waiting at a stoplight: ~750 rpm to about half of that, in a surging manner. It seems that the problem was getting worse in that the engine would quit sometimes at stoplights. It'd restart right away and drive out okay. Then the engine stumbled some when driving. It's stalled while idle in gear when slowing for a stop. Then one morning it didn't want to get up to speed at first, then it did. It's been a very intermittent issue- it comes and goes during my commute, but the issue persists during every commute. There was no check engine light, until it came on then went away, once this week. I had a code reader in the car, and got P0172.

I fairly recently swapped the EVAP canister and throttle body (cleaned it) with idle air control with used parts from the junkyard or my significant other's 3RZ. I was trying to figure out an EVAP code on the SO's car (still has an EVAP code) and eliminate the TB and IAC variables on mine.

I've looked at several threads over this time, and I have a bit of a plan. I started today with something nice and easy: taking a look at the mass airflow sensor and the air filter. The air filter is filthy and I don't really know what I'm looking for on the MAF. The bare wires (two shiny "goalposts" and an orange-colored(?) filament for the crossbar on one side, and a shiny wire or two on another side) don't look obviously fouled. I'm going to test drive it with the clean filter (from the JY). If that doesn't resolve it, then I'm probably going to try cleaning the MAS and/or swapping it out with the SO's and/or a JY one. I'm somewhat interested in seeing if I can confirm with what I think is called A-B-A testing, where I resolve the problem by replacing A with B and get the problem back be reinstalling A.

Last weekend I took a look at the spark plugs (look good), and swapped out one spark plug wire with a used one. One of my brothers said his 3RZ used to get carbon tracking on one of the spark plug wires (the ~6" long part between the wire and the boot) and it'd arc on the spark plug tube. He said that only caused stumbling with the engine under a load. I replaced both coil packs (auto parts store) a couple years ago. I don't think that any of these are part of the problem.

It also has an exhaust leak because one of the threads in the head is tripped out so there's one missing bolt for the header-style exhaust manifold. The cracked cast iron one probably leaked as much.

 

Check your fuel pressure/FPR operation.

 

I think the biggest clue is how intermittent the problem is. I've been messing with the EVAP system because I think problems with that could cause intermittent issues because I believe it operates (changes the inlet air) intermittently. I took off two (and then noticed the third) electrical connector and it seemed to be a little peppier when lightly accelerating from a stop and it seemed to be idling more solidly and at ~650 rpm versus ~750 rpm. Then idling at a stoplight it the idle surged down twice in a few seconds and the engine quit. I also blew through the hoses and pipe that goes from the EVAP canister to the air inlet box. I listened to the front two fuel injectors and they seem to be working good, but I couldn't get to the rear two. How do you access the rear two to listen to them? I had the alternator and battery checked a week or two ago and they auto parts store said both were bad. I put a JY alternator on and the charging system tested good, and I bought a new battery.

So, any good informed thoughts at what might be causing this intermittent problem?

Air in:
1. Would a faulty EVAP system likely cause this intermittent problem?
2. Would a vacuum leak likely cause this intermittent problem?

Fuel in:
3. Would a faulty MAF system likely cause this intermittent problem?
4. Would a faulty FPR system likely cause this intermittent problem?
5. Would a faulty fuel injector likely cause this intermittent problem?
6. Would a faulty A/F sensor likely cause this intermittent problem?
7. Would a faulty ECT sensor likely cause this intermittent problem?

Air out:
8. Would an exhaust leak at the manifold likely cause this intermittent problem?
9. Would a clogged catalytic converter likely cause this intermittent problem?

Electrical:
10. Would a faulty ground wire likely cause this intermittent problem?

 

I swapped MAFs and that seems to have solved the problem. I'm going to give it a little more time, but this was the first 10-mile drive without issues.

 

Cleaning the old MAF sensor didn't fix the problem. Replacing it with a dirty used one did.