StarOfJapan

I'm going to be as detailed and specific as possible, so this will most likely turn into a TL;DR post, but hopefully this will rule out/prevent users posting "fixes" that I already tried, or otherwise would be irrelevant. Thanks for reading:

About 3 weeks ago now, I tripped a CEL while driving on the freeway that read P0171 - System too lean. Found out that the hose clamp by the MAF sensor was loose, causing the hose to become disconnected from the engine air filter housing. I thought that was the end of my problems, as the CEL light went off. That is, until I drove again on the freeway. The second time I drove only half the distance as the first time, before my CEL tripped again, and the same code came up. A year ago, I cleaned my MAF sensor and throttle body to fix a low rpm issue, which it did solve. *Note* that was also why I thought I didn't properly tighten the bolt on the hose clamp, causing it to come loose. Again, the hose below the MAF was somewhat loose, but not coming off, I retightend it again. I went to Autozone, got some replacement MAF and TB cleaner (I moved from Hawaii to California during this time, so they obviously wouldn't allow me to ship flammable things), new air filter, and spark plugs (spark plugs needed to be done anyway, so why not do everything at the same time?). After doing this, the code (unsurprisingly) tripped again. Took it to my auto skills center on post, and they cleared the code, saying that perhaps it wasn't cleared right last time, and I drove it on the freeway again, sure enough it tripped. Then I took it back, they charged me $20 to check for EVAP issues, to which they found none, then tightened all hose clamps leading to TB, and said that maybe the air filter wasn't properly seated, causing it. I highly doubt that was the issue though. I haven't drove it again to see if the CEL trips yet again, but I'm certain it will anyway, so I wanted to ask now what anyone else thinks it could be. I also want to explain a few things below:

1. I am new to the area. The ONLY auto center we have on post messed up or possibly charged me but didn't even do my alignment after I replaced my struts/shocks, took 3+ hours to change a tire, etc. I have since done all my repair/maintenance work myself, and actually did my own repairs/maintenance back home. I have already asked acquaintances for help but none have been reliable. One told me to google cause he doesn't work with Asian cars (no help), another was supposed to come down and take a look at the car, but literally forgot, then wasted my whole day only to tell me at the end of the day that he won't make it back on post in time to do any work that day, and now he's too busy because he's not on vacation anymore T_T;; The only other person who would be able to help moved and is no longer here. No one else here seems to know for sure, all tell me that generally its a bad o2 sensor, but they let me know they are not mechanics, aren't certain, etc.

2. This area is VERY far from the NEAREST Toyota/Scion service center, and I only have this one car. I called them, explained to them (again, I'm new here) that should they take more than that same day to diagnose the issue with my car, I am without reliable transportation back home. There is a bus that runs, but I have already experienced that sometimes, even though it says it will come at a certain time, it doesn't. The bus doesn't just miss one route for ETA, but you could literally be stranded overnight if for some reason the bus doesn't come for the rest of the day. I already spoke with the service tech and even they explained that this specific code could be any number of issues, and they really couldn't tell unless they see it. They would charge me $125 for diagnostic of that one code, and only so long as no other codes come up. At that point, they (understandably) can only fix/replace a part they think will solve the problem, but if not, I will need to pay again for diagnostics, etc.

3. I am apprehensive about replacing my o2 sensor (upstream most likely would be the issue, if there is one, i think?) because after lurking forums, a lot of people have already said that for Scion tC's they replace it and it didn't solve the issue anyway. At $150 a pop, I am unsure that throwing this part or other pricey parts blindly at my car would be the wisest thing to do.

4. The only reason I took it to my auto skills center was because the first guy I spoke to said they had a smoke machine. I arrived there, waited 45 minutes because apparently the mechanic wasn't in, only to be told by the mechanic that the other guy was misinformed about having a smoke machine.

5. I want to believe that this is the correct order, but if I'm wrong, please tell me specifically and hopefully with pics/videos how to fix the p0171 code: 1) dirty MAF sensor (I already cleaned it). 2) check for vacuum leak (already explained I have no access to smoke machine, and I'm really scared to spray carb cleaner, and i don't smoke). If you have a guaranteed safe way to check for vacuum leaks at home, please give me detailed info about where and what to look for, and HOW to do this. 3) I know people say to check gaskets, o rings etc but I have no idea where and what to look for so again any help would be appreciated. 4) o2 sensor, and i think it would be the upstream one? I've also heard you can clean it, but I'm curious as to how to clean it. What I mean is that if you spray MAF cleaner on the wrong part you can break it because it's sensitive. Do I need to watch out for a specific piece like that on an o2 sensor? How would I know it needs replacing vs cleaning?

Additional info: At speeds over 65mph I lose power, or when climbing a hill, I notice I have to press down the gas more than normal. On cruise control at around 65mph+ when beginning to go uphill at a certain level of incline, not very steep mind you, my car jerks, engine sounds like it's straining. I don't hear any "Knocking" sound, which I think would sound like a pop, nor have I ever heard that from my car even with the old spark plugs. Mileage is roughly 122,000. If you need any additional information lmk and I will get back to you ASAP.

gjpjr84

unfortunately it looks like there could be any number of things that could cause your issues.

have you tried replacing the MAF altogether? check all your hoses for leaks?
a bad O2 sensor could still cause the code, the struggling uphill may point to an issue with fuel delivery, low fuel pressure/clogged fuel filter/bad injectors?


http://www.obd-codes.com/p0171

StarOfJapan

No I have not replaced the MAF. I am really not sure what hoses to check, other than the hose connecting the engine air filter (that the MAF sits on) to the TB. If you had pics and/or detailed how-to, that would be great. I was considering replacing the fuel pump at first, but just learned that I need to buy a pressure tester to see more or less which one to replace (I heard that can narrow down which parts to buy in the case of fuel-related issues). If the fuel pump is not the immediate cause of the P0171 code, then I would prefer to fix that on another payday, and just fix whatever is tripping the emissions-related code right now, being that I cannot pass smog with the CEL on. Can't really afford to throw random parts at my car right now tbh, and normally, being that this place is so isolated from even the nearest town over (which would mean a far trip to the NEAREST auto parts store), almost never drive the car far distances (3.5 miles one way, to work and back each day). Again, that's only here though, whilst when I was living in Hawaii, this car saw an equal share of highway vs city mileage.

gjpjr84

you can check the link I posted for what could be causing your CEL, or run a search on here for the error code to see what others have run into and maybe some more info.

i know you are trying to fix this as quickly and cheap as possible, but this doesn't appear to be an issue that you can diagnose easily without either taking it somewhere to check everything or checking/replacing each things yourself until you find the issue, especially if you aren't sure of what you're doing.