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Discussion Starter #21
Another update on this repair:

We first started last month with the intake manifold test with a can of starter fluid. After repeated attempts, there were no noticeable adjustments when spraying the fluid around the manifold area so we ruled that out.

Today we tested the heater resistance on the downstream oxygen sensor and received a reading between 15 and 16, which is within spec so we determined that sensor is functional. So then we moved to the upstream sensor, where we got a reading between 6 and 7. Since this should be in the 11-16 range, similar to the downstream, we are assuming this is faulty and is in need to replacement. So we are going to order the part through RockAuto and install it over the weekend. Looks simple enough and easily accessible from under the car.

One strange thing, though. Recently the CEL has just been intermittently been going on and off, with no rhyme or reason. Seems like one day it will go on and a few days later it will go off. Any thoughts as to why? Is there maybe still enough resistance in the upstream sensor to trigger inconsistent readings to the computer?

BTW, my apologies for the wide gaps between updates - given my son's schedule, we have been lucky to be able to look at the car for more than a couple of hours each month. Since we're hitting the end of his last extension for his emissions test, we are intent to wrap this up this coming weekend. Fingers crossed.
 

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Could be any number of reasons. During a driving cycle(s) the computer checks its sensors if operating within spec, if not, the CEL comes up. If during x number of driving cycle(s) the computer determines the sensor(s) operating within spec, it will clear out that code.If a sensor isn't operating properly within spec but still between driving cycle(s), it will log a pending code and if it continues it will go to a full fledged CEL.

The reason for intermittent CEL can be many:heat, weather, humidity, wiring/connectors, degrading/faulty electronics (ecu, coils, resisters, boards, etc.), fuel related*, elevation, vibrations, goblins, possessed spirits, UFOs, mischievous people.

*I have an intermittent CEL random misfire code (p0300?) that occurs when my gas is 1/2 or less full. Car will start up the 2nd time, everytime, but will stumble and run rough..then the CEL will come on about 15 seconds later. If start car again within few hours with 1/2 or less of gas, car starts right up. If I keep the gas more than 1/2 full, it will go away after about a week of driving and the car starts and fires up after the 1x attempt within couple cranks. I've pretty much narrowed it down to fuel delivery (vs ignition), and one of these years will replace the fuel pump, and its pressure regulator (and filter). This is just an example of an intermittent CEL where in my case, if there is more gas (1/2+) in the tank--somehow-someway (gravity/pressure...The Force--I dunno..lol) I don't have this issue.
 

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Discussion Starter #23
So I replaced the upstream sensor today. Actually removed pretty easily and it looks like it has seen better days:



That said, after driving for about 30 minutes, I got a pending P0171 fault code again on Torque. Hasn't triggered a CEL yet, but I'm guessing it's just a matter of time. <sigh>

So with the MAF and upstream sensor replaced, no apparent leaks around the manifold area, and normal resistance reads on the downstream sensor, any ideas as to where to look next? I know that acceleration in the car could feel smoother, as if its hesitating some, so perhaps something within the fuel system? I have read that issues with the fuel pump and fuel filter might be possible culprits.

As always, any guidance is appreciated.

- Mumford67
 

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Oh man, that sucks. I'm sorry. I know you have limited time(s) you are able to work on this car, I was seriously rooting for it to be solved.
This pretty much covers what you have already done but maybe check it out and re-do/recheck the lines/hoses in car: https://parts.olathetoyota.com/p0171-code-toyota

Everything is indicating up to pre-cat. Air leaks, pcv, 02, intake. Only thing I can think of is to re-check. Even air-filter. Use strong lighting and 2-3 pairs of eyes go over every inch of hose, intake, etc look for leaks/cracks/hissing--take it all off and put back on..maybe a clamp? check airbox and its hose for any cracks/leaks--flex the rubber (press down) so cracks become more evident. According to that link, check fuel injectors as well. you could try running some lucas fuel injector cleaner (get 2 bottles) and/or seafoam to see if that cleans it. they say to put a bottle in full tank of gas--screw that: use 1/4 to half tank so it's stronger (lucas says it's safe). All I can think of.

* Also maybe while the air filter hose is disconnected from the throttle body, maybe shoot some carb cleaner/engine cleaner around and just past the butterfly valve. Just manually turn the throttle linkage to open and clean the gunk with a good spraying and a rag/paper towel(s). That will remove any potential air resistance.

** And if that fails, I would go &&%%[email protected]@!!!!. In deciding if fuel pump/filter or the intake I would first go ahead and do the fuel filter and/or pump--easy job just time consuming. If that didn't work, I would remove/unbolt the intake (pretty easy), and replace the gasket (which is cheap).
 

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Hey, I don't know if you're still struggling with this engine code but I thought I'd jump in just in case anyone else is struggling with this same (particularly annoying) code. I got the car from my parents and they had put about 40k on the car without searching the source of the code on the advice of a mechanic because "this one isn't a big deal." Lol. Well anyway, I got the car and started to go through stuff. I replaced the PCV valve because A. It's cheap, and B. In some cases a clogged or bad valve can actually cause this code. I cleaned the MAF sensor, changed the air filters. None of that worked, so I went through the steps of checking vacuum lines, bought a Bluetooth scangauge and found my readings were similar to yours. I found some fairly inexpensive O2 sensors on Rock Auto that turned out to be densos and popped them in there and that turned out to be the problem for me even though in my case it seemed as though the voltage fluctuation was appropriate. Now I'm wondering how bad a shape the caddy is in because of all those miles of incorrect fuel mixture getting dumped in there....
P.s. - something else that could easily cause this code and isn't often mentioned is if there are any stuck open solenoids or broken vacuum hoses in your EVAP system. This would cause an unmetered air leak and you wouldn't see the broken hoses unless you're underneath the car by the gas tank. ***** either the Vent solenoid or the Purge solenoid being stuck open could theoretically cause this under the right condition. In my case I also had a rusted open vent hose on my fuel filler neck but because all the solenoids were working correctly there was no vacuum leak just an evap code. ANYWAY, That was long but hopefully helpful to someone.
 

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Discussion Starter #27
Thanks for the info, Daniel and l33ch.

Here's the update. Last month, I decided to have the car checked by a local mechanic I trust. He ran diagnostics and did a smoke test. While the intake manifold was sealed well, there was smoke coming out near the gas cap. He said that it shouldn't be happening but that that problem would likely trigger an evap code and note the P0171 code. He also said that he doubted that a bad fuel filter would trigger the P0171 code. His best guess was to have the injectors and fuel system flushed and then to run the car on premium gasoline for about a half a month. To make a long story short, in about an hour, the P0171 code came back and despite the mechanic telling us to give it time, it never cleared away. So, $250 later, we are still dealing with the problem. I'm beyond frustrated and now I'm feeling less than enthusiastic about my trusted mechanic. Even worse, another mechanic I've used multiple times in the past on my 96 RAV4 tried to tell me we couldn't just replace the fuel filter but that we would have to replace the entire tank. Knowing better, I'm now wondering if I have a local mechanic I can even trust with my son's car. Ughhh.

So, I've been thinking about going back to my original plan and replacing the fuel filter by myself (and the fuel pump while I'm in there). I've watched some videos and it definitely looks like a doable job.

But before I do...I hadn't run into mention of the PCV valve causing this problem before. It looks like checking/replacing it is a relatively simple procedure so I will tackle that this weekend. Like you said, it's a cheap solution and after all of the money I've sunk into fixing this problem so far (we're now near $500), it would be nice to have an uncomplicated, cheap solution vs another $60 part like the downstream O2 sensor or the fuel pump/filter assembly, both requiring removal of seats and a lot more effort than the PCV valve. Fingers crossed this $10 solution is the one.

So one more question - I know you said that the downstream O2 sensor shouldn't be causing the problem. But if the PCV valve is not the solution, do I attack that or the fuel pump/filter next? In other words, what is the more likely culprit?
 

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Hey, sorry to hear you still have problems with that silly code. It is an annoying one. I can't remember if you already did this, but vacuum hoses are very cheap so it may be worth replacing them all just for the heck of it. If you already have (again, I don't remember), I would say replace the downstream 02 sensor. I'm not an expert and I'm biased because those are what fixed it in my case but it is cheaper. If you're going to replace both the fuel filter and fuel pump it'll be a bit more expensive than an 02 sensor. Also, if the problem is the pump or the filter not allowing enough fuel flow I'd be surprised if you didn't have other problems that are related to a lack of adequate fuel supply. I just actually changed my fuel pump (and filter while I was in there) because the pump went bad after sitting for 2 weeks and I couldn't start the car. Let me know if you decide to tackle the pump and filter and I can send you links to the parts I used from Rock Auto. And for whatever it's worth, the 02 sensor I got from Rock Auto was the cheapest one but when it arrived it turned out to be a rebranded denso and has worked great. Don't feel like you need to shell out money for the best sensor because let's be honest, its an old car :p hope that was helpful. Again, I'm not an expert, just my two cents from dealing with my own dear echo
 

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Well PCV should be changed every so often (I can't remember when exactly--maybe every 30-50K?, but if it's black and nasty--change it), but if it's been awhile that is a cheap replacement (like 3-5 bucks). It can trigger rough engine operation and if clogged, could cause a back-leak to where the hose connects if that hose isn't in the best condition (sometimes mine will shift off a couple mm every so often..i need to change the clamp), and other evap/emission issues--not to mention impediment of how the engine is supposed to direct extra/lingering vapors in the engine for combustion,etc..so yeah it can affect operation of the car.

A new fuel pump and filter will cost you about 90+10 (pump and filter). The smoke coming out of gas cap..hmm.....Just out of curiosity, I would steal (er...borrow) a gas cap from another toyota..and see if it seals better. Gas Caps should not leak, and if leak-detector smoke came out = bad. That will cause evap/emission/fuel delivery issues. A new gas cap will cost 10-15 bucks--maybe? Like the HVAC system, the fuel system is a (should be) a sealed--PRESSURIZED--system. So if the cap doesn't seal all the way, pressure is less/lost w/ respect to fuel delivery.I'd like to say, that's the problem (after ALL you have done so far),(Why I suggesting stealing/borrow one from a Toyota/Scion/Lexus first so confirm so not out $15 if that isn't it).
 

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Discussion Starter #30
The smoke coming out of gas cap..hmm.....Just out of curiosity, I would steal (er...borrow) a gas cap from another toyota..and see if it seals better. Gas Caps should not leak, and if leak-detector smoke came out = bad. That will cause evap/emission/fuel delivery issues. A new gas cap will cost 10-15 bucks--maybe? Like the HVAC system, the fuel system is a (should be) a sealed--PRESSURIZED--system. So if the cap doesn't seal all the way, pressure is less/lost w/ respect to fuel delivery.I'd like to say, that's the problem (after ALL you have done so far),(Why I suggesting stealing/borrow one from a Toyota/Scion/Lexus first so confirm so not out $15 if that isn't it).
Some clarification here, as I was afraid I might not have been fully clear. The smoke didn't come from the gas cap itself. We viewed the smoke leaking out from underneath the car in the area around the top of the filler neck, near the gas cap area. Mechanic didn't seem immediately concerned as the computer wasn't picking up an evap system error code, though he did mention that there might be a potential issue with the charcoal cannister that would need to be checked later.
 

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Oh okay--Yes, if smoke was coming out from near that area, something isn't sealed: filler neck ,canister itself, or where lines connect to the canister. The canister unit itself should be a sealed unit. While there doesn't seem to be a a check engine code pointing to the evap system directly, it's possible, that the canister area that is leaking, would cause the fuel/air system/ecu to try to accommodate to the extreme of what would be consider "normal" (i.e. fuel trim, etc), but still not what it should be, so triggers a CEL under that system (o2, MAF, etc) even though those components might be fine.
 

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Discussion Starter #34
evegen1969 - We had a concentrated fuel system flush done in mid-February which hopefully should have cleared out any blockage in the fuel injectors.

So here are the latest updates:

A couple of weeks back, we replaced the PCV valve and gas cap. Checked the PCV hose to make sure it wasn't cracked. Took the car out for a spin. Within 5 minutes, the P0171 pending code was triggered. Kept clearing and rerunning scans while my son drove. Every 5 or so minutes, the pending code appeared. Today we replaced the downstream O2 sensor. This time, the PO171 code triggered within about 3 minutes and it took shorter periods of time for the pending code to appear. So it looks like the old sensor may not have been reading as well as it should have but it wasn't the source of the problem.

As a recap, to date we have replaced the MAF sensor, the up- and down-stream O2 sensors, the PCV valve, and the fuel cap. We had the system smoke tested, which showed no leaks in the manifold area, and the fuel system was flushed with a concentrated cleaning solution by the shop. The only potential sign is the smoke that slowly leaked from near the fuel filler neck (viewed from the underside of the car) during the smoke test.

My choices for the next step are either to follow-up on the evap system to see if it needs sealing, in case the P0171 is being triggered down the line as suggested by l33ch; to replace the fuel pump and filter; or to have the injectors checked to make sure they are functioning correctly. In the next day or so my son will request yet another one-month extension on the emissions test, so that gives me about 30 or so days for this next step. Thoughts?

FWIW, I have attached the latest log I collected through Torque today after the downstream O2 sensor replacement in case it might help. This is from some major road driving (about 40 mph) with stops/slowdowns for lights.

As always, any suggestions/help are appreciated.
 

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It's too late now, but I was thinking it would have been nice if the mechanic did the smoke test with the rear seat removed and the "sticky-tacked" cover plate removed to show the topside of the fuel pump. Just to see if any smoke came out there too (tank seal, or one of the evap lines that connects to the fuel pump, etc). or if it was just around the filler tube area. I don't like the fact any smoke is visible, so I would follow the smoke.

Hopefully others will chime in here. Your fuel trim is off the scale, and it still runs too lean--it's getting that air from somewhere. I don't think, contrary to what I said earlier, it's a fuel delivery issue as the car is trying to give it more fuel but still not enough to maintain ideal 14 to 1 fuel/air ratio (or whatever it is). Anyone else? Let's help get this guy's car fixed!
 

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Discussion Starter #36
So I called the shop where I took it in previously and gave him the update. Instead of asking me to bring it back in, he just suggested I take it to a Toyota dealership to see if it might be something related to an update of the computer. In his opinion, nothing he saw, including the smoke by the filler tube, signifies the amount of oxygen that would have to be registered by the MAF to cause those kind of high LTFT numbers.

Before I do that, I think I'm going to go ahead and recheck all of the hoses between the airbox and intake, as well as the vacuum hoses. Will also recheck the MAF to make sure it is properly seated and change out the air filter. While I don't expect it'll fix the issue, I just wanted to cover these bases before dealing with a dealer's shop.
 

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Discussion Starter #37 (Edited)
Hmmm...a potentially interesting development

So my son called me in a panic last night as his car was making weird noises and his overheat light was on. Headed out to see him, and besides a nearly empty radiator, he was also very low on oil (probably burned up from the overheating from little fluid). Added both, and noticed that the fluid was slowly but steadily dripping from the area near the manifold. Apparently the overheat light had been coming on for days and he had been adding coolant to turn it off, not considering something major was wrong by the fact that he was continually adding coolant (ugh!). I managed to get him running well enough to get the vehicle into another shop I use on a regular basis. My guess was a shot water pump. This was confirmed this morning by the mechanic.

While at it, I also asked them to give a stab at determining the cause of the persistent P0171 code. When running diagnostics, they found a coolant temp sensor fault code as well. Their deduction, if I understood it correctly, was that the CTS had been progressively getting worse over time, and while it hadn't triggered a fault code until now, it might have been sending incorrect temp info to the computer nonetheless, causing the computer to think the car was running warmer than it actually was. In reaction, the computer fed less less fuel than was needed, which in turn was triggering the lean code. So they replaced the CTS while replacing the water pump.

My son is up and running again and is keeping an eye on the dash. Keeping fingers crossed, even if I'm still hesitant to believe that this will be the solution. If the CEL doesn't reappear within the next 24 hours, however, I'm going to assume the mechanic was right since the P0171 code was usually triggered rather quickly (within about 5-10 minutes of driving).

Will keep everyone updated.

UPDATE: Never mind. The CEL came back. Will check this weekend to make sure it is still the P0171 code, though I suspect it is. :frown: Oh, well, back to the old drawing board.
 

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So my son called me in a panic last night as his car was making weird noises and his overheat light was on. Headed out to see him, and besides a nearly empty radiator, he was also very low on oil (probably burned up from the overheating from little fluid). Added both, and noticed that the fluid was slowly but steadily dripping from the area near the manifold. Apparently the overheat light had been coming on for days and he had been adding coolant to turn it off, not considering something major was wrong by the fact that he was continually adding coolant (ugh!). I managed to get him running well enough to get the vehicle into another shop I use on a regular basis. My guess was a shot water pump. This was confirmed this morning by the mechanic.

While at it, I also asked them to give a stab at determining the cause of the persistent P0171 code. When running diagnostics, they found a coolant temp sensor fault code as well. Their deduction, if I understood it correctly, was that the CTS had been progressively getting worse over time, and while it hadn't triggered a fault code until now, it might have been sending incorrect temp info to the computer nonetheless, causing the computer to think the car was running warmer than it actually was. In reaction, the computer fed less less fuel than was needed, which in turn was triggering the lean code. So they replaced the CTS while replacing the water pump.

My son is up and running again and is keeping an eye on the dash. Keeping fingers crossed, even if I'm still hesitant to believe that this will be the solution. If the CEL doesn't reappear within the next 24 hours, however, I'm going to assume the mechanic was right since the P0171 code was usually triggered rather quickly (within about 5-10 minutes of driving).

Will keep everyone updated.


Time for a new ride?

UPDATE: Never mind. The CEL came back. Will check this weekend to make sure it is still the P0171 code, though I suspect it is. :frown: Oh, well, back to the old drawing board.
 

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Discussion Starter #39
Time for a new ride?
Unfortunately not really an option. He had sunk most of his savings into buying the car and has had it for less than 18 months. And besides this one pain of a problem, the car has been running fine.

As for me....I just want a solution to this puzzle already.
 

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Discussion Starter #40
Next Steps

After doing some more research, I've decided that my next step will be to follow evgen1969's suggestion and replace the fuel injectors. Especially after reading this thread from another subboard here regarding a 2003 Corolla with a number of similar issues:

http://www.toyotanation.com/forum/132-corolla-9th-gen-1st-gen-matrix-2003-2008/1560930-yet-another-mysterious-lean-engine-p0171-code.html

What struck me most specifically was this quote:

1- In the past, when I wanted to reset the ECM, I disconnected the battery for several hours (usually overnight), allowing the engine to become cold. However, the most recent time I did a reset, I disconnected the battery for only 30 minutes after having driven the car, so that when I reconnected the battery with a reset ECM, the car was still warm. At that point, the car idled very roughly and almost stalled when I first started it with a hot engine but with reset fuel trims. In fact, it even threw a couple of codes (2 fuel injector misfires) and idled poorly until the long term fuel trim went back up to its abnormally high level. However, I have gotten the same codes before when I purposely stall the engine by artificially introducing a vacuum (such as disconnecting a hose). So while interesting, I don't know how helpful this information is diagnostically.
Going back to my earlier posts on this thread, that is the same thing I ran into each time I had disconnected the battery while cleaning that old MAF sensor. As per some people in the other thread, that seems to imply injector issues and one or more bad injectors could definitely introduce the lean code.

So I'm planning to buy this set of 4 remanufactured DENSO fuel injectors off of eBay and will install them. Seller ratings are decent and its cheaper than having just one of the current injectors inspected/cleaned. Looks like a simple enough job and I can always put the old injectors back in if these have any issues.

Will keep everyone updated.
 
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