2004 SR5 - 260K miles. I love my 4Runner, and I'm hoping to drive it forever. Very few issues mechanically thus far - just routine maintenance. I'm stumped by a P0171 code that popped up (lean bank 1). I've replaced O2 sensors recently.....both up and downstream. Prior to the 0171, I had a codes for the O2 sensors. Things were good for several months. Then a few days ago, P0171 popped up. Cleaned MAF (Haven't replaced it yet as it's $150). Checked vac lines and can't find any leaks. If I clear the code, it's right back on.....within a day or so. I've just picked up a set of spark plugs, as they are probably due anyway. Also picked up a PCV valve which I'll put on tonite with the new plugs.

The light just came on about 3 days ago.....but it's actually stumbling at idle today. It won't die, but I can tell it's running rough and isn't happy.

Is there anything else I'm missing? Other things to check?? It's my daily driver, and I'm really hoping this isn't the beginning of a wild goose chase......

Any help is much appreciated, Chris

Swap out the plugs, sounds like the motor may have blow by give her some fresh quality lube it's all good..

yea... I have never seen a bad sparkplug give a lean condition, though it wouldn't hurt to change if you haven't in a while I would hope it fixes the issue.

If it's stumbling at idle and is throwing a lean code it definitely sounds like some air is making its way in past the MAF or its not happy with the amount of fuel its getting (not very likely).

How are you checking for vacuum leaks? Just a visual? Get some carb cleaner or propane torch that and start spraying around with the engine running and see if it will rev up.. could be a small crack in something letting air get in.. just an idea.. .vacuum leaks can suck (no pun intended)

The lean codes are almost always related to a leak in the exhaust. Check your manifolds for minor cracking.

Thanks guys - I really do appreciate the replies.

Last night I swapped the plugs and the pcv. My plugs were pretty shot, so after pulling the 1st one.....I thought I may get lucky and have it fix my issue. But, alas, the code came back today on my way to work.....and it still stumbles somewhat at idle. I'm sure it'll be happier with new plugs, but I haven't found my issue yet.

To check for vac leaks....I've used small propane canister (for camping) with a valve and hose attached so I can "sniff" around specific areas. I haven't been able to find anything (at least not yet). I have an old CJ7 Jeep....and I've had luck using this method on the Jeep, but it's SO much easier to see everything and wrench on the CJ....Plus, no computer to fool with!!

I have not changed the MAF.....but I have cleaned it really well several times.

I'm curious, you mention air making it's way past the MAF - do you mean after the MAF? ie - is that where I should focus?

It's a bank 1 code - which if I'm not mistaken is on the passenger side on these V6's - is that correct? And does that necessarily mean the leak would be located on that side of the engine?

Someone else mentioned an exhaust leak - which could definitely be possible - and I haven't checked......not quite sure the best way to do that....

A few other questions....

Where is a good manifold vac source on these engines?? I have a vac gauge.....and thought it may be helpful to check what the readings are.....but I'm accustomed to working on my CJ and can't even find a manifold port on the 4Runner.

Is it possible that it is an O2 sensor? I have replaced all my sensors within the last year, so I'm not leaning that way....but I've heard that they can fail.

In my search for vacuum leaks, is there somewhere I'm missing? Or a common spot for air to creep in? Most of the vac hoses seem to be on the driver side, but I guess I could be overlooking something.

Thanks again for any help and for the replies above....

Also.....

I don't really understand fuel trims and OBDII diagnostics.....but I've been googling / Youtubing and maybe someone can help me with this.

I have a cheap scan tool that I've just used after my research, and my fuel trims at idle are really high. This makes sense to me that the ECM is trying to add fuel to compensate for the lean condition. Alot of the videos I've seen, show a drop in the Short Term Fuel trim with higher RPMS - which points to a vacuum leak. This also makes sense to me.....as there is less vac with the throttle plates open.

BUT - my short term trim barely drops at all. It's the same at 2500 rpms as it is at idle. So that makes me think NOT vac leak.....but either fuel delivery or a bad sensor - either A/F upstream sensor or MAF??

Is this correct??

For reference.....my short term is 19-20 and long term is around 28. From what I've read, they shouldn't be more than 10% combined....so I'm thinking the computer is correct about there being a problem.

Did I mention I hate computers??

I dealt with the P0171 fault code recently and also had LTFT of 20-30% and fixed it with a new A/F sensor on the passenger side.

I used this one:
Amazon.com: Denso 234-9047 Air Fuel Ratio Sensor : Automotive

If you want, you could try swapping the two upstream sensors to see if the fault code changes to P0174 for the driver's side.

...Except when they are related to actually being lean, as was my case...needed a new fuel pump at 170k miles. Rebooted the whole fuel system at that point, been fine since. The code pops up because the computer is detecting not enough fuel(or too much air) in bank 1 and has had to compensate over a certain point (I believe your ltft1 would be over +20 to trigger). So you need to ask why the ecu sees this? Vacuum leak? Down stream of the maf or even the manifold itself, you're pulling in air past the metering device which throws off the engine. Best way I have found to test that is with some type of smoke machine (I used to vape, so that was my solution) mmv. If you have the ability to check fuel pressure, that's a worthy test for fuel delivery. If pressure is low, like from a weak pump or a failing regulator, you will see both banks running lean typically. Since yours is related to one bank only, I'm thinking a vacuum leak at an injector seal or a bad injector. It's technically possible to have an issue affecting the whole engine but only getting the code on one side, do you have a the ability to monitor live data? Might be useful in diagnosis.

Thanks guys - I REALLY appreciate the feedback. It makes perfect sense - about the upstream A/F sensing more air (that's crept in after the MAF) and compensating by adding fuel on that bank. My bank 2 fuel trims are normal (+/- 1%)....so it seems that the issue is just on 1 side.

Do these engines have a valve (kinda like a schrader valve) inline somewhere that you can check / monitor fuel pressure? I don't own a gauge, but have borrowed a friends before to use on another vehicle. And I agree, that would be worth checking.

I think my next step is to focus again on leak detection - on the passenger side. I can probably borrow a smoke machine as well, so that'll be my priority.

I'm tempted to get a new A/F sensor on that side, just to rule it out. I may even be able to get it warrantied - not sure if rockauto / denso does that.....but it's less than a year old I'm fairly certain.

I think my MAF is fine after more research and head scratching. If it were bad....I'd expect to also have P0174 / bank 2 codes. AND.....using my scan tool....I'm reading between 5/g of air at idle and up to 10 or 11 grams at 2500 RPMs....so it seems to be operating correctly from what I've read.

81runner - my scanner can monitor live data stream - but I'm still learning how to use it. It's a cheap(er) tool, but it will show fuel trims, O2 voltage, etc. What live stream data specifically would I looking for to further diagnose this?

And - how difficult are the injector seals to replace?? Hoping this is something easy that I'm overlooking......that sounds complicated....

Key word being "almost". Unlike your case, OP describes only one side lean. How much was the new pump? Seems like a good PM measure as I am about to roll 200k..
Bank 1 is the passenger side. Swap your sensors as dj pointed out to see if it flags the other bank.
Most exhaust shops will do a free inspection if you bring it in and describe the problem. Typically their first approach is to use a modified stethoscope to listen for audible leaks while running.

I had what I’m thinking is a similar problem myself and could never find the solution until I replaced all the fuel injectors rubber grommet/seals. Some were cracked and old that would allow more air in than needed. Cheap fix but not the easiest to get to so make sure your boxes are checked first.

Check the connection between the exhaust manifold and the Y pipe on the side with the issue. There is a gasket in that connection. It is just down stream of the cat. If there is a leak there, down stream of the first O2 sensor and up stream of the second O2 sensor, you get the lean code because it sucks in O2 through the leak.

Derek V

Thanks guys - all really good suggestions. I"m headed out of town for a few days, so this will have to wait until Tues or Wednesday of next week. I hate leaving without solving it - but family calls. I'll report back with what I find and any other questions I have. And thanks again for the suggestions and help.

Back to work on this thing last night - spent about 2 hrs looking for leaks, still can't find anything. My friend with a smoke machine is out of town until after the first of the year....so last night I found an old cigar from a friends wedding....and tried my own "smoke" test. Does anyone have a vac diagram? I searched the forum, but couldn't find one. I'll sway the upstream A/F sensors tonight and see if I get any different results, but I'm still thinking I have a leak. Also - don't think I mentioned this before.....but the freeze frame data shows an idle RPM.....and that is definitely when she seems the least happy. All of which, makes me think I have a leak that I just haven't found yet.

Anyone have a diagram? I have the Haynes manual, and have searched here but haven't been able to dig one up....

Just wanted to say: same.