The Dreaded P0171 on my 2004 Tacoma 3.4L 4x4 Auto SOLVED!!!

I have the dreaded P0171 on my 2004 Tacoma 3.4L 4x4 Auto. I'm starting this post to document everything I have done and hopefully what fixed it.
I am including the scanner data below (freeze frame, I don't have live data on my scanner) so if anyone has similar values I hope it helps.
P0171 only seems to be appearing at idle or near idle, not under high speeds.

Any advice would be greatly appreciated. I've been doing hours of research to try and fix the problem myself. So far all repair have been done by me.

CEL: P0171
Date 12/30/2021 1/23/2021
Fuel Sys 1 CL CL
Fuel Sys 2 N/A N/A
Calc Load 13.3% 12.5%
ECT 187 Deg F 183 Deg F
STFT B1 20.3% 17.2%
LTFT B1 31.1% 33.6%
Eng RPM 786 790
Veh Speed 7 MPH 17 MPH
IAT 70 Deg F 70 Deg F


The following list of repairs were done right before P0171 popped up. P0171 did not appear beforehand:
- Replaced fuel filter. It has a very small leak, doesn’t drip at all but to the touch I can see residue on one flared fitting. I plan to fix this soon.
- Replaced all injectors with MotorWest remanufactured Denso Injectors.
- Replaced valve cover gaskets.
- Replaced upper and lower intake plenum gaskets.

After P0171 code appeared I have done the following:
- Replaced rear vacuum hose that connects from intake tube to back on engine. It has a pretty big crack at the end of it.
- Cleaning MAF sensor with MAF cleaner.
- Tightened down all hose clamps.
- Cleared codes with scanner but P0171 returned.

What I plan to do next:
- Re-install fuel filter flared connection to stop small leak.
Fixed small leak did not fix CEL issue.

- Replaced PCV hose. It has a very small crack at the end of it, maybe 1/8" long crack.
Replaced PCV hose did not fix CEL issue.

- Check brake vacuum booster for leak
Verified with vacuum gauge that booster had no leak, and still running lean.

- Check vacuum “Y” fitting at back of engine that ties into throttle body and behind wraps behind engine.
Y fitting checked and not cracked and secure in place.

WHAT FIXED IT / SOLUTION TO MY P0171:
I watched this Youtube video on P0171 which had almost exact symptoms like mine.
https://www.youtube.com/watch?v=SugtR4KMIDU&t=1249s

I purchased an OBD2 Scanner that provided data streaming and a vacuum gauge.
My STFT (Short term fuel trim) was not dropping below 19.5% at idle. Once I gave it gas it dropped and correct to +-0%.
MAF read 2.7 g/s (good reading) so I eliminated that. o2b1s1 was reading around 3.4 and when I gave it gas it began to drop so it was not a fuel issue.
So, I began testing vacuum on my vacuum hoses (brake booster, EVAP system). When capped off both did not correct the STFT.
So, I began spraying Start Fluid around the engine to see if the STFT would drop to +-0%.
The drastic change happened around my driver side valve cover. I replaced the VC gasket a week ago and the P0171 popped up after.

So I took the intake manifold off, and driver side valve cover off.
Re-seated the valve cover and I noticed it sat a bit lower than before because the spark plug tubes protruded out a bit more.
Put everything back and checked my STFT and its hovering around -2% to 2% at idle. That fixed the issue, no more vacuum leak.

Hope this helps someone in the future. I recommend watching the Youtube video. The guy did a great job of explaining P0171 and how to properly diagnose it.

 

I found your problem, non oem parts.

 

lol. Actually all the parts I've replaced have been with OEM toyota parts directly from my local dealership. Only parts I've purchases not from dealership were the remanufacturer Denso injectors from MotorWest.

 

So you took the valve covers off and now you have a surplus of air. My money's on a hose you forgot to reattach.

 

I will check my vacuum hoses again but I traced each one I had disconnected to make sure none were disconnected. Plus the truck runs fine and idles fine. After about 10 miles of the driving the P0171 code will return.

 

How's the main air intake tube?

 

Pro-tip: don't use starter fluid (or brake cleaner - it can kill you), use an unlit propane torch. You don't want to douse your aging vac lines or even your brand new valve cover gaskets in starter fluid...

 

verify that the brake vacuum booster isn't leaking by clamping off the hose or pulling it and plugging it. Check fro a vacuum leak where the brake master cylinder bolts onto the booster too. There is a seal that goes around the snout of the brake master cylinder into the booster.
If all OK there, then I would think that the injectors aren't quite right too.

 

I just googled this:

https://www.tacomaworld.com/threads/p0171-solved-finally.330233/

I just re-read it, it is for a 2nd gen tho, but maybe still something to look at.

 

Thanks for looking into it for me. I had read that thread a few days ago but ruled out the fuel pump because P0171 was not coming up before I did the injectors, valve covers, and fuel filter repairs. So I have isolated it to the engine bay or fuel filter. I will continue to provide updates. I will test the fuel pump once I have exhausted all other options.

 

It looks to be in decent condition but I plan to remove it and do a thorough inspections for cracks. It could be that when I removed it to do the injector repairs somewhere along the tube finally gave out and I missed it. I will continue to provide updates on this thread.

 

in addition look at the vacuum “Y” that feeds the ports on the back of the heads.

 

Hey Chacho! when I started to read this post I thought I posted it, and my senil brain was tripping me bad...
I got to add that before messing with the fuel filter, the light showed but it went away when the timing belt was changed.
A whole year, and plenty of money before replacing part number

$102 22204-22010 Intake Meter Sub-Assembly

22204-22010 Air flow meter. The original part that tested right, between the parameters stipulated from factory...well at least I trust my readings like the ones on a PCR test.
but is such a small, easy, cheap one that I replace it, knowing that I would be an idiot If it get fixed when done.
So here I am in front of a tacoma '00 that sound amazing, and rebaptized "The Idiot that drives that white tacoma with no check light on..."

 

simple explanation
https://parts.olathetoyota.com/p0171-code-toyota

 

Just wanted to provide an update on my P0171. This issue was the driver side valve cover was not seated properly. I explain how I diagnosed it in my main thread.
Thanks everyone for providing possible solutions.