Make sense of Long term fuel trim...

Hi People!
I'm new to this forum (I got my 2001 3.4L 6cyl tacoma recently) but not to car forums in general, thanks to all for your dedication!
I'm chasing a P0171 (lean) issue that started to appear a few weeks back. I've checked and cleaned the MAF and didn't find any visible vacuum leaks anywhere. I hooked up my bluetooth OBDII scanner, and traced a few things to try to figure out what's going on, and I'm completely puzzled by what I see:

This was taken during my morning commute (uphill) and I got a check engine light around frame 200, which is consistent with the LTFT being close to 40%! (I'm attaching the freeze frame below).
Note that the start and end of the trace are idling. in which case the LTFT and STFT are back to normal values, which seems to eliminate the vacuum leak hypothesis. The middle is at 50mph.

Here's what puzzles me: contrary to what I would expect, the LTFT (blue) varies a lot, while the STFT (orange) is more steady. I would expect the opposite.
This is the MAF reading for the same trip, which seems about normal.

Here's the Freeze frame information for the 0171 Fault:
------------------
Fuel Status = 0 byte
Engine Load = 43.922 %
Engine Coolant Temperature = 176 °F
Fuel Trim Bank 1 Short Term = 0 %
Fuel Trim Bank 1 Long Term = 39.062 %
Engine RPM = 2,006.25 rpm
Speed (OBD) = 51.574 mph
Intake Air Temperature = 48.2 °F

So, here are my questions:
- How do you guys explain that the LTFT is so erratic, while the STFT is much less. Could my OBDII reader be confusing the two? Or is that something that you guys see on your Tacomas?
- My understanding is that the LTFT increases when the STFT is too high for a while, but this isn't what I'm seeing here! Is that not how the Tacoma STFT/LTFT works?
- If I were to trust these readings (even though they don't make much sense) they would point to a fuel pump/filter issue (positive trim at high RPM/load in the middle of the trace, and normal trim at idle). What do you guys think?

Thanks for any help with this!
Jean

 

Doesn't really make sense to me either....I thought LTFT were an average of your STFT readings, so if your LTFT are high it would mean that your STFT are too. My LTFT always read about 5 or 6, I don't really pay much attention to the short trims because they jump around all over the place depending on throttle.

Whatever the case may be it has to be related to your P0171 code I imagine...that code is a bitch to track down. Mine ended up being a tiny vacuum leak. But if you're certain you don't have a leak and your MAF is good, I'd be looking at the spark plugs/wires and O2 sensors next.

 

Thanks for the response. Can I ask you why you'd prioritize plugs/wires (and O2) over fuel pump/filter? If I had a plug/wire issue, wouldn't I get misfire codes? (I guess not necessarily)...

 

True that would be something else to consider. The fuel pump should be relatively easy to check with the proper tools to check for resistance and flow. I just remember when I got that P0171 code, the shop(s) that looked at my truck replaced the O2 sensors, plugs, wires and MAF sensor and it was to no avail. Finally a master tech was called in and he was able to locate a small vacuum leak which turned out to be the issue. He was the one that told me the P0171 code was almost always a caused by vacuum leak in his experience.

 

Great initial info/analysis. Nice first post to TW, and welcome!

I don't have an answer for you necessarily, but I got a P0171 on my truck and did what is unfortunately the "normal" dance to try and figure it out, since it's reasonably hard to pin down.

I wrote the whole thing up here. Have a look, maybe do some of the same things I did, and feel free to ask questions!

Chasing the P0171 Engine Light

​

 

Man, that's a cautionary tale ... That's the thing about (some) shops, they'll just go ahead and change everything, until the problem is fixed. I'd like to avoid that if possible ...

 

Hey thanks for the welcome! I did read your blog entry before posting here! It helped me understand more about LTFT and STFT! Thanks a lot for that.
I also read carefully this one: https://www.motor.com/magazine-summary/fuel-trim-data-powerful-diagnostic-tool/ which has a lot of great info to help you sort out whether it's a vacuum leak, maf etc...

 

Yeah I would too, I only let them throw parts at it because it was under warranty at the time and didn't cost me anything (except being frustrated that my truck was in the shop for awhile while they couldn't figure it out).

Sensors can be checked as well to verify they're good before decided to replace them, I think you can pull up the sensor readings on your OBDII scanner or check their voltage with a multimeter to see if they're functioning properly.

You don't have an oiled air filter by chance do you (like one of those oiled K&Ns)? Those are notorious for ruining MAF sensors which would throw a code

 

I do have a K&N, and it was pretty dirty, so one of the things I did was to clean it thoroughly. I cleaned the MAF as well, but it wasn't very dirty (at all). But perhaps you're right, and the MAF is not working right. If it's under-reporting, I would definitely get a lean condition. It's hard to know whether it is or not, unless someone can tell me their reading at a given RPM for the same engine, and I can compare to mine. It's hard to find "normal" numbers online (at least I haven't found them)...

 

With how many years and miles our 1st gens have on them nowadays, vacuum leaks are very common (aging hoses, seals, etc.) which is why I think we see this code get talked about so often in this forum. Just sucks because there are so many places to check where a vacuum leak could be occurring, and it only takes a very small one to trigger the code.

What made you think to check the master cylinder? Was it hissing where the leak was?

 

Do you remember if you took traces of your LTFT and STFT? Where they abnormal during idle, or during high RPMS? The thinking is that if they're normal during low RPM, then it's not a vacuum leak since a vacuum leak has a larger impact at low RPM than at high RPM. I need to do a more thorough job of testing for vacuum leaks. There are a few tricks that I should use for that. I'll let you guys know.

 

With a ltft that friggen high (trying to add a BUNCH of fuel) you either have a wicked nasty vacuum leak (like the booster) or it’s fuel pressure related. That’s it. No wires, plugs, cat, nothing else. Well, possibly, possibly the bank 1 O2 but I really doubt it.

 

Crazy the stuff we learn here. Collectively we are better than most Toyota techs with years experience. Kinda like Yota whisperers...

 

I just checked that and it's not broken, I believe you mean the piece of plastic that connect the vacuum hose to the booster, correct? (not the booster to the master cylinder). But it's surprising how poorly secured to the booster the vacuum hose is, there's just this tiny arm that's pushing it. I know the vacuum forces it against the booster but it's still surprising, in my case it's not sitting super well on the booster, but it does not appear to leak (no hissing).

In any case, I pulled out the MAF again, and this time cleaned it more thoroughly, I had not seen the part of it that's hidden in the tube and upon inspection that part was quite dirty (dummy me! ). I put it back, and my numbers appear to be back to normal, at least during idling, revving, and short drive around the block. I'll trace it tomorrow on my way to work (uphill, highway) and post the traces. I'm keeping my fingers crossed that it was simply the MAF.
More tomorrow!

 

Here are the traces for my commute this morning.

The STFT and LTFT values are back to normal. By comparison, this is the trace from yesterday over the same commute:
A few things to note:
- I was surprised at how erratic the MAF trace is (esp when the RPM is constant), but I realize that the MAF will change drastically even at the same RPM depending on the throttle position.
- The MAF today tends to be significantly higher than it was yesterday. Toward frames 200/300 when I'm going uphill, the new values are around 30/40 while yesterday they were around 20/30. So the likely explanation is that my MAF was under-reporting which is consistent with a detected lean condition.

So the bottom line is: next time I clean my MAF I have to make sure I clean *all* sensors in it. not just the visible one!

Thanks to all of you who chimed in and helped, I really appreciate it.

 

Note:
This link: https://www.customtacos.com/tech.ol...06toyrm/06toypdf/06rmsrc/rm2006ta/0060027.pdf has data on expected MAF values. It's not for the first gen, so take it with a grain of salt.

 

STFT is what is happening "right now", LTFT is what has happened over a longer period of time. LTFT + STFT = total fuel trim. +- 5% total fuel trim is an acceptable range.

The Mass airflow sensor on the 3.4 motor is a tricky and complicated device. In addition to the sensor that actually measures the mass of air passing the sensor, the intake air temp sensor is built in it as well. There is no vacuum signal from the engine to the PCM so the PCM has no way of "knowing" the load on the engine... this is a calculated figure inside the PCM from throttle position, RPM, MAF etc etc. The B1S1 (pre cat sensor, either o2 or A/F sensor) is so sensitive that when the PCM is maxing out the fuel trims to get to the desire A/F ratio, it sets the CEL lean code. IIRC, the 4 cylinder motors use a MAP (manifold absolute Pressure) sensor so they are more "forgiving" to lean conditions.

If you are suspecting a vacuum leak set you scanner to monitor STFT and LTFT for B1S1. As an example lets say that STFT is +20% and LTFT is +25% ... that is the PCM is adding 20% more fuel to reach the desired AF ratio at idle. Open the throttle and watch the STFT and LTFT. If they start to go to 0% there is indeed a vacuum leak. With the engine at idle pulling a negative 18" of vacuum, normal, and the outside air pressure is apx a positive 14.5 psi that is a 32 inch vacuum difference. As the throttle opens the engine vacuum starts to approach atmospheric pressure and the difference inside the motor is less so there is less of a pressure difference for "false air" to enter.

 

Jeanl,
What OBD scanner are you using? I like that you can get the data back to your computer for plots. A lot of the ones that give plots on your phone are not helpful because they are too small.

 

I'm using a very cheap one, bluetooth, connects to my phone. What I do is I export the trace files written by torque from the phone to my computer. They're in csv (comma separated values) style, so from then on it's very easy to plot what you want, either in excel or in my case using python.

 

Maybe you or someone can help me with my situation, because I’m stumped.

A month ago maybe less my fuel trim that were usually are -+5 sometimes up to 9 Max, they changed to +25 +30,

After doing a smoke test and replacing all the vacuum lines and a fuel pressure test with no answers I decided to replace the fuel pump that had 300k and get an dealership OEM MAF.

THe results after the fuel pump and new MAF were perfect with fuel trims 0 to +3 or 4,
I DID also reset the ECU by disconnecting the battery overnight after the fuel pump.

A few days later and 300 miles the truck still dose not have any where as much power then it had before and the AFR at high rpms with the methonal spraying is very rich at 10. If the methis not spraying the AFR is good.
I can change out the nozzle to a smaller nozzle size to compensate for the difference in AFR and get it to 12.

So this evening I put my old MAF back in and the truck runs completely different as in excellent.

Has full power with AFR at 12.5 where it should be at full boost with the methonal spraying.

BUT the fuel trims are at +10 to +15 when full acceleration.
Idle is zero.

All this is from just changing out a new MAF for the old one with two very different results.
And the better overall performance power wise MAF by far, is showing long terms at 10-15.

What’s up with this?

Do I run the new MAF with perfect trims With obvious less power
OR
the MAF with high trims that makes much more power over the rpm range