Why would fuel trims drop slowly between ECU resets?

I'm driving a 2003 Tacoma 4x4 V6 with 170k miles.

So far I’ve cleaned the MAF/IAT sensor, and it is giving what appears to be accurate data. I’ve also tested and cleaned the injectors, replaced the fuel pressure regulator and fuel filter. ECT is reading around 176°F when driving around 30mph in 50°F weather.

If I unplug the battery to reset the ECU, my LTFT drops to 0, and my STFT stays around -1 to -2. Over several weeks, the LTFT incrementally increases each time go driving. After a month or so it will be so far in the negative that the check engine light will come on (P0172), and eventually I'll experience a rough idle and power surges on acceleration. Resetting the ECU starts the process over. If resetting the ECU were to immediately bring back the full negative fuel trims, I’d suspect that I had actual problems with fuel delivery, but because it’s so gradual, it almost seems like a problematic sensor feedback loop.

According to the live data, the upstream A/F ratio sensor appears responsive to changes in RPM, as well as pumping the brake to create a lean condition, but at idle it hangs around 3.25V. Since 3.3V is a normal reading, the A/F ratio sensor appears to be perpetually ever-so-slightly rich. The downstream O2 sensor seems to hang around 0.760V at idle.

The upstream A/F ratio sensor was replaced 10 years ago. The downstream O2 sensor is original.

Before I start replacing the sensors, do you guys agree with my train of thought here, or should I be trying something else first?

Thanks!

 

I would be inclined to think it is a vacuum leak. Probably a very small one since the change is so gradual. How far negative do the LTFTs get before setting the P0172 code?

 

If your LTFT is negative, then your ECU thinks that the engine is running rich, so it's removing fuel from the mixture.

Mike could be right with the vacuum leak, but I think that might introduce more air, resulting in positive LTFTs, since there'd be more fuel needed to get things "right."

A couple more things to check:

1. Generally, if you have (more) reasonable fuel trims at idle, and they get worse under load, then it's unlikely to be a vacuum leak; it's more likely the MAF. This seemed to describe my situation - my idle fuel trims were in the 0-10% range, and then get really bad at higher RPMs and loads.
2. MAF rate should be around 1 g/s per liter of displacement per 500 RPMs. That would mean that at ~705 rpm idle and 3.4L, it'd be around 4.75 - so a reading of 3.6g/s was a bit low.

I've found that cleaning the MAF doesn't always solve the problem; I've had to replace mine a couple of times to solve LTFT issues. Always use a denso MAF replacement, the other aftermarket ones just don't seem to work for long.

You might also check the A/F sensor (a lot of folks call this the O2 sensor). Certainly, it could be sending a mixture reading to your ECU that is "off," causing this to occur.

When I had this issue, I was chasing a P0171, which is essentially the same code. You can check out my troubleshooting here.

 

03/04's have the electronic TB's....are you able to watch throttle position and pedal sensor position while driving around watching live data..

these just as the 95~97's are a special subset, and need someone experienced with these...

The contacts on that pedal sensor is just a rheostat and degrades over time.

This may not be your problem....but all the 03/04 will eventually face this.

 

I did the propane test and didn't hear any RPM changes, but I haven't tried a smoke test yet. Are there instances where a vacuum leak would cause negative fuel trims? I replaced the PCV valve as a precautionary measure, but it didn't seem to have an affect.

It appears to throw the P0172 code once the LTFT stays beyond -20 for a bit.

 

I'll get data on the change in fuel trims under different RPMs and loads, thanks!

I replaced the MAF sensor with an OEM unit from last year and it didn't solve the issue, but because it was an Amazon purchase, I'm not 100% sure it's authentic, so I put the original back in.

My original, cleaned MAF sensor is reading 3.98g/s at 695RPM, so that does seem a bit low.. would a Denso unit from Rock Auto be the same as OEM, or would I be better off buying an OEM replacement?

 

I knew my throttle body looked different from other years, but I didn't know the specifics - thanks for the heads up, I'll go for a drive in a bit and monitor that data as well!

 

Here's a check list.......I'd pay attention to #2 and 3

How to Fix Negative Long-Term Fuel Trim: A Friendly Guide for Car Enthusiasts (blogproautomotive.com)


Wild guess would be check that ect temp sensor as engine heats up...
or even the injectors are shot....fuel reg, ....ect...

The egr ports on intake start clogging up around 160K....
but a lot of the lucky 5vz's never had egr.....

 

I don't believe I have an EGR. My ECT temp sensor is reading around 176°F at idle, which seems a bit low. I sent all the fuel injectors to MotorWest performance to get tested an cleaned, and replaced the fuel regulator with an OEM unit while I was in there. Replaced the air filter with a Denso. I cleaned the MAF, but it's possible that it's faulty.

 

good informative thread.

 

my 2.4 came with the pink oem injectors meant for the 2.7's

After putting in a cold air intake......my idle was like -3......-9 trim at hwy....
....so way too much fuel being dumped in there..

I ordered the brown densos from motor west actually meant for the 2rz.

Now I have zero to +1 at idle......jumps up to +8 trim at HWy
So now with CAI its running fairly lean.....with correct injectors.

 

A CAI seems like a good idea... that would give me more accurate IAT* readings. Right now my reading is only accurate once I've been moving for a while.

 

Based on everything in this thread, I would replace the O2 sensor. It seems to be off slightly, perpetually

 

I don't know why I never noticed this before, but there's a lump in the wiring harness near the MAF sensor connection, and on closer inspection it's chock-full of crimps.

I had P0102 (MAF Sensor voltage low) and P0113 (IAT sensor voltage high) codes a few weeks ago, but when I shut the car off, checked the MAF connection and restarted it, the CEL didn't come back on, so I thought it was a fluke.

I'm going to dig into this area and see if I have any loose connections. I appreciate everybody's help so far!

 

Two week update:

Under the electrical tape I discovered the MAF plug had been cut off the wiring harness and a new plug had been crimped in its place with cheap crimps. Two crimps were completely loose and pulled right out. I soldered the wires together and covered them in marine grade shrink tube. Fuel trims are now slightly positive, and the truck sounds so good at idle. No more stumbling. Looking forward to better fuel economy!

Thanks for all the help and ideas guys!