Troubleshooting P0171 DTC on 2000 4Runner 5VZ-FE

History:
I had the Check Engine Light turn on with a P0171 code a month ago. I pulled the ECM fuse and (not knowing it was a P0171 code at the time) tightened the gas cap since my wife just put gas in the truck and there seemed to be no pressure build-up when I released the cap. The CEL dissappeared and I thought that I had solved the problem. Engine has run fine throughout this problem.

I then drove it a couple more weeks and the CEL came back on. I tested it and found a P0171 code "System too Lean (Bank 1)". The last time I had this DTC show up it turned out to be a contaminated MAF meter from my oil-whetted air filter which I was able to cure by cleaning the MAF meter with no-residue electronic circuit cleaner.

I drove it for a few more days and the CEL went out again by itself. I decided to clean the MAF meter and hope the light would stay off.

I removed the air filter and replaced it with a paper filter to keep oil contamination from happening again (if it had any effect this time to begin with), removed the air cleaner housing from the truck (three bolts, a MAF meter connection, a wire clip, a small hose connection and the intake air hose. I then sprayed the MAF meter thermocouple and the inside of the MAF meter thoroughly with residue-free electronic cleaner, sprayed the MAF meter connection with the cleaner, and replaced everything in the truck. I then checked the remainder of the intake system for loose connections (none found).

A test drive immediately gave back the P0171 code. I drove back home and, with the help of my handy Actron CP9180, I downloaded the freeze-frame data from the ECM. It showed the following:

Freeze-Frame Data
Trouble Code P0171
Eng. Speed RPM 2118
Calc. Load % 51.3
Coolant temperature F 192
Inlet Air Temp F 89
ST FTRM1 % 0.7 (Short-term Fuel Trim 1)
LT FTRM1 % 35.9 (Long-term Fuel Trim 1)
ST FTRM3 % -100.6
LT FTRM3 % -100.6
VEH Speed MPH 63
FUEL SYS 1 Closed
FUEL SYS 2 N/A

To be continued as time permits....

There is a way to check if the MAF is functioning as should be. With a volt meter, you can check a couple of terminals in the MAF and blow through it to see if the voltage changes. I wound up having to replace the MAF on my 2000 Tacoma. As soon as I get my office cleaned up, I should be able to have access to my manuals again. I may even get to it tonight, so if I do, I'll shoot you the info on here.

Thanks for the input. I have a set of Toyota manuals for my truck. I just haven't got around to continue troubleshooting the truck. It's running fine and I have alot going on right now that keeps me from my garage. :)

New Results:

Deleted codes and obtained new Freeze frame data:

Freeze-Frame Data
Trouble Code P0171
Eng. Speed RPM 1906
Calc. Load % 38.4
Coolant temperature F 192
Inlet Air Temp F 122
ST FTRM1 % 1.5 (Short-term Fuel Trim 1)
LT FTRM1 % 39.8 (Long-term Fuel Trim 1)
ST FTRM3 % -100.6
LT FTRM3 % -100.6
VEH Speed MPH 37
FUEL SYS 1 Closed
FUEL SYS 2 N/A

Fuel Trim data total 41.3% indicating a problem - what a surprise! - (Total should be less than 35%)

Checked PCV valve and hose. PCV valve has good tight connections and hose is solid with good connections.

Tested MAF meter according to FSM:
http://i16.photobucket.com/albums/b8/rogersb/P0171%20Files/MAFTest.jpg

I removed the air box with the MAF meter, opened up the end with my deck plate and placed the air box over an air conditioner duct. I found a good thermometer, placed the thermometer bulb right next to the thermistor in the MAF meter, and turned the air conditioning in my house on. I moved the air box so that the a/c slowly cooled down the MAF meter. Once the a/c cooled the thermometer exactly to 68F, I measured the resistance between the THA and E2 terminals.

Resistance = 2.66 kOhms at 68F (Within specs)

Then, I connected the MAF meter to its connector in the truck, pulled back the seals on the VG and E2G terminals, turned on the ignition, and blew into the MAF meter. The voltage went wild, indicating a responsive MAF meter.

I then tested the ECT sensor according to the chart at the bottom of this image:
http://i16.photobucket.com/albums/b8/rogersb/P0171%20Files/ECTTest.jpg

Layed the thermometer on the engine next to the ECT sensor, removed the connector, and tested the resistance.

Resistance = 1.09 kohm at 110F (indicates good ECT sensor)

So, both the MAF sensor and the ECT sensor are likely to be good.

Further testing when I have more time....

Here is the diagnotic procedure as shown in the FSM:
http://i16.photobucket.com/albums/b8/rogersb/P0171%20Files/P0171-1.jpg

Update: My scanner wouldn't monitor the output of the A/F ratio sensor. I decided that the long-term fuel trim and MAF meter high resistance pointed to the MAF meter and bought one ($136). Installed it and no DTC code yet, having driven it all day - on and off. The old MAF meter was really dirty on the side I had a hard time getting to in order to clean it. Cleaning the MAF meter after removing it instead of cleaning it while mounted to the air box might have solved the problem. I'll clean the old MAF meter and reinstall it to see if that was the problem (However, I don't see how deposits on the thermistor could cause a change in circuit resistance).

Update - eight months later and still no DTC code. Replacing the MAF meter solved the problem.