I am in CA, have a '99 Toyota 4Runner w/ about 120K miles on it, and recently failed my smog test for the 2nd time. The first time the OBD fault code was, PO171(System too lean). So after doing some research, I thought the problem was a dirty or faulty MAF sensor. I thought I was showing all of the symptoms of having a dirty sensor and knew that it was a pretty common problem with the 4Runners. After the first smog test, all of the test results alone would not have failed the test though were close, but it failed only because the engine light was on. The test results from the 1st test are below:

%CO2 %O2 HC (PPM) CO(%) NO(PPM)
Test RPM MEAS MEAS MAX AVE MEAS MAX AVE MEAS MAX AVE MEAS
15 mph 1656 15.1 0.1 50 8 50 0.64 0.02 0.21 508 88 354
25 mph 1630 15.1 0 34 6 22 0.77 0.02 0.06 761 82 107

I know the O2 level was right on the verge of failing and the NO at 15 mph was high, but everything else checked out besides the engine light. The fault code along with these results is what prompted me to clean the MAF. I have had the car for about 60K miles and have never cleaned the sensor myself or ever had it replaced. Another disclosure which made me think even that this was the problem, the car would idle strangely sometimes at stops AND would not get the normal pickup and power when the pedal was on the floor around 70 MPH. So I used the specific CRC MAF cleaner. Sure enough when I took out the sensor, one side was clean and shiny, the other was black covered in crud. So I thanked my fortune that it wasn't something more serious and hoped that this was the problem. I put everything back together and made sure all of the hoses had good seals and no leaks and checked them with carb cleaner. I took off the negative terminal on the battery to reset the system and the engine light went off of course. The car was running better than it had before(didn't stutter at low speeds/stops and had more power). The only thing, the first mile or so of driving after this, at real low speeds, there is a slight hiccup when accelerating but once over about 20 MPH the power is there and the car runs great. So I take it back to the smog tester feeling more confident that I fixed the problem enough to pass the test. I fail this time straight out by the numbers. The results of the 2nd smog test are below:
%CO2 %O2 HC (PPM) CO(%) NO(PPM)
Test RPM MEAS MEAS MAX GP MEAS MAX GP MEAS MAX GP MEAS
15 mph 1652 14.8 0.4 50 276 9 0.64 1.94 0.02 508 1538 1941
25 mph 1637 14.9 0.4 34 226 6 0.77 2.08 0.01 761 1588 754

This time the HC AND the CO levels went down, but the NO went WAY up. A couple things that I have read since that may be the problem is a bad/dirty O2 sensor or a EGR problem. I was hoping that putting up the results of the two tests can help me figure out what the problem is from before and after cleaning the MAF sensor. Any help is VERY appreciated!

 

here ya go.
step one..go to dollar store and get rubbing alcohol
step two..poor into tank
step three...drive to smog testing location
step four...pass it.

 

Besides cleaning the MAF at 120k, what other maintenance have you previously done? I suspect nothing.

 

I change the oil every 3K miles, recently replaced the timing belt, and put on new brake shoes.

 

I just wanted to follow up with my post and possibly help someone else out in the future that is having the same problem. I figured out what the problem, which was a FAULTY MAF sensor. I cleaned it thoroughly and thought that since the other numbers went down that it was still working and there was some other problem with the car. Anyway, after getting some outside advice, I just replaced the sensor myself and passed the Smog inspection with flying colors.

 

Those numbers make my eyes buggy. If all you need to pass is Nox, check the ignition timing, if it has a distributor, make sure it's not over advanced. O2s seem to be awake now. Check EGR operation, lots of Toyota EGR control solenoids[Vacuum switching valve] EGR vaccum modulator.Rarely the valve itself. Fresh cat convertors clean nicely.

 

How many more people on the "95.5-2004 Tacomas & 96-2002 4Runners" forum will suggest to check the distributor?

 

Sorry, unlike you "great" dealer technicians, I don't memorize every models' powertrain management application. I am not lucky to work on the same thing every day. It would be great to have the factory figure out all the problems so I could just hang the parts. I only get to figure the problem out when I see the vehicle, then I do my best to forget it until the next one.

 

dude I just adjusted my distributor last week when I was doing the deckplate mod on my flux capacitor. Soo I am not sure where you are getting your info from.....