Troubleshooting a PO171

Subject- 1999 SR5 4WD Standard tranny 3.4 with 180K on the clock.

Problem- Code 171 almost every morning

Whats new or been done- New cat to clear that CEL, fairly new front O2 sensor. cleaned MAF, replaced valve cover gaskets and inspected all hoses or replaced a few, new PCV, new fuel filter.

How it runs- perfect, no shake or stall and idles smooth as can be. Fuel mileage is at 19+ with plenty of power. Couldn't ask for a better running motor.

What's happening is a code for Bank 1 lean will set and clear every day or two. I have watched the STFT - TP - MAF data and the STFT will spike (which sets the code) when I get to about 50+ MPH and hold a steady throttle. Fuel trim works at correct numbers until it heads north to +19.5 which is the limit of the graph. When over an overpass and the engine load going up did NOT see the STFT go back to correct range but when it started down hill, the trim came down to working in the correct range. MAF was 32g/s and TP was at 18% and stayed there before and after the trim #'s spiked. Was watching data and slowed to go through a small town. STFT was spiked until I kicked off the cruise. Data was normal until about a mile after I resumed speed when it spiked again. Again, even when this is happening, the truck runs great. WHAT IS IT?

There will be a credit for correct answers.

 

When I had that code I ended up replacing the maf sensor to fix it. Nothing else I did helped and the truck ran perfect at the time.

 

If you floor it and take her up to about 4500RPM, what does your MAF sensor read? My truck would hesitate at higher throttle levels and eventually gave P0171 (2003 3.4L). MAF was about 89 g/s at 4500RPM on the original MAF. When I replaced it, the reading went to about 115 g/s for 4500RPM. I never cleaned the original since it had 140K on it are <$70 for an original Denso at Amazon. . Your symptoms are different, but You might compare to see if your air flow is similar to the new MAF to decide if you want to try a new MAF.

 

as per http://www.obd-codes.com/p0171

A code P0171 may mean that one or more of the following has happened:

The MAF (Mass Air Flow) Sensor is dirty or faulty - Note: The use of "oiled" air filters may cause the MAF to become dirty if the filter is over-oiled. There is also an issue with some vehicles where the MAF sensors leak the silicone potting material used to protect the circuitry.

Failed or faulty oxygen sensor (bank 1, sensor 1)
Sticking/plugged or failed fuel injector
Low fuel pressure (possible plugged/dirty fuel filter!)
Exhaust leak between engine and first oxygen sensor

Read more at: http://www.obd-codes.com/p0171
Copyright OBD-Codes.com

these are the more likely candidates. try some mas air flow cleaner first and/or changing the fuel filter.

 

I did that and go about 90 g/s. I read where the MAF should read .8 X horsepower. I guess that might be worth a try. I'm just trying to understand the part where the STFT changes suddenly and the MAF is unchanged. Almost like something is loosing connection on a regular basis.

 

New MAF = no change

 

you have a vacuum leak between your MAF and the intake manifold ports of your head. what's weird is your code says lean, but you're actually running rich. your trims are adding ~19% of fuel due to the data coming form your o2 sensors.really you have no real issues aside from that. just vacuum leak. and it's not doing any real damage because the data from your o2 sensors are still adjusting the fuel trim accordingly. that's why you're still getting good mileage and performance. if anything, you just get a high idle. big woop.

i had this issue when I got my truck. I studied my fuel trims live for 2 years every day i drove with an elm327 and the torque app on my phone. figured out that the brake booster was one of the problems when I was parked and tried to push the pedal to the floor with no avail. then I changed the air intake boot cause it cracked soon after I bought the truck. those got everything down from 19% to about 5-7%. then I changed the rest of the vacuum tubes, pcv valve, and the pcv valve grommet and now my truck runs -+3% from 0 all the time. perfectly.

im 100% sure you are in the same boat as i was.

fun fact. your fuel trims will go as high as +- 25% before throwing a CEL.

 

Pulled the air filter after the MAF didn't help, went out for a drive yesterday and noticed the trim went to - 20. (NEG 20) What?
I have the intake boot off and the filter box out now. I snapped the MAF housing loose from the filter box and have JB weld setting up on it over night. I see some stress cracks in the boot but none look like they go through. Going to do a propane test today.

 

Update..........

Went to a friends shop that has a smoke machine and hooked her up. No smoke = no leaks. No smoke. As a last straw I picked up a new up-stream O2 sensor and changed it out. Old one was changed just before I bought the truck. Anyway....... it seems to have done the trick. It has a lot more response and swing than the old. Test drive shows the STFT #'s are staying + or - 4, which is great. No spikes.
Goes to show you that if you throw enough money at a problem you can fix it.