ok so i was driving home from work today and my car started to bog down....if you gave it gas it would want to die...but it would idle fine(for the most part)....so i left the car on the side of the road...the wife came and picked me up and we went home(about 5 minuites away) and grabbed my laptop...got back to the car...plugged in the computer and pulled code
P0171 POWERTRAIN: FUEL TRIM SYSTEM LEAN BANK 1
tried to start the car and it fired right up and ran without a problem all the way home...

so...whats this code actually mean and what could be wrong with the car???

 

Hmm, check your fuel injectors.. Might have one failing on you.. Also I would check the pressure at the rail if it happens again.. Maybe a bad fuel pump? I am voting injectors though..

 

Start with fuel pressure, should be 49psi, then injectors, and then fuel pump. I'd go with the walbro pump if it needs to be replaced. Could also be an O2 sensor. Pull the plugs and see how lean it's running.

 

What it means is that your PCM is adding more than 20% of fuel to the programmed amount for some reason. The PCM checks a lot of variable for a baseline fuel injector pulse width - TPS, RPM, , MAP, and IAT are the big ones, but also coolant temp, battery temp, etc. But in closed loop, this gets adjusted to keep the upstream O2 sensor happy, and it's only happy when it's reporting a 14.7:1 air/fuel ratio. So if the O2 sensor is detecting a "lean" condition, the PCM will compensate by adding more fuel. The PCM also keeps track of how much extra fuel is being added over time. Short Term Fuel Trim (STFT) only as long as the car is running. This helps keep your a/f steady as you vary rpm's and engine load driving around. Long-Term Fuel Trim (LTFT) data is only kept up to 4k rpm, and gets stored by the PCM for use the next time your car starts up and runs as an automatic correction to the baseline.

Anywho... If either of those gets to the point that it's adding more than 20% of fuel beyond the original baseline, you'll get a code for Fuel system lean. If more than 20% is getting pulled compared ot baseline, you'd get a rich code. It's been a while since I've looked up the specifics on this, but I'm sure I'm in the ballpark... someone with a FSM handy could double check it all...

So you might actually be running lean, but it could also be a faulty sensor. Coudl be the O2 sensor, but it could just as easily be the TPM, MAP, or IAT. I've even seen a faulty coolant temp cause a rich code, along with a nasty idle... The sensors may not be bad enough to throw codes for themselves, but bad enough to cause other problems.

I'd go through and start checking the voltage/resistance ranges of each sensor individually. Specs are in the FSM, Haynes/Chiltons, and even on Autozone's online help if I remember right... Replace anything that doesn't seem right. If your O2 sensor is more than 100k miles old, I'd replace it (unless it's a Bosch, which I'd replace after 50k... oh wait, I just did!).

Best of luck!

 

Check for a VACUUM leak. Lean conditions are caused by vacuum leaks alot. More air than expected. Do not rule this out. Check manifold vacuum should be ~20"hg. They covered most of the rest above.

 

i will check everything out later today if i have time....me and the wife are planning on sleeping all day..plus its supposed to have a heat index of 110 today so im gonna stay inside.....
but if i get a chance or it happens again then i will just start replacing stuff.....all the sensors are stock...the only stuff thats been replaced on the car..other then the suspension..has been the fuel pump..which i later found wasnt the problem, wire was pinched...
otherwise nothing has been replaced so its probally about time