I have an 04 trailblazer with the 4.2. So I tuned my truck with a new PCM from PCM of NC. Since I have had a P0171 code system too lean bank 1. In my tune I had them delete the cat, plus all the usual stuff. My troubleshooting has included checking for a vacuum leak, fuel pump pressure, I replaced my exhaust manifold and upstream O2 sensor, it has a fresh fuel filter and plugs. Is there anything else I can check to make sure it isn't my vehicle having a problem? I'm out of ideas and so is PCM of NC... thanks.
P0171 system too lean after PCM of NC tune
- Thread starter madmanvillain
- Start date
If your fuel delivery was marginal before the tune, and the tune is intended to run richer, the system may not be able to meet the new mixture requirement.
What are your Fuel Trims?
If you have a scanner to graph the O2 you can determine if your fuel delivery is the problem. Under WOT the O2 voltage must be above 0.9 volts. If it drops below that you are not getting enough fuel delivery. With the way the fuel pressure is checked the pump might be making just enough pressure to make spec pressure with no load, but under load the pump may not make enough. The above test will confirm adequate fuel delivery.
Otherwise you may have a vacuum leak, that would show up mostly at idle.
What are your Fuel Trims?
If you have a scanner to graph the O2 you can determine if your fuel delivery is the problem. Under WOT the O2 voltage must be above 0.9 volts. If it drops below that you are not getting enough fuel delivery. With the way the fuel pressure is checked the pump might be making just enough pressure to make spec pressure with no load, but under load the pump may not make enough. The above test will confirm adequate fuel delivery.
Otherwise you may have a vacuum leak, that would show up mostly at idle.
As said above.... You need to check your fuel trims. Check your intake bolts/gaskets. Vacumn leaks. Fuel filter couldn't hurt either. You may have fuel pressure but not good volume...
I have verified per the book my vacuum is good at idle. I'll verify my O2 voltage at WOT tonight. Intake is a pieced together one I made, gutted oem air box spectre filter and 3" abs to the throttle body, no resonator. All the lines reattached as well. I will also check the intake this weekend when I can have a little more down time with it.
A pretty much straight line for as long as you are WOT? Doesn't start to drop after a few seconds?madmanvillain said:
If so your fuel delivery should be OK.
In neutral....
Check your Short Term fuel trims at idle. Should be less than 10% +/-
If it remains close to zero then you likely don't have a vacuum leak at idle.
If it's OK at idle, then bring the RPM up to about 2500
If the trim is off here it may be a restriction in an injector.
Starts at 0.9 and drops to 0.8 and holds. I will check short term fuel trim in the morning when I won't get the cops called on me for revving in my driveway at 3am.
Threw my truck on a better reader. Stft looks decent. What was way off was my air fuel ratio. It was pegged at 14.6 and didn't change no matter the rpm or load or anything. Changed MAP sensor and on my short trip it seemed better but I'll know for sure with more time.
Correct AFR for E0 (100% gasoline) is around 14.7, E10 is around 14.2. your 14.7 is just a bit lean 3.5% for E10 or .5% rich for E0 , I assume your running E10. So your AFR is good.
Remember the PCM does about anything possible to correct an incorrect fuel burn. Beyond keeping the engine running, emissions are its main mission. The P0171 says your PCM is adding over 14% extra fuel to make up for a sensed lean condition. Since it's adding the fuel to correct the burn the O2 readings and any AFR measurements would show results of the correction. If your AFR is truly 14.7 then the PCM is doing it's job correctly to fix something wrong. I asked where you got the reading as that's important was it a wideband AFR or something else, from what I've seen the PCM does not report AFR. It reports LT and ST Trims. Those readings are critical to seeing what's happening. LT trims tell the percentage of fuel the PCM is adding/subtracting and ST trims show the results of the fuel addition (actual burn). ST s/b +/- 5% or lower , LT should not exceed +/- 10%. (0-5% is good) LT exceeding +14% throws the P0171.. for trivia a LT trim of -35% (pulling fuel) rich throws a P0172 the PCM tolerates pulling fuel much more liberally than adding.
Was the truck running error free before the tune, and was it untouched (no mods) between tunes ? . If so have you contacted your tuner.. as asked in post #4 does the tuner know about you custom air intake box. You could have been running 13% lean before the tune and the alterations took it over the mark, Any chance you know what LT trims were before the tune...
Remember the PCM does about anything possible to correct an incorrect fuel burn. Beyond keeping the engine running, emissions are its main mission. The P0171 says your PCM is adding over 14% extra fuel to make up for a sensed lean condition. Since it's adding the fuel to correct the burn the O2 readings and any AFR measurements would show results of the correction. If your AFR is truly 14.7 then the PCM is doing it's job correctly to fix something wrong. I asked where you got the reading as that's important was it a wideband AFR or something else, from what I've seen the PCM does not report AFR. It reports LT and ST Trims. Those readings are critical to seeing what's happening. LT trims tell the percentage of fuel the PCM is adding/subtracting and ST trims show the results of the fuel addition (actual burn). ST s/b +/- 5% or lower , LT should not exceed +/- 10%. (0-5% is good) LT exceeding +14% throws the P0171.. for trivia a LT trim of -35% (pulling fuel) rich throws a P0172 the PCM tolerates pulling fuel much more liberally than adding.
Was the truck running error free before the tune, and was it untouched (no mods) between tunes ? . If so have you contacted your tuner.. as asked in post #4 does the tuner know about you custom air intake box. You could have been running 13% lean before the tune and the alterations took it over the mark, Any chance you know what LT trims were before the tune...
Well the 14.7 is good at idle. At WOT it should drop and it wouldnt drop. Changed out the MAP sensor and it is fixed. Thanks for all the input I really appreciate it!
Glad you got it resolved.... Out of curiosity do you have before and after MAP readings from your scanner. MAP and RPM are used for calculating fuel mix in the '04. Yes you are correct the PCM should richen the mix during WOT called Performance Enhancement (PE Mode) . Depends on engine setup and tune on how much AFR drops. I run my Z28 to low 13's at WOT. Since OBD2 tuning software is so expensive ($500.00) and I have no real performance mods on my Envoy, I purchased my tune working with Lime-Swap.com, he set me up to enter PE at a lower throttle position, that along with some reduced torque management and timing tweaks woke it up off the line.. How do you like the PCM of NC tune ??
I've got mixed feelings. I am incredibly let down on the customer service I recieved. Wouldn't answer the phone or return messages for almost 2 weeks, couldn't work through any troubleshooting with me, and then they gave up on me saying it was an oddity and they had no idea what to do. So far the tune is working as planned, but I'd like to get more milage under it before a final thought.
