2001 Grand Prix Supercharged GTP with recurring P0420 and P0171

[rel3rd]

The MAF sensor AND front O2 sensor are both a month old.

When we first got car, it kinda bucked on light throttle take off from a dead stop. Was getting MAF code....can't remember number though.

I bought aftermarket MAF and cleared code...all seemed good to go. No bucking.
I did try another GM MAF sensor from junkyard and bucking returned, so I swapped the aftermarket one back on.

Few days later got the P0420 code, so just went ahead and swapped in a new Bosch O2 sensor. Figured it was probably original. (120K miles)

Car RUNS GREAT, but keeps getting P0171 and P0420 codes after roughly 4 or 5 drive cycles.

Fast forward to yesterday...

Bring home my scanner since it now has an ABS code.
I also do a live data scan and see front O2 is staying very low voltage at idle, like .120v or less, and rear O2 is staying right near the same....very little fluctuating, and DEFINITELY NOT any rapid, NORMAL O2 voltage movement...

I disabled MIL so it will pass emissions inspection (since it is due now).

But I am a little puzzled why it is idling so lean, and getting very little O2 sensor sweep????

I also noticed on HPTuner datalogs that STFT/LTFT's both stay pretty stagnant, which I thought was odd...

Any ideas?? On what to check, or look for??

Thanks.

[planethax]

Forget about P0420 for now and deal with P0171 first.
Verify O2s working and wiring. Unplug O2 you should have approx 450MV
Check for Vacuum leaks, this will cause a Lean condition.

My gut feeling you won't like.
I have seen the aftermarket MAF on the 3800s do this many times.
More air is entering the intake than is measured by MAF causing it to go lean.

Hard to tell more without a log or tune.

[rel3rd]

Forget about P0420 for now and deal with P0171 first.
Verify O2s working and wiring. Unplug O2 you should have approx 450MV
Check for Vacuum leaks, this will cause a Lean condition.

My gut feeling you won't like.
I have seen the aftermarket MAF on the 3800s do this many times.
More air is entering the intake than is measured by MAF causing it to go lean.

Hard to tell more without a log or tune.

I had hoped to upload a tune and log this morning, but this new fangled laptop doesn't seem to work right with the HPTuners....possible operator error, but being at work I have no time to troubleshoot this issue..lol.

I actually brought the car into work today because it also has a new ABS code as well, and I intend on doing some vacuum leak checks. I'll also verify the unplugged O2 voltage as well.

I am hoping it is a "simple" vacuum leak somewhere, but my old ears cannot "hear" one over the noise of the blower, since we added the so-called "cold air kit".

[rel3rd]

I am convinced that there are some deeper issues here. I spent about 15 minutes spraying Brakleen on anything and anywhere there could/would/should be vacuum at. Found two very, very minor leaks and replaced one hose, and clamped another. Again, both were very minor leaks.

Fired the car up and after a few minutes of warmup started scanning FT's and both O2's.

Had normal rapid movement of front and rear O2's, and movement/fluctuation of STFT's. I thought all was good.

Drove home but stopped at 2 places on the way, both for maybe 15 minutes each. After first stop all still seemed fine but after second stop when car started it was "loping" and idling rough, actually shaking the car.
Hooked scanner back up and O2's were both equal, and less than .100 volts, actually hanging near .070v and STFT's were both pegged out at 20.3%.....
If I held idle up slightly manually, they would both (O2's & STFT's) move but not at a normal curb idle speed....

So now I have no idea what is happening.

BTW this happened as seen on a regular diagnostic scanner not my HPTuners scanner so I still have no log unfortunately....

Any ideas???

I am still trying to post up a log and the tune, however the tune is 100% stock, aside form all fan temps being lowered 10 degrees and P0171 and P0420 codes being set to SES (unchecked)

[BrettBennett]

a few things lately I have see alot of dead 02's. they usally die lean try unplugging it and driving it the puts it in open loop #2 possibly a plugged cat do you get alot of KR? If it's bucking all the time remove the 02 and drive it(it's gonna be loud dont drive it long) If it stops bucking most likely the cat is plugged you can check back pressure though.

[rel3rd]

a few things lately I have see alot of dead 02's. they usally die lean try unplugging it and driving it the puts it in open loop #2 possibly a plugged cat do you get alot of KR? If it's bucking all the time remove the 02 and drive it(it's gonna be loud dont drive it long) If it stops bucking most likely the cat is plugged you can check back pressure though.

O2 is brand new, and as I posted in the reply above yours, it was reacting like it should (as was the rear O2 sensor), then on the 3rd start up, neither was moving like it was, or should.

The rough idle bucking is only happening at a stop, when first started and engine already warm.

No KR unless at WOT and 90+ degrees out. Even then, only 2 or so degrees at most.

I am having logistics problems getting the car in front of me to get a good scan to post.
I work daylight and my son (who's car it is) works 2nd shift.

[rel3rd]

Finally got tune and an idle scan posted.
As you can see, the O2 voltage is stagnant at a very low, and dead, voltage, but as soon as I rev the car a little higher, they seem to work fine

I cannot find any vacuum leaks.

[coachcrow56]

Did you ever find a solution to your problem?

[jaydubb71]

That bucking at idle problem (in my past history) was a funky MAF problem. Had this happen on my GTP before.

[rel3rd]

Did you ever find a solution to your problem?

That bucking at idle problem (in my past history) was a funky MAF problem. Had this happen on my GTP before.

New AC Delco maf sensor has the maf code gone and car running well. Still getting the evap code but it passes emissions test and son is OK with it running good so I haven't went any further into it.