Help us help you. By posting the year, make, model and engine near the beginning of your help request, followed by the symptoms (no start, high idle, misfire etc.) Along with any prevalent Diagnostic Trouble Codes, aka DTCs, other forum members will be able to help you get to a solution more quickly and easily!

2006 Trailblazer 4.2 P0171 - FIXED

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5 years 8 months ago - 5 years 7 months ago #22169 by Tutti57
Freeze frame shows code was set at idle at operating temp. I have no seen super high fuel trims at any point yet, with LT usually being around 8 and sometimes a total equaling around 15. I'm having a hard time interpreting the data on the fuel trims because they don't really seem to go one way or the other with rpms. O2 looks good, fuel pressure is at 57psi, I smoked the intake and found no leaks, IIRC the MAF g/s was around 180 under full throttle, air filter is clean, no EGRs on these engines. There is a TSB for clogged injectors, so I could do a balance test. I can try pinching the brake booster hose, and also look at the MAP sensor.

Any other ideas or common issues you've seen on these engines?
Last edit: 5 years 7 months ago by Tutti57.

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5 years 8 months ago #22177 by Tyler
Replied by Tyler on topic 2006 Trailblazer 4.2 P0171
Possible purge valve sticking open intermittently? But those almost always generate a P0496. You'd have to watch the tank pressure to catch it.

If you haven't already, maybe eyeball the MAF anyway. I've had debris stuck in the sensor that moves around and changes the reading accordingly.

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5 years 8 months ago #22237 by Tutti57
The MAF looks ok but I was going to check it with the scope too. I think the purge is a possibility too because of the fuel trims not always bring high. Thanks. I'll report back my findings.

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5 years 8 months ago #22243 by Tutti57
Here is some data. The wave form is the MAF. Thoughts?



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5 years 8 months ago #22254 by Chad
Replied by Chad on topic 2006 Trailblazer 4.2 P0171
Have you done a visual of all the vacuum hoses/connections, in addition to the smoke test?

"Knowledge is a weapon. Arm yourself, well, before going to do battle."
"Understanding a question is half an answer."

I have learned more by being wrong, than I have by being right. :-)

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5 years 8 months ago #22255 by Tutti57
As much as I can see. I know there are a few hoses under there. Does that data look look a vacuum leak? I see that ltft drops with room but the stft seems to adjust that back up immediately.

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5 years 8 months ago - 5 years 8 months ago #22284 by Tyler
Replied by Tyler on topic Re:2006 Trailblazer 4.2 P0171
Yeah, those trims aren't pretty, but still not bad enough. I don't immediately suspect a vacuum leak, as (to me) the total trim works out to around the same with the raised engine speed.

There's a couple PCV hoses underneath the intake 'box' that are easily missed if it's been reinstalled recently. But, leaving those off almost always generates a bigger trim issue than what you're seeing.

Any chance that E85 ended up in the tank at some point? If it did, and they've filled the tank with regular gas since then, you could be looking at a mix of the two. Fuel quality issues will tend to skew trims at all load ranges, in my experience.
Last edit: 5 years 8 months ago by Tyler. Reason: reasons, fight me

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5 years 8 months ago #22287 by Tutti57
All good thoughts. I remember the holding elbow hose and it didn't have any clamps on it, so I added two. I don't think it usually has them but it does now. I might have also found a cracked vacuum plug.

There are now some cam codes and I don't recall exactly which ones, maybe p0011? but the oil level was a little low and way past a change. Im doing a bunch of things on this truck but plan on starting it back up tomorrow to run the drive cycle and try to inspect it.

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5 years 8 months ago - 5 years 8 months ago #22296 by Tutti57
The cam codes are gone but fuel trims are all over the place. I'm starting to think the O2 might not be good.




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Last edit: 5 years 8 months ago by Tutti57.

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5 years 8 months ago #22300 by Tutti57
I kept thinking that I was hitting leaks with the propane but the trace above was done without propane...

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5 years 8 months ago #22308 by Tyler
Replied by Tyler on topic Re:2006 Trailblazer 4.2 P0171
And it's in closed loop, short term trims chasing the O2 during the same capture? Just making sure the PCM wasn't running some monitor while you were watching it. :silly:

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5 years 8 months ago #22353 by Tutti57
It was in closed loop. I'll try it again sometime this week to check again. I didn't think of the monitor thing.

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5 years 7 months ago #22362 by Tutti57
I think you were right about it running a monitor it something. I reset the fuel trims and took it for a 15 min drive and it's looking good now. Maybe I didn't give it enough time to reset. I got excited when I saw this picture going on but then it started acting a fool. I think it's good now. Thanks.



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5 years 7 months ago - 5 years 7 months ago #22416 by Tutti57
Not fixed! Fuel trims have been around 3% the past few days. Today the light is back on for p0171 again and ltft is up to 17-20% and stft is near zero. Engine is fully heated up. I hear the purge running and verified by touching it. I believe the last time I looked at the data it said the purge command was at 100% if that means anything.

If I give it some throttle, ltft drops down to 10% but stft goes up to 9%.

I think I'm going to pull the purge valve off tomorrow during lunch and vacuum test it.

Any other ideas it things I should recheck?

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Last edit: 5 years 7 months ago by Tutti57.

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5 years 7 months ago #22417 by Tutti57
Could I also test the purge by running the engine, closing the vent valve, opening the purge to let the engine pull some vacuum, then shut the valve and watch the tank pressure?

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5 years 7 months ago - 5 years 7 months ago #22424 by Tutti57
This morning I reset fuel trims and filled the tank. The evap monitor doesn't run until there is between 20 and 80% fuel in the tank. I drove it 25 miles and the ltft never got over 5%.

I'm going to watch mondes video in a minute but I feel like the trims went up recently after the tank dropped under 80% and the monitor was running.

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Last edit: 5 years 7 months ago by Tutti57.

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5 years 7 months ago #22452 by juergen.scholl
Tutti,
in your very first post you also mentioned the tsb on clogged injectors. I have seen the injectors clogged on thes 4.2 engines OFTEN. Did you ever do an injector pressure drop test?

An expert is someone who knows each time more on each time less, until he finally knows absolutely everything about absolutely nothing.
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5 years 7 months ago #22477 by Tutti57
That is on my list to check. I didn't go after that right away because the problem seems to be intermittent, but I am going to check then anyway. I'll get to the rest of the tests this week. Plans got a little messed up when my house water tank started leaking.

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5 years 7 months ago - 5 years 7 months ago #22539 by Tutti57
Oddly enough, I've driven it about 60 miles and it seems to be fine. Trims are looking good so far. I see that the evap monitor has not run though.

I did have a p0014 and saw that the actuator was not really moving when I out power to it, so I replaced it. After a few drive cycles I see that the code is pending again, so I'll have to look into that.
Last edit: 5 years 7 months ago by Tutti57.

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