2006 Trailblazer 4.2 P0171 - FIXED

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?

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.

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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Here is some data. The wave form is the MAF. Thoughts?



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Have you done a visual of all the vacuum hoses/connections, in addition to the smoke test?

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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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.

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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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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I kept thinking that I was hitting leaks with the propane but the trace above was done without propane...

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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:

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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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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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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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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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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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?

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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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.