Help needed with P0171 & P0174 running lean at idle. My fault!

Kapitein

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Hi folks,

Looking for some advice, I've come as far as I can go without further help from people more experienced with the platform.

A few weeks ago I replaced the oil pressure senor on my 07 Yukon XL 6.2. That all went great, but during that process I had to remove the two PCV/Evap lines that run down the side of the valve cover on the drivers side of the motor. One I believe is dirty air and goes from the inlet manifold to the valve cover, the other is a clean line that goes from the evap valve on the side of the engine, to a port on the rear.

After replacing the oil sensor, I put everything back together there was a really loud hissing noise, the motor ran like crap and the short term fuel trims were up at +50%. After playing with those two lines I managed to get the obvious hissing noise to go away, but at idle the STFTs would go from <1% up to +25ish%

This trips the CEL for P0171 and P0174 system too lean bank 1 and bank 2.

To cover my ass I ordered up the 2 lines and replaced them. Both had a broken or cracked boot/clip (see photo). This significantly helped but has not fixed the problem. At idle the STFTs still cycle up to around +10-15% (and then back to low values) which sets the CEL and triggers the stabilitrak warnings etc. The car also has a slightly rough idle.

Using live data I can see the problem is only at idle... at 2500rpm the STFTs are 0% and the warnings disappear and the car drives perfectly. So I have air ingress/vacuum on the intake side and its driving me nuts.

To cover all bases I have changed the whole inlet tubing an MAF sensor for a known good one I had on the shelf, made no difference.

This is obviously something I have caused when changing the pressure sensor, but can anyone offer advice on what to check next or do next... smoke test maybe?

See attachments for various screenshots and PIDS when the code(s) set.

Appreciate any and all input, thanks!
 

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donjetman

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if you removed the intake manifold? you may not have put it back together correctly?
 

Rocket Man

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My only suggestion is to go back and check your work, both the first work and the follow up work. Remove those 2 lines, check to see that they are seating and sealing properly, use a mechanics mirror and a flashlight to look around the area you were working in. Might be another vacuum hose or even a loose electrical connection to a fuel injector. It might be right in front of your face too. Maybe you pushed or pulled on something that didn't like being touched.:p
 

Foggy

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Agree with rocket man....
You may have inadvertently touched a vacuum/ air line that was already
in a deteriorated condition
That's a pretty big air leak for the FTtrims to go to 25* quickly
Triple check all that good stuff
 

Fless

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On the app that you're using, what do the three blue values above each PID value represent? L to R, I assume "low reading" "?????" "high reading"? What's the middle value?

Since those are STFTs, the current PID value is only for this ignition cycle since STFTs reset.

Screenshot_20221124-123608-1.png
 

B-train

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In your wiggling around under the hood, is it possible you knocked the PCV line off by the intake tube for the throttle body on the passenger side of the motor up front? Just throwing stuff at the wall at this point.....

I agree with all of the above as well. I've done the same job and it seems that you almost have to lay on the engine to get it done
 
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Kapitein

Kapitein

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Thanks for the input so far. I have been back today and double checked everything, and to be honest it was as expected... it looks fine!

I checked the brake booster vacuum line, and when I pushed on it, there was a slight click.... so I wondered immediately if I had found the problem....

Unfortunately not, although it seems the trims are slightly better. Its still running lean on bank 1 and 2 and its now recording short term trims at about -4% average.... thats enough to trigger the pending CEL/actual CEL and trigger the stabilitrak warnings at idle.

As soon as you are driving/giving gas the trims are perfect and it performs great. ECU PIDS are showing zero misfires. Its not a randomly occuring issue, im sure of it. This has to be down to me messing around in there.

What would you do as a next step? Replace the booster line just to be sure? Maybe it has a bad connection. If I listen when the car is idling I *think* I can hear a slight hissing noise above the sound of the valve-train etc, but its incredibly difficult to tell.

Smoke test at a mechanic?

I hate problems like this, especially when you cause them yourself. It was running great until I changed the pressure sensor.
 
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Kapitein

Kapitein

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On the app that you're using, what do the three blue values above each PID value represent? L to R, I assume "low reading" "?????" "high reading"? What's the middle value?

Since those are STFTs, the current PID value is only for this ignition cycle since STFTs reset.

View attachment 385942
The middle value is the average, since last engine start I presume.
 

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