P0171 help...

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sunliner

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My O6 Trailblazer just isn't happy unless the CEL is on...
I drove around for almost two years with an 0455 code, finally got to replacing the vent solenoid two weeks ago, haven't had the 0455 return yet.
However....about two days after I replaced the solenoid to fix the 0455 code, the CEL came back on. This time, it's the ol' P0171.
Had this come up about a year ago...cleaned the throttle body and checked my intake manifold bolts and it had not come back until this time.

The CEL went off and the code disappeared on it's own for a few days last week but it came back two days ago. Interesting...(?)

I took the AIR check valve off and found it quite dirty...took a chance and replaced it with a new one and reset the code last night.
This morning, wife wanted to go to the beach, so we got in the Trailblazer, drove for an hour and a half down the highway, and no check engine light (I swear the truck had more power than it ever has too...maybe my imagination). Anyway...we get back in the truck and as soon as I started it up, the CEL is on again. plugged in the scanner and of course it's the P0171. Just don't know what's up with this thing...
 
Have you ever changed the O2 sensor that is right off the engines exhaust manifold? Usually as it ages that sensor gets faulty and changing it usually helps fix this as long as you are not suffering from a vacuum leak anywhere in the system. With your scan tool, can you view a live data stream of the O2 sensor? If you can get the truck nice and hot up at operating temperatures and post a shot of the graph when you have it resting at idle. It will help us sometimes with figuring out if the sensor is faulty or if it may be alright.
 
will get on that and post back...thanks for the hint!
 
If you can't graph but you can read live data, observe that the O2S1 sensor has a properly-oscillating voltage. Should bounce between 0.1-0.9, should take a second or less to make a bounce.

And by bounce, I would mean go up to top voltage and back down again. One full oscillation. It may be difficult to observe without graphing or a very fast scan tool.
 
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While your at it take a look at the Short Term Fuel Trim it will validate the O2 readings. The Short term is percentage average of a perfect O2 reading. The Long Term trim tells you how much fuel the PCM is adding or pulling, the DTC says the LT trim % was to high (PCM was adding that % fuel to keep things operating) . The PCM bumps the LT trim number to keep the ST close to 0% as possible.
As said above it all assumes the O2 is reading correctly, how many miles on the O2 ??
 
well, I tried the O2 test. got a message saying 'no data returned from vehicle"
At idle with every thing warmed up, vehicle in park
Coolant is at 215
ST FTRM 1 is around 3.9 noted it bounces between 0.0->0.7-3.9 over the course of a few seconds
LT FTRM 2 is at 14.0

also if this means any thing, the CEL went off as I was driving just prior to pulling the above data. the code is still stored but the light is off.

as for the O2 sensors...I bought the truck when it had 72,000 miles on it, currently has about 103,000....for all I know, they have never been changed.
Thanks all for trying to help me with this
-Mike
 
Maybe you have a faulty sensor if it can't read data from it. Can you read data from the Bank 1 sensor 2 O2 sensor?
 
in case the link to the movie doesn't work, looks like the highest the O2S11 v went was .76
O2S12 .780-.810
 
It looks a little slow at responding but nothing that I would say to throw a red flag at. What I wonder though is since you mentioned you had the no data returned error, I wonder if you have an intermittent connection issue with the O2 sensor, whether it be that the connection is corroded or you have a wire that broke through and is grounding out, something is flaky there.
 
kickass audio said:
It looks a little slow at responding but nothing that I would say to throw a red flag at. What I wonder though is since you mentioned you had the no data returned error, I wonder if you have an intermittent connection issue with the O2 sensor, whether it be that the connection is corroded or you have a wire that broke through and is grounding out, something is flaky there.
Click to expand...
just to clarify...when I got that message, it was when I had selected "O2 Monitor Test" on the scan tool-everytime I selected it. I'm still learning how to use the scanner... the video I shot was just the performance data, no problems at all with it showing there.
 
That LT fuel trim says the PCM is adding 14% extra fuel to keep the AFR correct (ST trim at 1%) .
That's the cause of your DTC.
Since you don't have any P030X codes it looks like all cylinders are firing so I would rule out a cylinder not firing. Check your Fuel Pressure first.
If the O2 was reading incorrectly then the PCM would be adding the fuel in error, that would cause in a rich condition, black plugs, possible rich exhaust smell.
Still could be an intake leak, however small air leaks show lean at low RPM and get better as RPM increases, more throttle body air flow so the small air leak gets less impacting.
Is that LT at +14% in all engine operating ranges, does it get better or worse at higher RPM and loads. FP would get worse at loads, vac leak would get better at higher RPM.
 
our local car genius at work said it's time for ' a bottle of Techron and some 'spirited' driving down the interstate"

But seriously...I'll have to explore the fuel pressure issue now that you've brought it up.

The truck sometimes feels like it's idling just a little bit rough when cold, certainly not as bad as some cars I've had but a little rough. 'warm' idle is pretty smooth

Since I got the EVAP system working, it really seems like the truck has a lot more pick up, she used to be pretty weak on acceleration, especially on a long grade in the heat. Both this truck and my old Blazer take off like rockets when the outside temps hit about 60-65.
 
I think the reaction time of the scan tool is too slow to accurately determine whether the upstream switch rate is satisfactory.
 
Checked vacuum today ..not easy to get the hose on since mine doesn't have the fitting on the side of the airbox...anyway, the vacuum at warm idle read right at 17 in/Hg, from what I understand is the minimum normal reading.
Something else I just really paid attention to is noise coming from the brake pedal area when the pedal is depressed...see the link below...could this be a factor? Had the brake booster croak in my Blazer once and it sounded a lot like this if I remember correctly...
http://smg.photobucket.com/user/sunliner/media/Mikes%20Pics/CAM00908_zps933986b6.mp4.html
 
To check the brake booster, unhook the vacuum hose and plug the end of it.
Check your intake manifold vacuum and see if it improves. Just a suggestion.
 
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Texan said:
To check the brake booster, unhook the vacuum hose and plug the end of it.
Check your intake manifold vacuum and see if it improves. Just a suggestion.
Click to expand...
Well, it appears that I'm an idiot (wife was right..).
I had been checking the vacuum with an extra piece of hose since the port on the manifold is in a tight spot. the extra hose kinks, throwing off my measurements. Got the gauge nozzle on there better today, and it reads 20 in/Hg, both with and without the brake booster hooked up. Even with the brake booster hissing at me, still looks like I've got good vacuum.
Now to find a fuel pressure gauge and check that angle out...thanks to all for trying to help.
 
Just a wild hunch, check your intake manifold bolts. Some have been known to become loose.
 
Mooseman said:
Just a wild hunch, check your intake manifold bolts. Some have been known to become loose.
Click to expand...
first thing I checked; had found them loose last year but they were still snug this time...couldn't be that easy
 
The LT FTRM goes to around 11-12% when I'm on the Hwy ~70 mph. Back to 14 at idle and
"around town" speeds.
Going to try to check that fuel pressure. Even though my idling vacuum looks good, still planning on doing the brake booster...now that I notice the hissing, it drives me nuts. Thanks again for the advice.
bobdec said:
That LT fuel trim says the PCM is adding 14% extra fuel to keep the AFR correct (ST trim at 1%) .
That's the cause of your DTC.
...
Is that LT at +14% in all engine operating ranges, does it get better or worse at higher RPM and loads. FP would get worse at loads, vac leak would get better at higher RPM.
Click to expand...
 
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