P0171

steve-in-ga

Original poster
Member
May 16, 2014
8
My wife's '04 4.2L I6 trailblazer has started setting the P0171 lean on bank 1 code. I am trying to determine if this is too much air or not enough fuel. I used my reader to clear the code and look at fuel trims at idle with the vehicle warm. The LTFT is solid at 8.2% and the STFT oscillates between +5% and -3%. The RPMS are around 600.

While driving, the code will be set and the RPMS will go to around 900 at idle and will not return to 600. If I clear the code, they will immediately drop back to 600 at idle. I also noticed that if at idle with the code cleared, when I turn on the AC compressor, the code will immediately be set and the RPMS jump to 900+.

Any advice on how to determine if this a lack of fuel issue or too much air issue?
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
Welcome! I'm not the specialist on these, but the LTFT needs to go close to 25% to set the code. Looking at the numbers while driving or datalogging them for later analysis might be useful.

My memory seems to tell me that owners have more problems with vacuum leaks than leaky injectors, so you might slap a vacuum gauge on it. Intake manifold bolts are notorious for getting loose onthe I6, but they're also notoriously difficult to get to except by feel or through the wheel well. Also the brake booster diaphragm is a suspect for leaking - pull the hose and plug it off and see what changes. And loose, cracked or disconnected cavuum hoses, of course.

How many miles?

Any other maintenance history?
 

Robbabob

Member
Dec 10, 2012
1,096
Welcome to the Nation!

Seems to be quite a few of these codes lately. Our platforms are getting old.

Several possibilities, easiest and common is that vacuum check. You're in good hands here, neighbor.
 
  • Like
Reactions: UsernameIsTaken

bobdec

Member
Apr 19, 2013
233
Agree vac leak is most common. Usually a vac leak (if small) will have less impact as RPM and load increase. A lean caused by a fuel delivery problem (filter, pump, injector) will usually get worse under load. Also since MAP and RPM are used to calculate fueling in the '04 a marginal MAP could be a culprit. With key on engine off do BARO (barometric pressure) and MAP (Manifold Absolute Pressure) both read the same (usually 97-100Kpa depending on weather/altitude). At idle your PCM has to add 8% fuel to keep the ST Trims close to 0% (-3 to +5). As roadie says would be interesting to see what trims look like under load, 8% is not really that bad, not enough to set the DTC it must get worse at specific times or conditions.
 

steve-in-ga

Original poster
Member
May 16, 2014
8
I hooked up the scanner and drove the car, as RPMs got higher the LT trim got worse calling for more and more fuel. The O2 sensor seems to be operatong correctly. As you all stated, I suspect a fuel delivery problem over a vacuum leak at this point. I am going to get a fuel guage and and see what that reading is. I think I am also going to clean the throttle body and check the MAP sensor.

Does that seem like a sound plan with the readings I am geting at this point?
 

KNBlazer

Member
Feb 8, 2012
811
What about your Fuel Pressure Regulator? Sometimes the easiest things are overlooked....
 

IllogicTC

Member
Dec 30, 2013
3,452
KNBlazer said:
What about your Fuel Pressure Regulator? Sometimes the easiest things are overlooked....
I support this suggestion. FPRs are a common enough part needing swapped around here (they're not necessarily trouble parts, just tend to fail more often than other parts, relatively speaking).
 

steve-in-ga

Original poster
Member
May 16, 2014
8
I checked the fuel pressure without the vehicle started and it is 52 which is in the range. When I start the vehicle, it goes to 55 which according to the manual it should drop if the fuel pressure regulator is good. I pulled the vacuum line with the vehicle running and it made no difference in the fuel pressure. So I checked the vacuum at the resinator and there is no pressure. According to the manual, there should be 10-14 untis of vacuum. I hooked up my vacuum pump to the fuel pressure regualor and every time I hit it the fuel pressure drops.

I think the FPR is fine, I just have no vacuum at the port on the resinotor where the vacuum line connects to the FPR.

Where does the vaccum at the resinator come from that goes to the FPR?
 

steve-in-ga

Original poster
Member
May 16, 2014
8
My wife has had a few things done to it at a local garage so I do not know the answer to that question. What kind of parts are you referring to?
 

KNBlazer

Member
Feb 8, 2012
811
steve-in-ga said:
My wife has had a few things done to it at a local garage so I do not know the answer to that question. What kind of parts are you referring to?
Just trying to build a timeline of when exactly it was that the problem was noticed...other members report that sometimes when using non oem parts create other issues as they don't work well with our trucks...
 

steve-in-ga

Original poster
Member
May 16, 2014
8
nothing has been replaced in a year or so.

not having any vacuum from the resinator vacuum port to the fuel pressure regulator is a definite problem as fuel flow is restricted. I need to figure out why there is no vacuum at that port.
 

Texan

Member
Jan 14, 2014
622
If you look to the right from the port on the resonator, there is a port on the intake manifold. On my 03 short wheelbase TB that port has a rubber cover.
 

thisguy1985

Member
May 17, 2014
7
steve-in-ga said:
nothing has been replaced in a year or so.

not having any vacuum from the resinator vacuum port to the fuel pressure regulator is a definite problem as fuel flow is restricted. I need to figure out why there is no vacuum at that port.

i just seen this, it explains why theres no vacuum at the resonator.
http://gmtnation.com/forums/topic/10911-fuel-pressure-regulator/

im also getting the p0171 code and goin crazy tryin to figure out why :confused:
 

TrailBoss

Member
Oct 27, 2018
1
USA
My wife's '04 4.2L I6 trailblazer has started setting the P0171 lean on bank 1 code. I am trying to determine if this is too much air or not enough fuel. I used my reader to clear the code and look at fuel trims at idle with the vehicle warm. The LTFT is solid at 8.2% and the STFT oscillates between +5% and -3%. The RPMS are around 600.

While driving, the code will be set and the RPMS will go to around 900 at idle and will not return to 600. If I clear the code, they will immediately drop back to 600 at idle. I also noticed that if at idle with the code cleared, when I turn on the AC compressor, the code will immediately be set and the RPMS jump to 900+.

Any advice on how to determine if this a lack of fuel issue or too much air issue?
With my 2006 Trailblazer I6 I had had the P0171 code on and off for awhile, but it eventually became a permanent thing. Both the method of spraying carb cleaner around and the use of a smoke machine indicated that the intake manifold was leaking. I decided to just tighten the bolts and see if that would solve the problem instead of replacing the manifold gasket. There are ten bolts and the ones closest to the front of the engine were not loose, but as I moved further back the bolts became progressively looser. Towards the middle of the manifold the bolts were definitely loose requiring perhaps a quarter turn to tighten them, the ones toward the rear took perhaps a full turn, and the very last one probably two full turns.

I used a variety of 1/4" drive tools to get the job done., including a variety of extensions including wobbler extensions, universal joint, and regular and 10mm deep socket.

I did unbolt the ECU to improve access, as well as removing the large vacuum hose going to the brake booster. The front ones would be much easier if the alternator was removed, but I left it on. I also disconnected the battery because toward the rear of the engine the hot terminal of the starter is fairly close to where you will be working with your sockets.

Each bolt posed its own particular challenges. I suggest you do not do this when you are pressed for time.

The problem is solved!
 

LikeEnvoy

Member
Apr 17, 2012
128
Winnipeg, MB
Thanks TrailBoss, I'll try this on my Envoy.
I started getting P0171 last winter (on and off a few times).
It cleared up when spring came but this winter it's back again.
CEL comes on for a while then goes off, days later. Seems to come back whenever the temps go lower than -4°C or so.
I swapped the MAF sensor but no help. Checked vacuum hoses all connected.
Torque app shows LTFT around 29%. Revving to about 2000 RPM drops the LTFT immediately to about 14%, but as soon as revs drop the LTFT jumps back up to 27 or 29%.

I just cleaned the throttle body- very dirty and also looked sticky on the back side. Intake manifold seems sticky/oily when I look inside.
A vacuum gauge on Torque reads about 49 to 51 inHg at idle. Revving to 2000 RPM, the vacuum jumps a bit then settles back to the same as idle.
MAP also does this: jumps a bit as revving, but settles down to idle value again.
So I will try to access the manifold bolts and see.
\
By the way, The top of the manifold seems to have a "seam" for left and right halves. There seems to be a foam gasket sticking out of this, around the throttle body area, and the gasket is loose; it can be pushed into the seam but it springs out. Surely the manifold doesn't rely on a foam gasket for sealing two halves?? I tried wedging this in with rubber bungs but there's no difference in the LTFT values.
 

Attachments

  • Gasket Out.jpg
    Gasket Out.jpg
    795.8 KB · Views: 13
  • Gasket PushedIn.jpg
    Gasket PushedIn.jpg
    462.4 KB · Views: 12

Mooseman

Moderator
Dec 4, 2011
25,257
Ottawa, ON
Surely the manifold doesn't rely on a foam gasket for sealing two halves??
That's a foam insulator for heat and noise abatement. I'm pretty sure your manifold is fine. Never heard of one of these failing before.
 

LikeEnvoy

Member
Apr 17, 2012
128
Winnipeg, MB
Thanks MM. I'll ignore that then.

I went for those manifold bolts this morning. Yup- all the rear bolts were loose. Couldn't get a torque wrench in for the back few but gave a decent estimate compared to how it felt with a torque wrench on the centre bolts. Started up and Long Term Fuel Trim was immediately way down to about 3.9% then came down to 1.6%. Engine is not warmed up fully yet but that's enough to tell me my problem is fixed. It was always about 27 to 29% before, and went down when I revved.
Now the 1.6% goes UP to about 10 when I rev.

Trouble is... my 3/8 extension and 10mm socket dropped after the first bolt and I just can't find the damned thing. I took the wheel off-nothing. Put up on ramps and crawled underneath- nothing.
Where the heck do these things hide? I gave only one dull thud when it dropped.😥
 
  • Like
Reactions: Mooseman

Mooseman

Moderator
Dec 4, 2011
25,257
Ottawa, ON
So you are definitely on the right track. For the tool, I know what you mean. It probably wedged itself somewhere. One popular spot is on the starter.

You would be better served using a 1/4" dr. socket and ratchet instead. I think I did the best with just a deep socket on those rear ones.
 

LikeEnvoy

Member
Apr 17, 2012
128
Winnipeg, MB
Yes- 1/4" tools worked better. Guess what...I just found my socket and extension! It was right where you said- under the starter, and resting on a bracket there. I couldn't see in there yesterday but today I used my cheap endoscope(from china) with my tablet and caught a glimpse of something shiny. Stood on a box and reached right down under that starter and felt something wiggle. Yay!
 

mrrsm

Lifetime VIP Donor
Supporting Donor
Member
Oct 22, 2015
7,639
Tampa Bay Area
There is NOTHING better than an Intrepid Mechanic who is HONEST about his successes AND his failures when attempting to diagnose a problem that starts with him using New, Non-OEM Parts without clinical information ...and ends up with him doing an enormous amount of work to solve a P0171 Code issue. Much can be learned from watching THIS Video:

 
  • Like
Reactions: JerryIrons

Forum Statistics

Threads
23,272
Posts
637,482
Members
18,472
Latest member
MissCrutcher

Members Online