NEED HELP P0171 Lean Code

Reed

Original poster
Member
Sep 21, 2015
21
2003 TB EXT 4.2l is throwing code P0171 after driving for about 20 minutes at speeds of 55-70. The vehicle has almost 160K miles on it, with a remanufactured engine that only has 20K miles on it. Over the past 3 months this is what has happened.
I was getting a code P0340 (Camshaft position sensor circuit malfunction). I replaced the sensor and a bad ground wire from the sensor to the PCM, which solved the problem.
Shortly after I had a misfire on #2 cylinder P0302. No spark, replaced coil pack. Code gone.
Next I got a lean code P0171, along with a multiple misfire code P0300. Also had P0128 engine temp too low. I had a slight leak on the valve cover, above the VVT actuator solenoid, so I pulled the generator, PCM, intake manifold, valve cover, fuel rail, and cooling fan off. Checked the timing chain and made sure it was still tensioned and on the cam gear marks, removed and cleaned fuel injectors, installed new gasket on throttle body, valve cover, and intake. Cleaned the air temp sensor, throttle body, resonator, and intake manifold. While I had it torn apart, I also had the generator tested, and I rebuilt the starter.
For the engine temp too low P0128. I checked the ECT sensor and thermostat, and replaced the thermostat, which solved the temperature problem. Fuel pressure at 56psi, and only drod ps to 54psi after 30 minutes. Fuel pump and fuel filter only have about 7000 miles. Battery at 12.7 volts.
While I was at it; I also replaced the MAP sensor, tested and replaced VVT actuator solenoid, Fuel pressure regulator, the vacuum hose going into the passenger side of the intake manifold, new iridium spark plugs, plus Oil and filter.
After driving it for a while, with engine hot, now I'm back to the P0171 code only. Fast startup with engine cold or hot, slight stumble while at idle, but no misfire codes. Idle is back up to 800-900 RPMs from normal 600 RPMs when engine is warm. Everything points to a vacuum leak, but I've listened, pulled hoses and plugged holes, sprayed carb cleaner everywhere with no change (brake booster, booster hose to intake manifold, PCV hose, valve cover vent hose, air intake, air filter hose to resonator, resonator to throttle body. I loosened the exhaust between the manifold and the catalytic converter, and no change in engine performance.
The upstream O2 sensor has power to the signal side and the heater side, and has continuity. Only ohm'd the heater side of the downstream sensor, and it was good.
I live in the country, and don't have a scope.
Any ideas? Will a small leak in the valve cover gasket possibly cause a lean condition? Is it likely an O2 sensor?
 

gmcman

Member
Dec 12, 2011
4,656
Did you get the P0171 code before or after cleaning the fuel Injectors?

I would want to guess it possibly could be an intake manifold leak.

Did you replace the intake gasket? You could check all the bolts again, be aware they only take 89 inch pounds of torque.

Here's the bolt pattern again...however I'm sure you may already know this.

10 bolts in all

20190620_190048.jpg


Did you check the molded hose between intake runners 3&4?
 
Last edited:

Reed

Original poster
Member
Sep 21, 2015
21
I had the code prior to cleaning the injectors. I replaced intake, valve cover, and throttle body gaskets. I removed the intake manifold again, and checked to make sure a gasket wasn't rolled or out of track. re-installed, and torqued to 89 in lb. double checked both the PCV hose and brake booster hose on intake manifold. Sprayed both hoses well with carb cleaner, at both ends. No change in idle.
I don't see your bolt pattern, but I torqued them starting from the middle moving toward the ends.
 

gmcman

Member
Dec 12, 2011
4,656
@Reed sorry about the pic, I had to go back and edit.

There's also a port on the front of the intake manifold facing the passenger side. This port is used for the LWB models but on some SWB models it will flow vacuum. Usually there's a cover on the port and can get brittle and /or just not be there. Something else to check.

Screenshot_20190620-230504_Chrome.jpg

Wouldn't hurt to also double check the vacuum hose at the TB. I would trace that line down to the EVAP solenoid under the intake manifold, front of starter.
 

limequat

Member
Dec 8, 2011
520
Hmm.
I would think not vacuum leak. From your detailed posts, it's obvious you know how to swing a wrench. To trip a lean code, the trims have to be > 25%. So it seems doubtful that you'd miss it.
Plus, even if it was sucking in air through a leak, the MAP sensor would see it and correct.

Is it REALLY lean or does it just think it's lean? The "slight stumble at idle" sounds like a true lean condition, but could be other things too. What do the plugs look like? Are compression numbers good?
 
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Reed

Original poster
Member
Sep 21, 2015
21
Thanks for your input guys.
I replaced the vacuum hose attached to the passenger side of the intake manifold before the last code (P0171), because the rubber elbow was split. I have not chased the EVAP vacuum hose, because we all know what is entailed to gain access to it. That may be my next move, because I have had no success finding out where the problem lies.
The CEL popped up while going through multiple start/stop cycles, under these conditions: Level road, outdoor temp 104 degrees, AC on, 3/4 throttle accelerating to 55-60 mph, with occasional pedal down to kick in overdrive up to 75 mph. I put a scanner on it today, and got the following Freeze Frame readings from when the code set:
Fuel system CL
ECT 187
STF 18.8%
LTFT 3.9%
MAP 23Hg
RPM 2775
Vehicle speed 35
 

gmcman

Member
Dec 12, 2011
4,656
Your short term fuel trip is pretty high, it's having to add a lot of fuel. Could be due to the acceleration, but mine doesn't go anywhere near that on acceleration.



I see you have a new pump, but have you verified your fuel pressure since this code popped up? Did you by chance hook the fuel filter up backwards? I don't think a fuel filter hooked up backwards would pose an issue, may or may not have the filtration capacity in reverse...not sure.

If you have a bluetooth OBD reader, the Torque app would show a good deal of info in regards to fuel trims.
 

Reed

Original poster
Member
Sep 21, 2015
21
I checked the fuel pressure again today. Only 46 psi while priming, with key "ON," then drops to 42 psi, and holds there without any loss. No change in pressure (46 psi) while accelerating, but the needle had some shake. I cleared the code, and took it for a drive. No codes set, but it still has some stumble at idle, and poor acceleration.
Here are the scanner readings:
At idle 620 RPM:
STFT -8 to -14%
LTFT -3.9% (steady)
O2S1 .05 to .85v
O2S2 .05 to .81
ECT 183
Accelerating in Park at 2500 RPM:
STFT -3%
LTFT +18%
While driving appoximate RPM's 1800:
STFT -3 to +3
LTFT 18 to 22%

STFT increased to +6 to 8% with increase in throttle, LTFT dropped to +12%
Foot off throttle, coasting, STFT approximately -3% to +3%, LTFT +3%
The upstream O2 sensor seemed to be doing what it is supposed to do, jumping from .08 to .87v.
The *downstream sensor was jumping as well, instead of holding steady around 5v. It was reading .1 to .75 v, mostly around 6.5 to 7v while driving at a steady speed. I assume because of the high fuel trim sending extra fuel to the cat, trying to make up for the lean condition. But, I'm not sure why it would send such a low voltage at times?
*Note: I have an adapter installed between the exhaust pipe and the O2 sensor.
**The muffler was replaced, with the reman engine, and now has a rattle at times (baffle?), but the cat seems to be good.

I don't know if the new pressure regulator could be the cause of the lower fuel pressure, if the fuel filter is getting clogged, or if the fuel pump is going out? The fuel pressure reading I took last week was at 56 psi, prior to replacing the fuel pressure regulator.

Limequat, I have not pulled the plugs since I replaced them this week (ACDelco iridium plugs). The old plugs were dry, and no carbon buildup, but they were platinum plugs, and didn't have any continuity from end to end of electrodes. I did pull all of the coils, one at a time, and tested for spark, before new plugs. I also have not checked compression. Remanufactured engine with only 20,000 miles on it.

So, what do you guys think? Fuel system (pump, filter, regulator)? O2 sensor(s)? Or vacuum leak?
 

gmcman

Member
Dec 12, 2011
4,656
Is your fuel pressure test port at the fuel filter on that model year? If so, should be on the OUT of the fuel filter.
 

Reed

Original poster
Member
Sep 21, 2015
21
gmcman, it is at the fuel filter. I just reached under enough to reach the port, so I didn't get a close look, but based on your info of the test port being on the "Out," are you saying my problem could very well be the fuel filter?
 

gmcman

Member
Dec 12, 2011
4,656
I don't completely believe it's your fuel filter, I'm just going on the possibility that if installed in reverse, could you run the risk of running the fuel through in such a manner that would yield a significant reduction in filtering capacity and likely restrict the filter in short order.

I honestly have no idea, never seen one inside. I would just want to be sure that part of your diagnostic is covered.

I don't have the specs handy, but I believe the minimum pressure is somewhere between 50-53 PSI. Doesn't mean it won't run on 42 PSI, but that's definitely out of spec.

Do you have your old FPR? Perhaps give that a try.

Sounds like you aren't getting enough fuel, and less likely a vacuum leak. I would speculate with a vacuum leak, you would still have decent acceleration....hard to say.

Have you put a vacuum gauge on the TB port yet? Curious if the needle is steady or fluctuating. By fluctuating I mean shaking back and forth, not slowly wandering.
 
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gmcman

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Dec 12, 2011
4,656
I went over the thread again, just curious...did you replace your air filter? Again, just something else to look into.

I recently changed mine and it didn't look too bad, turned out it was almost completely clogged with, what I believe was fine road brine dust. I didn't realize how clogged it was until I tried to shine a flashlight through it.
 

gmcman

Member
Dec 12, 2011
4,656
What O2 sensor did you use?

Can you get a temp of the inlet and outlet of the cat after about 5 miles of hwy driving?

Have you tried clearing the codes?

The LTFT is setup to keep the STFT at or close to 0.

If your long term is showing +18-22% then it surely is adding a good bit of fuel.

I would put a vacuum gauge and tester on the evap line going into your TB.

How erratic is the idle if you pull the hose from the TB? What happens to the trims when you pull the line? Also, try plugging that port at the TB and see what the trims do.
 

gmcman

Member
Dec 12, 2011
4,656
Using a vacuum pump, I was able to hold a vacuum on the evap tube that goes into the TB.

If you can, check that yours holds a vacuum. Mine bleed down just slightly...maybe 1" per 30 sec... and that was from the friction fit of the tube in the connector.


Screenshot_20190628-155351_Gallery.jpg
 

gmcman

Member
Dec 12, 2011
4,656
@Reed any updates?

Something else to check is your brake booster check valve. You mentioned the booster but not the check valve.
 

Reed

Original poster
Member
Sep 21, 2015
21
Sorry, I've been away for a while. Finally back to work on the Trailblazer.
So far I have replaced the spark plugs, gaskets (valve cover/throttle body/intake manifold) fuel pressure regulator, FPR vacuum hose connector, fuel pump. fuel filter, vacuum line to intake manifold, muffler, catalytic converter, MAP sensor, thermostat, coolant, air filter, oil/filter, CMP sensor, and VVT solenoid. I cleaned the throttle body and fuel injectors, and installed new filters/plenums/O-rings on injectors. Plugs are clean, and no oil leak down from valve cover gasket.
I put a vacuum pump on the EGR valve, MAP sensor, and fuel pressure regulator, and they all hold a vacuum. I tested the brake booster check valve, and it only allows airflow one way.
After installing new exhaust, cat, and injector kit, and putting everything back on the TB, I started it up. It started fast, idled at 1100 rpm's until reaching closed loop, and dropped to normal 620 rpm's. It still had some misfire at low idle, but nothing noticeable at higher rpm's. While driving; it ran without any noticeable misfires, and had good power, with slight hesitation when snapping the throttle. However, the long term fuel trim was still at +14 to 20%, and the short term was -3 to +6%. After driving at 35 to 75 mph for 30 minutes, when I put it in Park, the idle remained at 900 rpm's, and would not drop back down. NO CODES popped up, but I haven't taken it for a second drive yet, which could cause the PCM to throw a code, if certain criteria is met.
This is driving me crazy. If it were an intake leak, wouldn't it likely start out running at high RPM's and possibly go away when the engine warmed up? If it were a vacuum leak, wouldn't it run at a high idle at all times?
Maybe it's the exhaust gasket? Maybe it's weak spark on a coil, but why a high idle?
Any input would be greatly appreciated!
 

Reed

Original poster
Member
Sep 21, 2015
21
I forgot to add O2 sensor data in my last post.
Both sensors are still fluctuating from high to low, although I have replaced the cat and muffler.
The downstream sensor holds around .6 to .7 volts at steady throttle, with occasional drop to .1 volt, and rise to .8 volts. It always drops low (around .05v) when accelerating before rising back up.
 

Maverick6587

Member
Dec 16, 2018
730
Sterling Heights, Michigan
Did you replace that hose that's connected to the intake manifold and the brake booster?
The parts that you replaced, were they all ACDelco, Bosch, or Delphi?

I see you said that the spark plugs were, I just wanted to see if everything else was as well. Also, did you replace the engine air filter and the gasket for the throttle body? I know it usually comes with the intake manifold gasket, but not always.

A quick and easy test would be spraying soap and water around everything on the engine (avoiding electronic connections). Just to test for any leaks anywhere.

If there's no result there, I would guess it would be either the upstream O2 sensor or the MAF sensor.
 

gmcman

Member
Dec 12, 2011
4,656
The 900 RPM thing should point us to the issue.....this is definitely one for the books.

With the engine not running, look into the TB with the key on and take a pic of the butterfly please. I'm curious to it's resting position...also very curious if the thin edge of the butterfly is clean all the way around... this is critical

Have you replaced your gas cap yet? Or had it checked?

The STFT will fluctuate back and forth across the "0" line, this is normal....+- 7 is not a huge cause for concern... 20% LTFT is not normal.

Check your kick panel above the footwell and ensure the gas pedal doesn't have anything resting against it.

Still need this info:

Have you checked the vacuum like I showed you with the hand pump at the evap line at the TB?

What about the vacuum ports I showed with the red arrows?

Sorry to ask again, but I didn't see a response.

I'm very curious to the vacuum at idle...using a gauge, is the needle steady or vibrating? Steady would be not moving to very slight movement back and forth.
 

gmcman

Member
Dec 12, 2011
4,656
I replaced all my coils recently with no codes and made a noticeable difference. One thing I noticed is I could feel a prominent tapping on top of one of my coils...couldn't explain it.... new coil eliminated that.


The only things you haven't addressed are your EVAP components. Start with the gas cap, then check vacuum in the line from the TB.

I'm not sure when the solenoid under your intake is open or closed, part-time or full-time.

When the valve allows vacuum to flow, and you have a leak further back, this could be your issue. Just not sure if you have a leak, and/or is it in the line from the TB, or to your canister, or vent valve.
 

gmcman

Member
Dec 12, 2011
4,656
Two different threads here on P0171 and I was thinking of the P0440 code I was dealing with...still a possible vacuum issue but not likely the evap system. Sorry for any confusion...have a lot on my plate recently.

The fact the idle changes after you drive it makes me not rule out an evap component just not high on the list.

When you replaced the intake gasket, did you leave it intact? Did you trim any material between the runners?
 

mrrsm

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Reed

Original poster
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Sep 21, 2015
21
SOLVED
I took the Trailblazer to a repair shop, and they told me I had done everything, and to bring it back when the miss got worse. Driving with a misfire wasn't acceptable for me.
Having good compression and fuel delivery, the only thing left was spark, so I decided to replace coils on plugs. although I had spark from the old coils. No more stumble, and idle is back to normal. Apparently, weak spark was the culprit.
 

gmcman

Member
Dec 12, 2011
4,656
That's good to hear. :thumbsup: With the price of these coils being very reasonable, it's definitely a wise choice to change all of them. I didnt have any lean codes but changing all of them resulted in a much better running engine.
 
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zaid3ssaf

Member
Jan 1, 2020
151
Midwest
Sorry for reviving an old thread. but sharing the info just in case. For me, I found a leak in this little hose tripping off P0171
1592968016697.png
 
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