P0171 and P0172, idle at 900 rpm, engine running hot.

will5857

Original poster
Member
Sep 2, 2014
33
Hi everyone, I'm new here. Transferred over from trailvoy. I have a recurring problem that I'm hoping to get some help with. I've seen some similar discussions, but my case is a little different so I thought I'd start a new thread.

First off, 2003 trailblazer LT 4x4 4.2 I6. 102,000 miles.

I'll start with a little history, bought the car with 45000 miles, I've had the P0171 code pop up a few times since I've had the vehicle. The past couple times, changing the upstream O2 sensor fixed the problem. Each time the code occurred, it would be accompanied by high idle RPMs, specifically 900 RPMs. Just replaced the Upstream O2 sensor 10000 miles ago with an AC Delco sensor. That fixed the problem until now.

Now I'm getting the two codes P0171 and P0172. The engine will idle high at 900 for a period of time and then go down to normal after a bit of driving. The check engine light will come on and then go off. The engine is also running a bit hot, when the idle is high. It's about 2 ticks to the right of 210. Normally it's always just about right on 210.

I took the vehicle to a mechanic and they said both O2 sensors were operating normally. They suggested there was a vacuum leak somewhere and that I should bring back in a week and they will take another look. They also cleaned the throttle body.

I don't know a great deal about fixing engines so I don't know that I'll be able to diagnose anything on my own. I'm debating whether to take the vehicle to the dealership (not excited about that) or go back to the same mechanic.

Any suggestions or help would be appreciated. Thanks!

-Will

P.S. I forgot to add that the first time I read the codes about 3 days ago, there were about 24 codes that came up. I erased them and took the car to the mechanic. The only codes that came back up were the two I mentioned. Unfortunately I didn't write them all down, but there were some weird ones in there that didn't make any sense, like a brake light malfunction and all my brake lights work fine... Could there be some electrical problem or problem with the computer?
 

IllogicTC

Member
Dec 30, 2013
3,452
Looks like you have a good scanner, it's picking up on more than just Pxxxx codes, which leads me to believe some things such as brake light malfunction may be stored, historical codes. Maybe a brake light got changed a while back, it's still sitting as a trouble code in the history.

A vacuum leak is not out of the question. A common problem with this platform is that the intake manifold bolts work their way loose over time - 89 inch-pounds and no thread-locker compound doesn't seem the best at keeping it on, I guess! There are 10 bolts which hold the intake manifold on, and they must all be torqued to 89 inch-pounds. I'd say it's likely you'll find one or more which are loose.
 

will5857

Original poster
Member
Sep 2, 2014
33
Thanks for the reply. I don't recall if the intake manifold has to be removed to clean the throttle body... If so, they would have re tightened today when it was cleaned. Does it?
 

Texan

Member
Jan 14, 2014
622
Short answer - no - the intake manifold does not come off to clean TB.
 

gmcman

Member
Dec 12, 2011
4,656
Do you have a cap on this vacuum port?

Also the PCV vacuum hose between the intake runners of CYL 3 & 4 can crack over time.
 

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will5857

Original poster
Member
Sep 2, 2014
33
regarding the high idle, I did a little more research and a couple of possible fixes are the alternator plug or the ETC sensor? Is that correct? Also is the high idle related to the P0171 and P0172 codes?
 

will5857

Original poster
Member
Sep 2, 2014
33
gmcman said:
Do you have a cap on this vacuum port?

Also the PCV vacuum hose between the intake runners of CYL 3 & 4 can crack over time.
Yes, I checked that cap and it is there. I have not checked that hose. Could you possibly post a picture locating it?
 

Texan

Member
Jan 14, 2014
622
Did the person who cleaned the TB remove it from the intake manifold and clean the back side?
 

gmcman

Member
Dec 12, 2011
4,656
will5857 said:
Could you possibly post a picture locating it?
Lower right, #6

Do you possibly also have an exhaust leak?

Many factors here, definately start with the intake manifold bolts and all the vacuum hoses. Could be leaky injectors, have you run any good cleaners through your fuel sysytem?
 

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gmcman

Member
Dec 12, 2011
4,656
I would also try a TB relearn, you could also possibly have some crud on the edge of the butterfly of the TB. If you clean the edges, it's critical you get the ENTIRE edge. Any residue could keep the TB from closing and the gap created by the cleaned area will allow too much air to pass resulting in poor idle control.
 

gmcman

Member
Dec 12, 2011
4,656
Sorry to keep adding posts. You stated this was fixed when you changed the O2 sensor....any chance the wiring could be loose or dirty? Does the harness snap into place?
 

will5857

Original poster
Member
Sep 2, 2014
33
gmcman said:
Lower right, #6

Do you possibly also have an exhaust leak?

Many factors here, definately start with the intake manifold bolts and all the vacuum hoses. Could be leaky injectors, have you run any good cleaners through your fuel sysytem?
No visible cracks in the PCV hose... but I'll have them take a closer look. And no I haven't run any cleaners through recently... I suppose it wouldn't hurt.

Also, how do I check for an exhaust leak?
 

will5857

Original poster
Member
Sep 2, 2014
33
gmcman said:
Sorry to keep adding posts. You stated this was fixed when you changed the O2 sensor....any chance the wiring could be loose or dirty? Does the harness snap into place?
The mechanic said both O2 sensors were functioning properly... But yes the wires do look a little dirty.
 

gmcman

Member
Dec 12, 2011
4,656
I would pull the TB, and ensure it's clean on both sides, also check for the gasket.

Check all vacuum hoses, even put a vacuum gauge on it of you can.
 

IllogicTC

Member
Dec 30, 2013
3,452
Bolt positions

sb4s1w.png
 

gmcman

Member
Dec 12, 2011
4,656
I know I sound like a broken record but what I would recommend strongly is a can of bg44k and let it go for about a tank it's possible you can have a whole set of leaky injectors and your system cannot lean itself out enough to compensate which is why you're getting a rich mixture.

Sometimes it doesn't help, but in the end it's good for the system. Doing this requires no wrenches or shop fees, just let it run for a tank and see what happens.
 
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will5857

Original poster
Member
Sep 2, 2014
33
Alright, so I have some updates. I went to Advance Auto and read my codes. they read as follows:

HISTORY
U1255 - Class 2 Communication Malfunction (twice)
U1055 - (twice)
C0300 - Rear propshaft signal circuit malfunction
C0305 - Front propshaft speed sensor system malfunction
C0306 - Motor A/B circuit malfunction
C0327 - Encoder circuit
C0359 - 4Lo discrete output circuit malfunction
C0550 - ECU malfunciton
B0770 - AWD indicator circuit malfunction
B0775 - 4Hi indicatior circuit malfunction
B0785 - 2Hi indicator circuit malfunction
B0790 - Transfer case neutral indicator circuit
U1301 - class 2 data link high
P0172 - Bank 1 system system too rich

CURRENT HISTORY MIL
P0171 - Bank 1 system too lean

INTERMITTENT
P0130 - Oxygen Sensor circuit Bank 1 sensor 1

CONFIRMED
P0171
P0172

PENDING
P0171

At the time I read the codes, the idle was at 900 and the smell of gas was present.

I erased all the codes and the idle went back to normal and the smell of gas went away. After about 10 minutes of driving, the idle was back at 900 and the smell of gas came back.

Could my ECU just be crap? How do i find out if it is?
 

IllogicTC

Member
Dec 30, 2013
3,452
That P0171 seems awful persistent. It may also lead to overheating; I do know that running lean can really make an exhaust manifold glow!
 

will5857

Original poster
Member
Sep 2, 2014
33
I forgot to add, read the O2 Bank 1:

MOD $10
MAX -----
MEAS .345 (v)
MIN -----

And there was a snapshot stored:
Engine RPM - 1995
Calc. Load% - 84.7
MAP HG - 20.1
Coolant Temp - 201
ST FTRM1% - 19.5
LT FTRM1% - 27.3
Speed - 66 MPH
 

bobdec

Member
Apr 19, 2013
233
IMO you are going to need a decent scanner or android w/aldl (Torq app and bluetooth ALDL) capability to chase this down. Otherwise you will be running to Advanced Auto for a scan each time you try something. P0171 is to much air (vac leak), bad O2 , or not enough fuel. Your LT trim of 27% is bad and you need to monitor it after changing or checking things. As an example here is a long thread that ended up being the O-Ring between the TB and intake. http://gmtnation.com/forums/topic/11665-do-i-need-to-reset-the-ecu-or-clear-codes-when-replacing-o2-sensor/ . It lists a lot of info and checks for a lean DTC.

I would also power off the PCM (pull the two fuses) for a min or so to reset all the DTC's, history ETC. You have a lot of history and extraneous DTC,s it's good to start with a clean PCM prior to analyzing a problem. And then go after the first important thing that fails.. But you need to get the LT trim average down near +/- 8% or so.. Reseting PCM will get LT trims down for a few mins , but if problem is still there the trims will climb back up.
 
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will5857

Original poster
Member
Sep 2, 2014
33
Thanks bobdec. I cleared all the history and the code that keeps popping up is P0171, too lean. In the past I've changed th o2 sensor and the fast idle issue has been taken care of. But I took the truck into a mechanic and he said the o2 is fine. He thinks it's most likely a vacuum leak. He thinks probably the intake manifold gasket. He recommended I keep driving until winter when a vacuum leak would be easier to detect. He said he could not visually find or hear a leak anywhere. The check engine light comes on at highway speeds and that triggers the high idle. After driving around town for a little bit the idle goes down to normal and the check engine light goes off.
 

will5857

Original poster
Member
Sep 2, 2014
33
I did, they seemed a bit loose. I didn't have any way to measure how tight they were though. And i Couldn't reach all of them. I'm embarrassed to admit it but I'm and idiot and snapped one of them. I tightened them when the engine was still a bit warm, so I think that's why it snapped. So pissed at myself. So that may be the problem. I'm scared to find out how much it will be to fix that.
 

gmcman

Member
Dec 12, 2011
4,656
Depending on what bolt but with the intake removed they are not too hard to get to.

Do you have any freeze frame data from the stored lean code? Could very well be a vacuum leak and I would surely try to snug all of them....remember, only 89 inch pounds which is just barely past snug.

Something else that comes to mind is your fuel filter...have you ever replaced it? If it's at least 2 years old with your symptoms I would change it anyway.
 

will5857

Original poster
Member
Sep 2, 2014
33
gmcman said:
Depending on what bolt but with the intake removed they are not too hard to get to.

Do you have any freeze frame data from the stored lean code? Could very well be a vacuum leak and I would surely try to snug all of them....remember, only 89 inch pounds which is just barely past snug.

Something else that comes to mind is your fuel filter...have you ever replaced it? If it's at least 2 years old with your symptoms I would change it anyway.
Engine RPM - 1995
Calc. Load% - 84.7
MAP HG - 20.1
Coolant Temp - 201
ST FTRM1% - 19.5
LT FTRM1% - 27.3
Speed - 66 MPH

Like I said though, I have no way to measure the pounds. And I snapped a bolt.

I have not replaced the fuel filter. Is it hard to do?
 

will5857

Original poster
Member
Sep 2, 2014
33
I found this tip online. Has anyone tried this/think it would work?
Diagnostic Tip:
For a suspected vacuum leak, note the fuel trims at idle and increase engine speed to 2500 RPM and hold. If the STFT immediately decreases and moves to acceptable levels and the LTFT slowly starts to come back down, you have a vacuum leak. After the repair, reset the KAM and start the vehicle. Monitor the fuel trims to make sure they are within the normal ranges. It could take up to 10 miles of driving for an accurate LTFT reading.
 

gmcman

Member
Dec 12, 2011
4,656
Fuel filter is easy. Pull Fuel pump relay, crank engine for about 5 Seconds and ensure no start. It's located under drivers door inside frame rail. Use Phillips screw to release strap then use long nose pliers to squeeze tabs on fuel lines, with tabs squeezed, pull line from filter..

Make sure you wear safety glasses or just be careful... and also drain residual fuel from line with Schrader valve at bottom of fuel line under filter before you pull the line off.
 

gmcman

Member
Dec 12, 2011
4,656
You do need to get the intake all sealed up though. Under any load, the PCM relies on accurate MAP values to determine other parameters especially fuel delivery. If there's a leak then the PCM will see a lower value than what's actually happening resulting in less fuel delivery, ie, a lean condition.
 

will5857

Original poster
Member
Sep 2, 2014
33
So, I did a fuel trim test and monitored the short term and long term fuel trims at idle and at 2500 RPM. I started the test when the engine was idling normally. A P0171 code had been triggered already but the check engine light was not on, it had gone off. I took a video of the test and will try to upload it later.

At idle, the short term fuel trim was down in the -10 to -19% range and the long term was up at 21%. I increased the RPM to 2500 and the short term evened out around 0, and the long term went down to 18%. After about 3 minutes at 2500 RPM there was a sudden jump in both the short term and long term trims. The short term jumped from around 0 to 19% and the long term jumped up to 27%. When I went back to idle, the RPM were at 900 and both trims did not move.

I also did a couple tests where I monitored the fuel trims and created false lean and rich conditions. I did this after I cleared the codes and the engine was idling normally. The computer did not seem to react to the lean and rich conditions, short term fuel trim stayed the same.

That immediate jump in both fuel trims at 2500 RPM seems odd to me, but what do I know... And it seems to be what triggered the 900 RPM idle. I'm no mechanic, and correct me if I'm wrong, but it seems like something is faulty, either the PCM or o2 sensors or something... It seems like when the engine was idling normally, the short term fuel trim was reacting correctly down in the negative percentile, making up for the highly positive long term percentage. As soon as that jump in both trims happened suddenly, the computer was reacting to a lean condition, or what it thought was one. When I erased all the codes, the idle immediately goes back to normal and the short term fuel trim was back down in the negative percentile making up for the high long term percentage, seemingly acting normally. I'd be willing to bet if I reset the PCM and the LT fuel trim, the short term would go to a normal range. Then that sudden jump in short term (computer sensing a lean condition) would happen at some point which would increase the long term as well. And this leads me to believe it's not a vacuum leak. Any ideas? Thanks in Advance.
 

gmcman

Member
Dec 12, 2011
4,656
You must first ensure that the intake manifold is not leaking. MAP information influences air/fuel ratio, ignition timing.

If you step on the gas, and the PCM sees a light load, it will adjust accordingly. However in your case, you could have a moderate load and some of the vacuum that needs to operate the MAP sensor is possibly escaping...thus resulting in a false MAP reading.

The MAP sensor sends vacuum readings from the intake to the PCM.

You can't really properly chase your codes with a possible leaking intake manifold.
 

will5857

Original poster
Member
Sep 2, 2014
33
gmcman said:
You must first ensure that the intake manifold is not leaking. MAP information influences air/fuel ratio, ignition timing.

If you step on the gas, and the PCM sees a light load, it will adjust accordingly. However in your case, you could have a moderate load and some of the vacuum that needs to operate the MAP sensor is possibly escaping...thus resulting in a false MAP reading.

The MAP sensor sends vacuum readings from the intake to the PCM.

You can't really properly chase your codes with a possible leaking intake manifold.
Thanks, but is this in response to my latest post?
 

gmcman

Member
Dec 12, 2011
4,656
Yes.

Trims should be within roughly .7 of each other...and generally less than 10 and greater than .-10....yours are off the chart.

If you have a vacuum leak then the engine is receiving unmetered air and is being compensated by more fuel from the O2 sensors....thus likely being the cause for a high idle.

Take a propane torch and let the gas escape. ...no ignition, and wave it around the intake manifold at the head. Does the RPM's increase?
 

gmcman

Member
Dec 12, 2011
4,656
It's possible you have a flaky sensor, but since your intake bolts were loose and now one is broken I would start there.

My Sunday morning is looking open from about 8 to noon. I would go by the dealer and get a bolt and Advanced has the intake gasket with the TB gasket as a set. If the bolt is not all the way in the back or front I may be able to get it out.
 

808_LS_EXT

Member
Aug 28, 2014
305
Check out this youtube video...

https://www.youtube.com/watch?v=ViL7RX_22A4
 

will5857

Original poster
Member
Sep 2, 2014
33
That is exactly what's happening to my vehicle. Interesting, thanks for posting that video. That may be the culprit. Is that valve hard to replace?

Actually I just checked my engine and I can't locate the valve. that video was for a 2006 and mine's a 2003... Does the 2003 have it?
 

gmcman

Member
Dec 12, 2011
4,656
I think they started on 03.

That's a well done video, very informative.

I don't think it would explain the high idle though.
 

will5857

Original poster
Member
Sep 2, 2014
33
Yeah I checked and I definitely don't have that valve. The symptoms are exactly the same though. I do have the exhaust cam actuator solenoid. Is that related?
 

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