NEED HELP P0171 Code causing high idle and poor fuel mileage!

[ghost_leader07]

Hello Gentlemen,

I apologize ahead for opening this thread again I’m sure it has been discussed numerous times but I’m struggling to find the culprit regarding my P0171 code, I have 2004 Trailblazer LT and here’s what I have done so far:

-Vacuum leak checked with a smoke machine and none were found.

-SAIS was deleted and installed a block off plate.

-Replaced O2 upstream sensor.

-Replaced MAP Sensor.

-Tried to tight down intake manifold bolts to the best of my ability and I’ve managed to tighten most of them down however on there was one bolt towards the front of the engine (before the ECM mount) was almost rounded off it seems like someone was on it before (probably the previous owner) so i haven't ruled out the intake manifold yet.

-Cleaned TB and changed the gasket.

-Replaced fuel filter.

-Fuel pressure tested (My readings were 49PSI with key ON and 52PSI with car on idle) also did a leak down test for 10 minutes and no pressure drops.

Here's my some of my live data:



P.S.
There's another issue regarding the P0171 which is the idle change usually my truck idles around 600rpm however since this code came up my idle raised to about 900rpm and stays there causing the truck to feel rushed when shift to drive.

Any help is greatly appreciated,

 

[budwich]

the first O2 voltage graph does not look right although it could be the time scale that makes that way. The "normal" graph should be a "wave" pattern oscillating about 4.5-5 v ish area.

Of course, the FT confirm your "rich run" because they are set to dump more fuel because of a response of seeing "lean" somewhere... probably the o2 sensor.

 

[Mooseman]

Any leaks at the exhaust manifold? You could check for vacuum leaks using carb cleaner, especially at the intake manifold.

But as @budwich said, that O2 sensor reading is wrong. It's almost as if it's just out in the open air. Was it the same way with the old one? What brand did you use?

 

[ghost_leader07]

the first O2 voltage graph does not look right although it could be the time scale that makes that way. The "normal" graph should be a "wave" pattern oscillating about 4.5-5 v ish area.

Of course, the FT confirm your "rich run" because they are set to dump more fuel because of a response of seeing "lean" somewhere... probably the o2 sensor.

Sorry about the graph it didn't show the reading correctly i have managed to use a detailed graph and it shows that my o2 sensor is working properly volt values fluctuate between 0.1-2 to 0.8-9 in as you mentioned a wave pattern, I'll attach a photo of the graph as soon as I get home.

 

[ghost_leader07]

Any leaks at the exhaust manifold

I've removed the heat shield and checked for cracks on exhaust manifold but i didn't find anything visible

You could check for vacuum leaks using carb cleaner, especially at the intake manifold.

As for vaccum leaks i didn't find anything but I'll check the intake manifold today with a crab cleaner and get back to you.

 

[gmcman]

Be careful with those intake manifold bolts, they only require 89 inch pounds and will break on you, then you have a real pita to deal with.

Check your TB butterfly and see if it's secure, meaning lightly try to open it and see if it feels loose. Should feel like it has resistance against an electric motor, not flopping around, some have been known to break a tooth off inside.

Your coolant temp is a little low if that's the hot reading, shouldn't make it run at 900 RPM though unless it was still in open loop. Is your coolant temp steady, does it fluctuate wildly?

When the RPM is up near 900, take a pair of pliers and pinch your brake booster hose. Also check the evap solenoid vacuum hose, you could only have the leak when it commands for evap flow into the TB, but I would suspect that would throw another code. When the RPM is up, pull the evap hose and plug the port, does the RPM return to normal?

Check the molded hose between intake runners 3&4.

Check the port on the intake near the LF which faces to the right. Some SWB models will have vacuum flow and some will not, should be capped off.

When did all this start, before or after your O2 sensor change?

 

[ghost_leader07]

Be careful with those intake manifold bolts, they only require 89 inch pounds and will break on you, then you have a real pita to deal with.

I understand but as i mentioned one of the bolts was almost rounded i guess a pita is incoming

Check your TB butterfly and see if it's secure, meaning lightly try to open it and see if it feels loose. Should feel like it has resistance against an electric motor, not flopping around, some have been known to break a tooth off inside.

I did clean the TB and the butterfly seem to be in good condition and it doesn't move unless i give a good pressure.

Your coolant temp is a little low if that's the hot reading, shouldn't make it run at 900 RPM though unless it was still in open loop. Is your coolant temp steady, does it fluctuate wildly?

Yes my coolant temp is steady and doesn't fluctuate that much, my coolant temp is usually around 190F.

When the RPM is up near 900, take a pair of pliers and pinch your brake booster hose.

I'll try that and see if my idle changes.

Also check the evap solenoid vacuum hose

Where is that hose located?

Check the molded hose between intake runners 3&4.

is that the one covering the intake manifold bolts in the middle?

Check the port on the intake near the LF which faces to the right. Some SWB models will have vacuum flow and some will not, should be capped off.

I did check it and it was capped off no vacuum leaks there.

When did all this start, before or after your O2 sensor change?

it has been on and off for a while before changing the O2 sensor, speaking of which here's my live data reading for both of my up and downstream sensors:

 

[gmcman]

The evap hose is connected to the base of the TB. You need to lift the small plastic tab on the plastic connector before you pull the hose.

 

[ghost_leader07]

The evap hose is connected to the base of the TB. You need to lift the small plastic tab on the plastic connector before you pull the hose.

OK thanks, now that i have a couple of things to check I'll come back to you when I'm done with tests.

A question regarding my fuel pressure i have mentioned that my fuel pressure is at 49 PSI when ignition ON and around 52 PSI when car is running is my fuel pressure good? I'd like to eliminate the fuel side on the issue and focus on the air side.

Thanks again!

 

[gmcman]

Not sure of the spec, but 52 sounds very close.

Don't rule out a clogged/ bad injector which would change the pulse width of the injectors to compensate not getting enough atomized fuel.

You can monitor your injector pulse width in Torque, but I suspect they would be uniformly high if the PCM calls for more fuel.

Does it stumble from a stop when trying to accelerate?

 

[ghost_leader07]

You can monitor your injector pulse width in Torque

what values should i be looking for?

Does it stumble from a stop when trying to accelerate?

when i try to accelerate normally it seem to be fine no stumbling but when i do a hard acceleration it does stumble for like a second like an overdrive buildup before kicking in not sure if it's normal.

 

[ghost_leader07]

[UPDATE]
Just got around to check the intake manifold today with carb cleaner and started spraying under the TB and just as i was dumping a lot of crab cleaner around that area my RPM dropped quite noticeably for a second and then the RPM goes up I've managed to replicate it maybe twice or three times but not right away and used about a bottle and half of carb cleaner i don't know why i couldn't target the same area where the RPM dropped I'm thinking i could've tried spraying from the bottom side of the intake but i couldn't wiggle through because the spray tube was too short to get in there however i think it's obvious that there's some air getting inside the intake manifold somewhere otherwise my RPM shouldn't drop am i right?

I've already bought gaskets for the manifold now i just need to get it off the engine and seems like a lot of work especially with that rounded bolt I've mentioned speaking of which any ideas or tricks on how to get that damn bolt off?

 

[Mooseman]

If it's really that rounded, you might need a twist bolt remover, like this:

Used one like this on a 10mm bolt on another vehicle in a tight spot and it worked great.

 

[gmcman]

You will find it easier, if not necessary, to remove the fuel pressure regulator before you start maneuvering the intake out, not that hard.

Remove your negative battery cable before you begin, there will be a lot of wrenching near the alternator's positive post on the rear.

Honestly, knowing what I know now, if I had to remove my intake, I would change the thermostat and coolant temp sensor while I was at it.

A 1/4" ratchet and short extensions will be your friend for some of the intake bolts.

 

[ghost_leader07]

Sorry i haven't got the time to start on removing the intake manifold yet but i need to have enough knowledge before working on..... I'll post an update after I'm done.

I would change the thermostat and coolant temp sensor while I was at it.

I've replaced the thermostat recently but I didn't touch the ECTS is it necessary to replace it though? i don't have any issues so far.

Also thanks for your insight.

 

[Mooseman]

It's not necessary but because of its proximity to the thermostat, it's easier to do it now while you're there than later.

 

[NJTB]

Sage advise on the ECTS. I was checking mine with a scanner and the coolant temp was around 170, said to myself time for a thermostat.
Put one in, scanned it again, still 170. Replaced the coolant sensor, all's good.
Just an FYI, use some teflon tape on the sensor, mine leaked, wound up doing the job 3x till it was right.

 

[ghost_leader07]

It's not necessary but because of its proximity to the thermostat, it's easier to do it now while you're there than later.

Yeah that makes sense i'll order new one and throw in there as well, Thanks!

 

[gmcman]

You will need a 18MM cutaway socket like this, will make replacing that sensor a more pleasant experience.

You could take an angle grinder to a deepwell socket and make a slot. Please wear eye protection if you do, sockets are very hard metal.

I don't have a link, but looks like this.

 

[Mooseman]

Bah, I'd cut the wire to use a regular deep well socket to remove the old one and then use an open ended wrench to install the new one.

 

[budwich]

I understand but as i mentioned one of the bolts was almost rounded i guess a pita is incoming

I did clean the TB and the butterfly seem to be in good condition and it doesn't move unless i give a good pressure.

Yes my coolant temp is steady and doesn't fluctuate that much, my coolant temp is usually around 190F.

I'll try that and see if my idle changes.

Where is that hose located?

is that the one covering the intake manifold bolts in the middle?

I did check it and it was capped off no vacuum leaks there.

it has been on and off for a while before changing the O2 sensor, speaking of which here's my live data reading for both of my up and downstream sensors:

View attachment 99847

your upstream has "improved" (ie. better) but the downstream looks "funny". Of course, I don't believe the downstream has any impact on fueling (having said that, I have read in places that some system do have operational firmware that does have some "interest" in the downstream values... basically to ultimately "help" that system improve its "environmental impact" a bit).
IF you look at the traces, you can see something happening between the two. For instance, look at the first yellow down spike. At that point, the red appears to be following... it should have been rising to follow its "pattern" but instead, it also down turned. Same with the second yellow down spike.... and possibly with the others. Not sure what it means at this point. Hopefully, your current "R&R effort" will have some impact.

 

[gmcman]

Bah, I'd cut the wire to use a regular deep well socket to remove the old one and then use an open ended wrench to install the new one.

Yeah, you're right, the fact the intake will be removed should offer all the clearance necessary.