P0174

[hmclaughlin67]

I have a 2005 Yukon XL with the 5.3. Today I got a code for P0174, or fuel trim lean bank 2. I was wondering how I possibly had a vacuum leak when I’ve checked this engine and worked on it up and down since I bought the car in September, but I remembered reading how the MAF sensor is a common cause. I went to go pull mine, and I noticed that the air restrictor that attaches to the side of my air box, right above the filter, is completely gone. This leaves a big hole in my air box and is definitely letting a bunch of dirty air into my intake hose. What are the odds this is my problem and it just made it so my MAF sensor got dirty quicker? It’s late, so I couldn’t get a good look at the sensor but I’m picking up a cleaner tomorrow morning and will update on if the sensor was disgusting or not. Temporary fix is just going to be to duct tape the hole shut and order the part as a replacement.

 

[strutaeng]

Usually, MAF misreporting would affect both banks (a single MAF sensor is used for both banks) so it's probably not the issue. It doesn't hurt to clean it. If you have a scanner with live data you can see if the MAF values jive.

My suburban LQ4 had been running very lean on both banks, 20-25%. I had cleaned the MAF sensor but didn't do anything. I noticed the reported MAF values were around 4.8 grams/second and it should be higher. I temporarily swapped the MAF from my other truck that has a 5.3 and idle MAF values are now around 6.0 gr/sec. Fuel trims dropped to like 10-15% at idle, which I'm still trying to figure out if I have a vacuum leak, because cruising they are close to 5-7%.

Another thing that can cause lean on one side is 02 sensor reporting inaccurately.

 

[Fless]

To throw another variable at you, an unbalanced fuel injector can affect one side (and sometimes both). Fix that air box (especially if the hole is after the MAF), clean the MAF and observe the MAF readings, observe O2 sensor performance and, if that doesn't help, do an injector balance test on all injectors.

 

[hmclaughlin67]

I have a 2005 Yukon XL with the 5.3. Today I got a code for P0174, or fuel trim lean bank 2. I was wondering how I possibly had a vacuum leak when I’ve checked this engine and worked on it up and down since I bought the car in September, but I remembered reading how the MAF sensor is a common cause. I went to go pull mine, and I noticed that the air restrictor that attaches to the side of my air box, right above the filter, is completely gone. This leaves a big hole in my air box and is definitely letting a bunch of dirty air into my intake hose. What are the odds this is my problem and it just made it so my MAF sensor got dirty quicker? It’s late, so I couldn’t get a good look at the sensor but I’m picking up a cleaner tomorrow morning and will update on if the sensor was disgusting or not. Temporary fix is just going to be to duct tape the hole shut and order the part as a replacement.

Update: I pulled the MAF sensor and the whole thing was covered in a brownish/gray sludge. I hosed it down with MAF cleaner, literally the whole can, and wiped everything to the best of my ability. Will update again if it works or not, I have an 8 hour round trip tonight so plenty of room to test with.

 

[Doubeleive]

Update: I pulled the MAF sensor and the whole thing was covered in a brownish/gray sludge. I hosed it down with MAF cleaner, literally the whole can, and wiped everything to the best of my ability. Will update again if it works or not, I have an 8 hour round trip tonight so plenty of room to test with.

it's very possible, those MAF's can make all kinds of wierd stuff happen everything from a-z
having a bad maf would be like drilling a hole in your skull and then go walking around like everything is normal.
and I can tell you that with the CAI i have on my 18 with just the removal of a small adapter that changes the airflow it throws a lean code, put the adapter back in, no code.

 

[hmclaughlin67]

it's very possible, those MAF's can make all kinds of wierd stuff happen everything from a-z
having a bad maf would be like drilling a hole in your skull and then go walking around like everything is normal.
and I can tell you that with the CAI i have on my 18 with just the removal of a small adapter that changes the airflow it throws a lean code, put the adapter back in, no code.

All was well for 160 miles/2.5 hours of driving, but then the code came back up out of nowhere. Engine still sounds perfectly healthy with no lack of power. Beginning to lean towards one of the injectors being slightly clogged. May pick up some Techron later and give it a shot. For now i just cleared the code and will wait to see if it comes back for the remaining 5 hours I have to drive tonight.

 

[Doubeleive]

All was well for 160 miles/2.5 hours of driving, but then the code came back up out of nowhere. Engine still sounds perfectly healthy with no lack of power. Beginning to lean towards one of the injectors being slightly clogged. May pick up some Techron later and give it a shot. For now i just cleared the code and will wait to see if it comes back for the remaining 5 hours I have to drive tonight.

How many miles on the vehicle?

 

[hmclaughlin67]

How many miles on the vehicle?

just ticked over 203k on this trip, will be at 203.5 by the time i’m back home

 

[hmclaughlin67]

How many miles on the vehicle?

worth noting i bought it at 198k a few months ago, and i’ve done a TON of “catching up” maintenance since then. i’m about $4k into repairs, and thats only parts and no need to hire someone else to do anything as of yet.

 

[strutaeng]

Definitely can be a dead or intermittently sticking injector. Injector balance test is best tool to diagnose that. Maybe pull the plugs on that bank to inspect them?

On the O2 sensors, you could swap the left and right (upstream only) to see if it's the one mis-reporting, if you don't mind crawling underneath and getting a bit dirty. And see if the problem switches with the other bank. Kinda of a pain, but that only costs some of your time. Just make sure truck is warm, not hot.

 

[Doubeleive]

just ticked over 203k on this trip, will be at 203.5 by the time i’m back home

do you have a scanner that can read the maf data?, it might be worth doing a drive record and see what the maf data shows.
from my own personal experience the maf's on the gmt800's can start going goofy around 160k miles, after that they are questionable in my book, had more than a couple go bad before. not saying go drop more money on one but I would at least see what it is showing data wise. also check what the fuel trims are

 

[Doubeleive]

Definitely can be a dead or intermittently sticking injector. Injector balance test is best tool to diagnose that. Maybe pull the plugs on that bank to inspect them?

On the O2 sensors, you could swap the left and right (upstream only) to see if it's the one mis-reporting, if you don't mind crawling underneath and getting a bit dirty. And see if the problem switches with the other bank. Kinda of a pain, but that only costs some of your time. Just make sure truck is warm, not hot.

yep that's another thing to try

 

[hmclaughlin67]

do you have a scanner that can read the maf data?, it might be worth doing a drive record and see what the maf data shows.
from my own personal experience the maf's on the gmt800's can start going goofy around 160k miles, after that they are questionable in my book, had more than a couple go bad before. not saying go drop more money on one but I would at least see what it is showing data wise. also check what the fuel trims are

sits around 1.2lb/min at idle while warm

 

[Fless]

sits around 1.2lb/min at idle while warm

It would be easier for everyone if you have it display in g/s. 1.2lb/min = just over 9 g/s if my conversion is right; that's a lot for your 5.3. Should be around 5.3 g/s at idle, about equal to the displacement in liters.

Also, to clean injectors, if it's a Flex Fuel engine you could run some E85 in it if that's available in your area.

 

[Doubeleive]

@swathdiver @Fless does that seem low? Or high lol

 

[Fless]

9 g/s seems high. At idle it should be closer to the displacement in liters.

EDIT: https://www.motorverso.com/what-should-maf-read-at-idle/

 

[strutaeng]

Yeah, 9 gr/sec definitely seems too high to me as well.

Now, I thought underreporting caused lean conditions and overreporting caused rich conditions. Is that correct? Or I got that reversed?

Seems the opposite of OP's situation?

 

[Doubeleive]

Yeah, 9 gr/sec definitely seems too high to me as well.

Now, I thought underreporting caused lean conditions and overreporting caused rich conditions. Is that correct? Or I got that reversed?

Seems the opposite of OP's situation?

that's what i thought but I don't know a lot about the maf other than the mf's can cause a headache, I have had them cause all kinds of codes, misfire, lean, bad cat, throttle body, vacum leak, shifting problems, you name it.

 

[Marky Dissod]

LEAN means pcm / ecm sees too much O2. Correction is to add more fuel.
RICH means pcm / ecm sees too much HC. Correction is to subtract fuel.

In other words, if it's running rich, it THINKS it was running lean before it applied corrections.
It's @ 9 grams / sec because that what it takes to make the pre-cat O2 sensors happy.

MAF sensor is not the boss. It just says how much air flows past it. It can easily be fooled / tricked / misguided / just plain wrong.
Pre-cat O2 sensors are the boss. If they see lean, fuel will get added. If they see rich, fuel will get subtracted.

Check for vacuum leakage / gaps between MAF sensor and pre-cat O2 sensors (besides the hole you just taped shut).

 

[strutaeng]

that's what i thought but I don't know a lot about the maf other than the mf's can cause a headache, I have had them cause all kinds of codes, misfire, lean, bad cat, throttle body, vacum leak, shifting problems, you name it.

Ok, looks like we're correct:

https://www.motortrend.com/how-to/1402-understanding-maf-sensors-technically-speaking/

I believe I mentioned that an issue with the MAF would cause both banks to be lean or rich, which is not the OP's case. But definitely MAF values seem out of range and need to be investigated further...

And the rule that there could be more than one problem always seems to apply on these types of issues. I guess try to solve the problem by taking it step by step, fixing known problems first.