2015 Chevy Tahoe 5.3 P0171 & P0174

Jason Owner/Technician Walnut Creek, California Posted   Latest   Edited  
Question
Driveability
2015 Chevrolet Tahoe LS 5.3L (C L83) 6-spd (6L80)—1GNSCAKC3FR647226
P0171 - System Too Lean Bank 1
P0174 - System Too Lean Bank 2
High Fuel Trims

Hello Folks, I have this Tahoe that I am stuck on. This is my first time asking for help here so please bear with me.

The customer just purchased this vehicle has no repair or service history on it. He said the check engine light came on 5days after he purchased it. The codes have been cleared a few times and return within a few days. 

What I found is that the fuel trims are very high under all conditions. Based off of what I am seeing I believe vacuum leaks and breathing issues are ruled out. I am leaning towards a fuel related issue. I am almost thinking an injector problem maybe? This is a GDI setup and this is where I am having an issue determining what the next step should be. I am not really familiar with diagnosing this system yet.

The engine runs smooth and has a lot of power. I haven't performed any intrusive testing yet as I really wanted to glean as much info out of the scan data as possible. I have attached the snap on file of my test drives. I apologize for the disorganized data. I was trying to grab as much as possible.

Any thoughts or ideas would be greatly appreciated.

Thanks,

Jason

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Gary Owner/Technician
Hydes, Maryland
Gary Default
 

Check the alcohol content of the fuel, may need to be reset, There is a PIP on those vehicles about the customers running E85 and setting those codes, also smoke test cold for any air leaks

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Jason Owner/Technician
Walnut Creek, California
Jason Default
 

I forgot to mention that I did reset the fuel composition. It indicated 10% before and after. I also reset the trims before and after with no change. I'm not really sure where they would have gotten e85 around here. I cant say that it is very common. I just smoke tested it. No leaks. I blocked off the pcv line and the purge lines as well with no change in trims. The way the trims read though…

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Samuel Owner/Technician
Williamston, Michigan
Samuel Default
 

What is MAF reading at idle and what room?

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Jason Owner/Technician
Walnut Creek, California
Jason Default
 

Hot idle in park is about 4-5gps. At WOT im hitting 100% calculated load.

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Jeff Technician
West Columbia, South Carolina
Jeff Default
 

While vehicle is warm shut engine down and turn key back on with engine off fast enough not to loose connection with scan tool. Graph fuel rail pressure pid, the only way it comes out of the rail is if the injector opens and heat in the rail will make the pressure rise over a few minutes. If it doesn’t then you have a high pressure leak but that would make the mixture rich not lean. if it’s high…

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Jason Owner/Technician
Walnut Creek, California
Jason Default
 

I definitely wasn't thinking carbon because this thing runs great and has lots of power.

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Scott Owner/Technician
Troy, Missouri
Scott Default
 

Have you checked fuel quality? High alcohol content in the fuel can cause this. Have you disconnected the MAF sensor and tried that?

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Jason Owner/Technician
Walnut Creek, California
Jason Default
 

I'll take a fuel sample next. I did check the maf. looked good. My tech pulled his good maf off of his personal vehicle and tested with no change.

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Scott Owner/Technician
Troy, Missouri
Scott Default
 

Everybody "checks the maf". Unless you define HOW you checked it and what your results were, it is a useless statement. Most people don't know how to check it. Remember, the sensor is only one part of the system Did you disconnect the sensor and see how that affected the trims?

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Jason Owner/Technician
Walnut Creek, California
Jason Default
 

I did disconnect and didnt see a change. I was basing the fact that I thought it was good off of what I was seeing with the calculated load at wide open throttle. We did substitute with a known good just to rule it out completely. I will recheck on monday when I get the vehicle back in and I can spend more time with it.

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Scott Owner/Technician
Troy, Missouri
Scott Default
 

IF you disconnected the sensor and it remained in closed loop with no difference in fuel trims, then you have completely ruled out the possibility of a skewed MAF input signal causing your problem. You can move past it to find other things. I would check fuel pressure and quality next.

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Geoff Diagnostician
Lahaina, Hawaii
Geoff Default
 

Hi Jason, I'm looking at the scanshare​.​io stuff that Phil converted your shoptream files to. There is long idle period at 500 RPM, MAF = 8 g/s and the MAP = 15 in-Hg. Your BARO is the same as here. MAP should be 10-12" at idle. which would be 18-20" of engine vacuum. What does MAP read on your buddy's truck? Skewed MAP will make all PCM calculations be wrong.

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David Owner/Technician
St. Albert, Alberta
David Default
 

Injector problem likely would not affect both banks like this. Maybe a fuel supply issue. I would be watching fuel pressure under load.

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Stephen Technician
Gallatin, Tennessee
Stephen Default
 

I’m wondering why bank 2 injectors pressure drop an average 20% more than bank 1?

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Jason Owner/Technician
Walnut Creek, California
Jason Default
 

That was my question also, but I dont think I would see such trim changes on both banks.

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Stephen Technician
Gallatin, Tennessee
Stephen Default
 

Injectors, fuel pressure and Injector drivers are all that is in effect for the injector drop test. No sensors or fuel quality will effect them. The side to side difference may no be THE problem but, I sure think it is part of the problem and it is reproducible so , it can be further tested as to cause.

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Lonny Diagnostician
Largo, Florida
Lonny Default
 

I would verify visually that there is not an exhaust leak. had an issue with a 08 Silverado 5.3 that had hair line fractures in the manifolds. replaced manifolds and never returned.

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Jason Owner/Technician
Walnut Creek, California
Jason Default
 

I did listen closely, I don't here any leaks, however, it has an aftermarket muffler on it. Cats are original. the muffler is welded in where the factory muffler should be.

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Scott Owner/Technician
Troy, Missouri
Scott Default
 

Is this going to affect both banks equally?

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Philip Educator
Hemet, California
Philip Default
 

Thanks for posting your PID files. scanshare​.​io/share/tLqS61rv… You are lean across the boards. But, at WOT you do go rich enough at least to keep the O2 Sensors above 600mv. However, on your way to WOT the O2 sensors go lean. scanshare​.​io/share/tLqS61rv…; It sure looks like someone added E85. Its worth the time to first ask the customer where he got that tank of…

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Jason Owner/Technician
Walnut Creek, California
Jason Default
 

I will take a fuel sample on Monday. I also didnt see a Maf issue based on the data. My tech swapped Mafs from his known good one out of his personal truck just to see. No change at all.

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Todd Diagnostician
Macomb, Michigan
Todd Default
   

Is the air intake ducting and filter original? If not, you'll want to reinstall original and re-evaluate. Also, what is the desired fuel rail pressure (I saw that its making 855 psi, but need to know what the ECM is commanding)? If you haven't already done it, I would also smoke test the intake (also watch around the fuel injectors, if someone had them out but didn't replace the seals it may be…

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Jason Owner/Technician
Walnut Creek, California
Jason Default
   

Yes air ducting is original, I found a bulletin that pertained to the ducting, but that all looks good. filter is a good factory filter. It appears as though the desired and actual fuel pressures stay pretty close to each other. I added a file that has high pressure fuel pressures

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Albin Diagnostician
Leavenworth, Washington
Albin Default
 

Since you are using a snap on scan tool, hook it up, graph : fuel trims both long and short, oxygen sensor voltages, throttle position, engine RPM, MAF volume & engine load. Now take it out on a FRTD, and post up the data files. Using this information, finding your problem should not be very difficult.

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Jason Owner/Technician
Walnut Creek, California
Jason Default
 

FRTD? I will gather more info on monday and repost. Thank you for the info guys!

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Philip Educator
Hemet, California
Philip Default
 

He's already done that, Albin. Here's the scan share links: scanshare​.​io/share/tLqS61rv… scanshare​.​io/share/pbC3h_qx… (no road test on this one it appears but there is on the one above including a WOT run)

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Geoff Diagnostician
Lahaina, Hawaii
Geoff Default
   

There's a long idle stretch in that first one, Phil, at 500 rpm and MAF = 8 g/sec. And the MAP = 15 the whole time. That strikes me as off/odd. ps, I think Bob B. said Jim's son invented ScanShare?

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Philip Educator
Hemet, California
Philip Default
 

Yes, that's correct on the origin of Scanshare. I like it much better than ShopStream. Wouldn't you figure if the MAF were over-calculating airflow, we should see negative fuel trim? I agree that the numbers are unusual but as a known=good has already been tried without success, I'd say it could only be a problem with the way the air is going through the MAF rather than the sensor itself and it…

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Geoff Diagnostician
Lahaina, Hawaii
Geoff Default
   

Agreed, normally a high MAF would give too much fuel, and FT would adjust Neg, however with the MAP that wrong, at the same time, too, all "normal" bets are off. It probably needs injectors, they are dribbling un-cumbustible globs, causing wash-down, low vacuum (high MAP), and extra clean air at the O2.....so the PCM is adding more fuel-globs....

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Paul Mechanic
Harvey, Illinois
Paul Default
 

Find out if customer filled tank with premium? I had sold a customer one of my cars that I had driven for almost eight years with no cel. She came back almost a week later with it lit, pulled 171 and 174. Ford product so I flipped in a known good test MAF-no change. I happened to ask if she had put fuel in it yet and she said yes, premium-nothing's to good for my baby. Told her to run it out and…

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Albin Diagnostician
Leavenworth, Washington
Albin Default
 

I'm at home and have time to look at your data. I found what I was looking for. I was able to find data you had grabbed at idle, cruise and on a WOT run. I have put some notes on the captures to raise some questions and point out what I am seeing. diag​.​net/file/f4fyabluw… ​ At idle speed of 500 RPM I would expect to see MAF of around 6 G?S on this 5.3 engine. Yours is showing 8…

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Helpful
Randall Technician
Kunkletown, Pennsylvania
Randall Default
 

Jason, Recently I had an 2011 GMC Acadia 3.6 GDI with lean codes. I did pressurize exhaust, and do the soap bubble test as outlined in the GM service bulletin. There was a bad weld on the rear bank cat. However the problem was carbon on the combustion side of the injectors. We replaced all 6 and walnut shell cleaned the intake valves. The owner had receipts for two recent injector cleanings…

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Jason Owner/Technician
Walnut Creek, California
Jason Update
 

Thank you for all of the thoughtful replies. I came in today to spend little time with this one before the Monday rush. As Albin suggested I rechecked for exhaust leaks. I stuck some rags in the tail pipe and had no change in fuel trims. Next I hooked up a fuel gauge and took a fuel sample. It looks like the sample has a high alcohol %. According to TSB …e this vehicle is capable…

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Todd Diagnostician
Macomb, Michigan
Todd Default
 

Jason, Glad to hear you've nailed down the diagnosis. I looked at the bulletin you mentioned and you're right, it says the 2015 Tahoe with the L83 engine is E85 capable. However, the VIN also has RPO FH0, which indicates it is only E10 capable. I then looked at the parts catalog, and it shows the 2015 Tahoe L83 only has the fuel composition sensor if it has RPO FHS, which this vehicle does not.

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Jason Owner/Technician
Walnut Creek, California
Jason Default
 

I checked for a Flex fuel sensor and it does not have it. I guess my first clue should have been the fact that the fuel composition sensor PID reads 0hz on my scan tool.

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Paul Mechanic
Harvey, Illinois
Paul Default
 

Did you query the customer of he indeed filled with a higher blend of ethanol? Even mistakenly? Would make the most sense given the codes set five days after purchase.

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Jason Owner/Technician
Walnut Creek, California
Jason Default
 

I talked to the owner of the vehicle, apparently he loaned it to his sister after he purchased the vehicle. She lives in Fresno and wasn't sure if she intentionally added E85 or not. I do know that it is available there. He opted to run the fuel level down for a couple days. He'll bring it back for a follow up and a smog, and I'll update at that point. Thanks again for all of the guidance…

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