P0171 CEL pops up only when climbing on-ramp to highway speed. (solved) (1 Viewer)

[KFunk740]

Anyone had the CEL (P0171, lean in bank 1) only pop on right after getting on the highway up to speed? It then stays lit until I disconnect battery for ~5 minutes, goes away, then goes around town and up big hills fine for several days, then pops right back on after I go from low speed and get it up to speed on a highway on-ramp.
Anyone had that particular failure mode and which part fixed it?

176k miles on LC. I went ahead and swapped air filter and fuel filter (needed done), sealed one possible vacuum leak. Did a quick spray with electronic cleaner on the MAF, but may try harder with that. Got a new PCV to pop in in the next few days. Wondering if a long vacuum drag on that PCV overcomes a weakened spring/ball or whatever is inside it which develops in to lean condition only on those on-ramps.
If that doesn't solve it, I'm getting to more expensive parts to try (MAF, O2 sensor, etc.), buy a fuel pressure guage, etc. If someone else had that same on-ramp failure mode for say the MAF or 02 to help narrow it down, that'd help me out.
I certainly have exhaust leaks downstream near the middle muffler, but those have been there a while, and are way downstream from O2 sensor. No obvious leak near manifold, only ticking I can hear is probably cold valves. I've also done a can of seafoam in tank several months ago.

Thanks for help.

edit: Solved, I think. Smoking gun found. Bank 2, Sensor 1 oxygen sensor was loose in threads. Apparently codes can lie as well, as fuel trims indicated lean issue on Bank 2, despite P0171 saying issue on Bank 1.

 

[abuck99]

Not ruling out a fueling problem but that lean code might point to an air leak: I'd look at lines and connections for breaks, cracks etc. Check/listen for manifold or exhaust leak on that side. You can pull the 02 sensor & MAF and test it very easily with a basic multimeter.

If you have access to Techstream you might monitor long & short term fuel trims

 

[KFunk740]

Yeah I'm aware of those things and haven't yet found any issues. I'll have to look up O2 and MAF sensor testing procedures soon I bet. I haven't yet tried getting the bootlegged Techstream running.
I'm just severely limited on time I can spend working on anything, had some hope I could narrow it down based on the distinctive on-ramp failure mode.

 

[Alibaster]

Is it on ramp failure or is that when you’re completing the test cycle? You are probably already aware but I believe in order for the CEL to pop the drive cycle needs to be complete. So it might be possible that the on ramp is the final test in the drive cycle.


And for what it’s worth I had an equivalent code in my daily driver g35x and it turned out to be a tiny crack in the drivers die cat where it bolts on. I must have tweaked it driving like an a****** on s***ty roads. It was easy to spot from the dark black soot right around where it cracked. I threw some high temp gasket maker on it and it hasn’t come back in three months now.

 

[KFunk740]

I was unaware of what you meant by 'drive cycle,' new one to me. I'm more of a carburetor type guy, and don't know as much about EFI. I googled a bit, and I suppose its possible that it was stored from earlier and just completed its cycle then. Not sure, seems like I'd done all the required stuff a few times, just not that quickly on on-ramps.
In my experience with this LC, I've had the P0171 pop immediately after a failure and without completing the drive cycle. After going to get my new permanent plates and heading home, the P0171 popped on the highway, and the failure was quite obvious when I rolled into my garage. Strong fuel smell and fuel dripping onto the concrete from a rusted out line near tank. Replaced line, and light went away on its own.
This may be a different type of trigger though.

 

[Alibaster]

If you have an obd scanner, even the cheap ones, will alert you as to what systems have passed the drive cycle testing. Not specific to your situation now maybe, but if I am buying a used car I’m that guy and I bring my scanner and see if they pulled the battery. Pulling the battery resets the CEL until the cycle for whatever system is completed.

 

[KFunk740]

Does this tell anybody anything? I'm struggling to figure out anything with Techstream. It appears that this must have freeze framed when I got on the highway at 63mph and tripped the CEL. I saved this data while sitting idle in the driveway.

It has a new MAF, PCV, fuel filter, and so forth. No new O2 sensor yet, wondering if maybe I should go ahead and get fuel pump.

 

[KFunk740]

Bump, anybody, please? I need this thing reliable for a trip soon with kids.

From that freeze frame, it seems to me that it indeed is grabbing that only whenever I get good highway speeds and high demand like I thought. Any vacuum leak would show up at idle, and be overcome if engine revs. I tried reading fuel trims at idle, and they were all near zero. That stft #2 jumps quick to like 20 if I rev it, but I don’t know if that means anything. I thought the code was for bank#1. I need to do some more learning.
I’m still under impression of not enough volume delivered from fuel pump, maybe even the sock/pre-filter around it is clogged with crap.
I don’t have much time to investigate anything, got 2 kids in diapers in the house, plus work and grad school.

 

[robgv]

did you ever figure this out? I too have this reoccurring P0171 code.

I have replaced the following with OEM parts:

vacuum hoses
PCV grommet/fitting/hose
fuel filter
spark plugs
fuel pressure regulator
gas cap
Driver side, 1st O2 sensor nearest engine

Additionally, I cleaned the MAF and unplugged battery (this worked, but the CEL came back on after driving on the highway). I must admit, I have put in some non-91 octane in the vehicle several times. My next purchases might be a new MAF and/or fuel pump.

Not sure what to do here...

 

[Eicca]

Whatever is happening is only affecting one half of the engine. That means it has to be at the injectors, in the head, or in the exhaust on just that side. Everything else, like a garden variety vacuum leak, plugged fuel filter, or bad MAF, would affect both banks equally.

 

[KFunk740]

I have not fixed it yet. I have driven it over 2K miles ignoring it, hah.
I did show that freeze frame to my mechanic friend, who knows EFI far better than I, and he said it looked like fuel supply issue and thats its actually lean on both banks, but the computer was handling it OK. He said check fuel supply stuff and a good dose of injector cleaner. He seemed to think I could drive it just fine without too many issues. I'm getting my usual terrible 12 to 14 MPG, so it can't be too bad.

The things I've replaced are PCV hose, MAF, fuel filter, fuel pump, left rear 02 sensor gasket (blown out), and lots of downstream exhaust gaskets that were leaking anyways. I've also tried a lot of good fuel injector cleaner (Seafoam, Chevron Techron concentrated, BG-44K).
I couldn't find any leak around intake or injectors by spraying starter fluid around, no effect on idle. Spraying starter fluid around did make light go away for a little while. I figure some got into the air filter or otherwise and made it burn a little excess oxygen for some time and made it seem rich.
The pump probably didn't need replaced, but its the one failing part I know that could leave me stranded. Anything else is probably just emissions bull****.
Sock at the end was a a little dirty, so you could clean that for no investment.
I should probably try a smoke test for finding vacuum leak, but don't have a smoke generator and I don't smoke cigarettes or vape. I tried incense, but the fan just blew things around way too much. Another possibility is an exhaust manifold gasket leak, but I don't see much evidence of it, and I'm not eager to pull that off and break some studs in process.
I'm pretty sure most people hear a ticking for exhaust manifold leaks, and I don't hear that.
I have a new left front O2 sensor ready to change, but I couldn't get mine loose without taking more drastic action. Should probably put torch to it, maybe cut wire and put 6 point deep socket over it. Not convinced it would help though, since if it failed, it should trigger its own CEL.
I do need to change plugs sometimes for just maintenance, and my valve cover gaskets are leaking, probably unrelated.
Yeah, I may just keep ignoring it too, unless you come up with a miracle solution.

 

[Eicca]

I find what the mechanic said very questionable. Your screenshot shows bank 1 well within spec and bank 2 is waaaaay off.

 

[KFunk740]

Why isn't bank 2 triggering a CEL, though?

 

[Eicca]

Good point, since the code is for bank 1. It should be throwing P0174 for bank 2.

Although, when I had a nasty vacuum leak it only tripped P0171 even though both banks were super lean.

I would trust the raw data more than the codes. If you can, post a few more screenshots of fuel trims under various conditions (idling, city driving, highway driving, WOT, etc)

 

[KFunk740]

What do you think about trying this for vacuum leaks? It's a DIY smoke machine with oil in a paint can, connected to air compressor, blowing smoke into the intake hose (engine off), and look for smoke emerging from engine somewhere.

I thought smoke machines were used to see smoke getting sucked into vacuum leaks, but this looks far more convincing.

 

[KFunk740]

I went to move the LC a little while go, and by some miracle the CEL wasn’t lit up. No idea what the hell happened, hoping it stays off for good. We’ll see.
The only thing that’s happened with it is that it sat for 5 or 6 days, before which I put it in 4L for a little while to pull my riding mower out of the mud. I hadn’t used 4L in quite some time, so no idea how that might have changed anything.
Maybe pull a mower out of the mud or go wheeling? Or the fuel injectors finally cleaned themselves, or it’ll light back up tomorrow.

 

[KFunk740]

OK, some developments.

So, i think the CEL is solved. But, the cats, perhaps not so good. I’m pretty sure what solved the cel wasn’t running in 4 low, but it was the BG 44k super snake oil I put in it a week or two ago. It was the only real change I’d done in a while. I did add it with like a 1/4 tank of gas in the tank, so it was super concentrated. Then it probably finally unclogged injectors, and took a drive cycle or two to clear the cel after that (I don’t drive the LC much, just weekend trips usually).
So, yesterday I finally took it on highway and ran it up to speed a couple times, and no light popping on this time.
The last time getting on the highway though, I turn on the ect pwr button to help with merging, and by the time I got up to speed, there was an overwhelming methane-like rotten egg smell. After figuring out it was definitely coming from LC, and real bad, I pulled off highway to check stuff out.
Battery not overcharging, battery not hot, no smell under hood. Cats didn’t look cherry red by the time I looked, but I didn’t have temp gun and wasn’t about to touch them.
I kind of figure it’s been forcing it to run rich for the last 2k+ miles, and built up a lot of hydrocarbons and crap in the cats, and now that it’s back running lean, it’s burning all that s*** out. Hopefully they’re not damaged, and I can just keep running it a little more and get the excess burnt off. It sure smells nasty, though.

 

[robgv]

A new MAF fixed my problem

 

[KFunk740]

OK, revisiting this thread, as it turns out that I did not fix it last year, as the CEL came back a while, went away at some point, and came back again.

I think I finally found the smoking gun and fixed it. I had to learn how to read the fuel trims, and thats when I figured out that Bank 1 is actually OK and Bank 2 was real lean. Yep, P0171 was a lie from my computer. FTs looked fine at idle, then Bank 2 went way lean when under heavy load. If it had been a vacuum leak, it would've been lean at idle and went away under load. Anyways, at some point I had tried new fuel filter, pump, new fuel injectors, and FPR (wooooo, wasted money!).
So, now that I realized P0171 was lying about Bank 1, I went ahead to try and swap out Bank 2 upstream oxygen sensor, even though it seemed to not be throwing a specific code for sensor problem. Well, I was figuring out how to best get in position to get some arm strength to loosen that sensor, and when the socket wiggled over the sensor I thought it was slipping and too big. Nope, oxygen sensor was loose in its threads. Could wiggle it by hand until it tightened up a little in either direction. So, there we go, sensor working fine but allowing fresh air past threads intermittently. I went ahead and put a new sensor on, since I had one sitting next to me, but figure the looseness was the issue.
Went ahead and took it on the road, and Bank 2 fuel trims were fine. Drove it to work today and ran it under load a few times, no CEL.

So there you go, P0171 can lie about which Bank is the problem. Learn how to read FTs. Check how tight your O2 sensors are. I know my Bank 1 upstream sensor is too damn tight, since I checked it first, but hadn't looked at Bank 2 until a couple days ago.