Misfires on one bank

[Fless]

Not sure if you looked at short-term or long-term fuel trims, but if the STFTs are negative (like you noted) at idle, then the ECM is taking away fuel -- that's a rich condition. Suspect that purge valve on the engine, it may be staying open when it should be responding to a commanded state.

A vacuum leak would show positive STFTs at idle, returning nearer to zero at higher rpms, absent any other issues.

Your screen shot of the MAF and rpm readings don't tell us much since the MAF values should be initially checked at hot idle, not elevated rpms.

 

[rdezs]

Ah, I missed the LTFT/STFT readings. Purge valve sticking open....surprised there's no problem starting.

 

[Fless]

Ah, I missed the LTFT/STFT readings. Purge valve sticking open....surprised there's no problem starting.

Right, at this point it's just something to check.

 

[Potius]

Will check that next. Maf was at 6 g/s idle 600 rpms. 11.6 g/s at 1200 rpms and 18ish g/s at 1800 rpms. Maf readings seem to be within range and rise proportionally to the rpms ruling that out (hopefully) still cleaned it earlier. Haven’t looked at purge valve at all yet so will check that next.

 

[Potius]

Here is 2 pics I took earlier today of fuel trims. Side note when I checked for vacuum leaks I did a smoke test from a few different locations. Also tried the propane test all around and for S&G also did the carb choke cleaner spray test all around.

 

[Fless]

I wouldn't keep looking for a vacuum leak -- just the opposite. While LTFTs at -14 aren't awful, when they're good they will be closer to 0. -25 LTFTs for bank 2 (passenger side) are what trigger the money light, and that shows that there is way too much fuel on that bank. ST and LT fuel trims for each bank should be added together for the overall fuel trim value: in your latest 2nd pic the Bank 1 overall FT is not too bad at -11% (still a bit high), but the Bank 2 overall trim is -25%.

I would suspect a leaky or over-fueling injector, especially if aftermarket injectors are in use. A leaky injector might cause hard starts, but minor leaking or over-fueling might not. An injector balance test could confirm or disprove that as a cause.

If it's been rich for a while -- which seems to be indicated by the LTFTs -- there could be fuel vapors in the intake, crankcase, and oil so that would keep things rich until the cause is corrected and it has time to burn off the vapors.

Did we ever see what the running fuel pressure is, or did I miss that?

Also, what composition of gas (E10 - 10% ethanol, E85, etc.) is it using and what does the scanner report for the calculated ethanol/alcohol percentage? Are those ethanol percentages similar, say, within 10% of each other?

 

[Potius]

I don’t seem to have the injector balance on the scanner for the Tahoe but the running fuel pressure was 55psi. Gas is e10 standard cheap pump 85 octane. Scanner says 10% as well. Has been running lean since last oil change about 2000 miles ago so probably plenty of fuel in oil lol. I’ll change that in the next few days. Thought it might have been injectors first so changed all 8 with new genuine gm ones. No change at all in anything. On another note was in the Tahoe a bit earlier and remembered that I have had a range afm disabler installed and has been for past 60k. I’m wondering if it’s blocked anything from being seen so figure I’ll unplug it and drive around tomorrow and see how she acts

 

[West 1]

I repaired a Mercedes E350 with a One Bank lean condition. After a boatload of hours testing I found a prior mechanic had somehow let a small bit of plastic get into the fuel rail on that side. Once removed the car ran like new. Embarrassed to say i had changed all 3 injectors and chased many other avenues before finding the plastic piece in that fuel rail. It was just enough to slow the flow and varied so sometimes it blocked fuel and other time fuel was fine.

 

[Fless]

Injector balance is a test that is done, it's not a scanner PID. Kind of wish GM had a power balance test like the Fords do.

Where did you purchase the Genuine GM injectors?

Well, I'm confused. The current (negative) fuel trims indicate a RICH condition, where the ECM is taking away fuel. Not sure where the lean condition diag comes in, but I haven't read through all of this. I don't really have anything more than that.

 

[West 1]

I did a semi power balance on my Yukon 6.2L this week, chasing a sometimes miss at idle. All fuel trims on mine are perfect well under 5% all the time STFT and LTFT. My scan tool allows shut off of one injector at a time. It does not measure the RPM drop so you need to do that seat of the pants but in my case the #1 injector has less RPM drop than the other 7 so I think I found the source of my sometimes rough idle. Worked like a power balance test sort of. Gave me what I needed.

Might offer information to use on this engine issue.

A lean cylinder will miss at idle, gas fires off easier in a rich condition.

 

[Potius]

Well took off the purge valve solenoid and tried to blow through it but wasn’t able to indicating that it was good and still closing. Going to try a power balance tomorrow and see if any noticeable difference in cylinders 3&4 where the misfires are. Got the new injectors from rock auto but made sure to keep my old working just fine ones as well which are also oem. I even remembered to number them so can always swap out both those with known good ones just to rule that out as well

 

[Marky Dissod]

... even remembered to number injectors so can always swap out both those with known good ones just to rule that out as well

In an ideal world, if the part numbers are the same, then all eight injectors should be and perform identically.
In the real world, since each side has its own O2 sensor, actual goal is 2 teams of four injectors (L vs R) that are evenly matched against each other.
Some firms that clean/rebuild fuel injectors include where each injector ought to go as part of their performance report.

 

[Potius]

Ok so fun update. Unplugged the range obd2 afm disabler for first time in 5 years and went driving around town. She runs a lot rougher with it unplugged and has harder misfires. Also seems it was blocking some codes being plugged in cause I got a few lol. Got a
P3400- afm issue though it’s on opposite bank as misfire
P0300 random mis but scanner shows it on 4
P0420&p0430- assume caused by steady misfire
P0106- MAP performance
P0171-bank 1 lean
P0174- bank 2 lean
P0175- bank 2 rich
P0573- brake switch circuit
Stabilitrac warning popped on twice also and caused blinking CEL. This is starting to seem like when I was chasing a misfire on my 2015 Silverado finally after checking literally everything else found a dead lifter

 

[rdezs]

I consider risky to unplug a disabler thousands of miles after it was plugged in.... No data to back that up, but left unused for so long would make me very cautious.

I'm assuming you plugged it back in and you're back to misfires on only the two cylinders? Might as well pull the valve covers and check for a collapsed lifter I guess.

After rereading everything, I lean towards a couple bad fuel injectors..... But now you have to check on those lifters. I also wonder about the disabler itself. No idea what happens when they have an internal failure.

Interesting.... While unplugging the disabler may have further convoluted the situation, by the same token it may actually be pointing to the problem.

I noted your compression reading was 150? That's at the very low end of what I'd expect. A partially collapsed lifter could give you the low compression reading....but.... On two opposing cylinders at the same time? That seems doubtful. Especially since one is an AFM cylinder and the other is not.

 

[Doubeleive]

before you go digging into the engine do yourself a favor and do a backpressure test, pull the forward 02 sensor out and stick a fitting in there with a gauge, it should read no more than 5psi.
I had similar issue's and after a boatload of money, discovered the exhaust was plugged. which may or may not have been the entire problem but we gutted that 3rd cat out and she runs like a top again. the reason the 3rd got gutted is because I blew all the crap out from the forward cats and it clogged up the 3rd. but either of the front ones could be clogged/burnt/melted and causing backpressure.
I had 10psi at idle and 15+ at throttle
harbor freight is your friend

 

[07burban88]

what about a sticking pcv valve or weak coil or spark plugs?

 

[Doubeleive]

also if you have not already done so
1 at a time
swap plugs wires, see IF the problem follows
swap coils, ditto
swap plugs, ditto
if no change then check inspect the coil/injector wire harness

 

[Potius]

Wires are new genuine gm tested resistance even first. Plugs are newish at about 10k miles. Tried swapping coil packs and plugs back and forth with no change. Changed pcv early on with brand new one. Going to check injector wiring and clean up all ground points tomorrow for good measure as well as the back pressure test. If nothing else valve covers coming off this weekend and maybe the head for good measure. I can’t think of a single thing else that hasn’t been mentioned here and/or checked tested. At this point finding a partially collapsed afm lifter on #4 will almost be a relief

 

[Fless]

What PCV did you change? It's an integral part of the DS valve cover.

 

[Doubeleive]

Wires are new genuine gm tested resistance even first. Plugs are newish at about 10k miles. Tried swapping coil packs and plugs back and forth with no change. Changed pcv early on with brand new one. Going to check injector wiring and clean up all ground points tomorrow for good measure as well as the back pressure test. If nothing else valve covers coming off this weekend and maybe the head for good measure. I can’t think of a single thing else that hasn’t been mentioned here and/or checked tested. At this point finding a partially collapsed afm lifter on #4 will almost be a relief

Newish doesn't mean diddly we can't even tell you how many times it has turned out to be the problem..... knock-off, cracked, miss gap, so on so forth.
the diagnostic tree is on purpose, skip a step and you'll kick yourself later if that's what is was.