Misfires on one bank

Hello everyone I’m having an interesting issue. 2007 Tahoe LTZ 5.3 flex with 255k (unknown if engine was replaced but it’s clean with great compression) I’m having misfires on basically bank 2. Started with a p0300 and steady misfire on cyl 4 at idle with the occasional misfire on 2. Started by swapping coils plugs wires with other cylinders (6 and 1). No change at all. Scanners telling me fuel cells at 37 in red stuck open and running lean with high ltft’s around 25. Well checked maf and cleaned. Smoked test and no vacuum leaks. Wires replaced under warranty with new set of oem just because they were free(warranty I tore one taking it off) checked spark plugs and look good. Compression checked cylinder 4 and steady 152. So figured with 255k replace injectors. I replaced the injectors and it seems smoother but now I’m getting steady misfires on cylinders 2,4,6. Misfires only happen at idle and go away around 1500rpm.
So basically I just ordered a fuel pressure test kit cause mine didn’t have the adapter 3/8 for the gm. My next step I was thinking was to test fuel pressure to rule out weak pump(though figured that would act up under load not idle). Also I do have a shot exhaust I will be replacing. I got exhaust leaks at multiple points most notably I’m thinking at passenger side manifold which might also be cause. So I guess I’m asking what do y’all think? Am I on right path?

Welcome to the Forum from NH.

Lots of knowledgeable folks here who freely share their knowledge, experiences, and perspectives. Knowledge is power.

I hope that you will become a participating member in the Forum's discussions.

Pics of the truck, please.

I cannot personally assist you with your issue, but other members of this Forum much more knowledgeable than me in this area will chime in. Replies may be slow today, given that it is a holiday.

Clean MAF (GENTLY!) & change fuel filter (if that's still a thing).
Were I in the mood to fire the parts derringer, I'd still check the crank & cam sensors for accuracy before replacing them,
since they are chiefly responsible for checking misfires.

Did you dispose of old injectors?

And yes, fix that exhaust manifold leak.

All misfires on one bank? Hoping the exhaust leak is it..
Really big numbers for fuel trim, check O2 sensor output also.
Keep us posted!

Since you mention misfire only happens at idle, and very lean on same bank...

Does your scanner have the ability to monitor live data? What happens to the fuel trims when you rev to 2,000rpm? 3,000rpm? If not, look at the freezeframe data to see what RPM the code was set.

If fuel trims improve (get closer to zero), then that points to vacuum leak. That test is almost always correct to determine if you have a vacuum leak.

A fuel delivery issue will usually manifest itself with lean conditions on both banks, within 1-2% of each other because the fuel line delivers to both banks off the same line.

Additional tests can be listening to the offending injectors with a stethoscope and a fuel injector balance test. Some of the more advanced scanners will have that feature.

High fuel trims at idle that get significantly lower at higher rpms can also be caused by exhaust leaks. Since you know about those, I'd recommend to stop wasting money on guesses and do the maintenance. A lot of this can be diagnosed without parts changes, unless the parts (read: exhaust) are clearly faulty.

Save your old parts, especially injectors. They may be better than inexpensive aftermarket ones, if that's what was installed.

After you fix the exhaust leak, if the problem still exists then check the voltage feeds to the injectors and coils. Since the problem is on one bank, you only need to look at things for that bank. Systemic fixes like MAF, TB, fuel pump. filters, etc would show up on all cyliniders.

Thank you guys for the input. I do have a nice bidirectional scan tool to monitor data. When raising rpms to 1500-2k fuel trims almost even out status changes to closed and the 02 sensors seem to read correctly. Idling I have the obvious misfire status stays mostly in open and bank 2 sensor 2 seems to not move around that much unlike the other 3. I save all old parts btw lol never know and I have plenty of storage. (Still got a lot of parts from vehicles I don’t even own anymore lol).
I have the new exhaust manifold and resonator flex pipe ordered so will change those out once it gets here and will update. Side note last few days when I go to start it in morning it hesitates for a second.

I tried to add a video of scanner but I can’t upload videos for some reason
https://files.fm/u/m2xp8acmr8
Anyway the vid shows when I give it gas and raise rpms to over 1500 both o2 sensors for bank 1 and the sensor for bank 2 sensor 1 all jump around a lot ranging from 50-800. Bank 2 sensor 2 only moves around from 790-850

There is the possibility your bank 2 sensor 1 is the culprit.
Since generally the primary O2's determine fueling, and secondary is used to verify cat function.
(I have seen a thread where the secondary O2 was somehow affecting fueling)

Your bank 2 sensor 1 only goes above .5V a couple times, whereas your bank 1 sensor 1 goes over .5V twice as frequently.
(hard to say for certain without a longer period, plus your fuel trims being maxxed out, assuming you haven't reset them...)

If bank 2 sensor 1 is going bad and reading lean, computer injects too much fuel..
Bank 2 sensor 2 could be good, constantly reading rich because it actually is.

Here’s a bit longer of a video of 02’shttps://files.fm/u/gs8kkat2c5

The values would be much easier to compare if you could graph them, along with rpm. Is that possible?

My vote is also the O2 sensor

Unplug the bank 2 sensor 1 and drive it, if it runs normally you found it; the check engine light will come on but it will run fine if that sensor was sending a false lean

If that's it resist the temptation to change them all, new sensors all suck; you might have to buy several before getting one to work right

Good luck!

I like @RET423's suggestion of unplugging sensor to see if misfires stop.. Nice and easy!
Though my thinking regarding replacement is... I don't want to deal with another O2 down the road, so I may replace all at once! (easier now since you'll have the exhaust out to deal with the leaks)

In the last video, both bank 1 sensors were rich all the time.. And both bank 2 sensors were lean until you hit the gas pedal.
Not sure if it's the refresh rate, or as @Fless mentioned vacuum leak? (since it's somewhat reversed from your first video)

I had issues like yours and eventually every cylinder on bank 2 stopped firing completely! Although each cylinder has its own coil the entire bank of coils shares a common ground. Basically the ground connected to the block and the eyelet was seriously corroded. I've heard it happening to others from the rust belt. IIRC it connected to the block on the driver's side near the front of the engine. I put a new eyelet on years ago and been running like a champ since.

KerryBoehm said:

I had issues like yours and eventually every cylinder on bank 2 stopped firing completely! Although each cylinder has its own coil the entire bank of coils shares a common ground. Basically the ground connected to the block and the eyelet was seriously corroded. I've heard it happening to others from the rust belt. IIRC it connected to the block on the driver's side near the front of the engine. I put a new eyelet on years ago and been running like a champ since.

Click to expand...

I second that. I was thinking either that: a bad or almost bad ground, or you're experiencing what many others have in their GM trucks at some point: rodents. They love wires. One of the ingredients that goes into wire insulation is peanut oil. I got 2 questions for you:
1) Do you ever park outside at night?
2) Do you ever eat in your truck?
If yes to both, there's a real good chance of it. Rodents chewing your wires. And from your description, it fits the bill.

So been awhile but I got an update. After teaching myself how to weld I installed an entire new exhaust system to replace the old one with bunch of leaks. Misfires mostly stopped afterwards turned out the gasket from header to cats was completely gone. Now cyl 2 misfires maybe 1-2 times every few minutes at idle according to scanner. My mpg went from 12.5 to 15.9 so that’s a plus. Only thing right now is my ltft’s. My stft’s are usually around 0 but my ltft’s stay around -18%. I’ve smoke tested for vacuum leaks as well as used propane to check and just for S&G also checked with carb cleaner and can’t seem to find any leaks. Did just get a cel for fuel tank pressure sensor. Would that sensor affect fuel trims? Last thing on scanner my Fuel Cell Trim is red and reads 37

As to the grounds and wiring I live in northern Maine so mice and corrosion is a constant problem but due to that I keep a constant eye on it so can safely say that has been ruled out

So are your ltft -18, and stft 37?

Tank pressure sensor is mostly to check evap operation, which shouldn't affect fuel trims much, I think...

What does your tank pressure read? (volts)

Well, considering that the prior lean condition had the ECU adding fuel, there's a possibility that the oil now contains excess fuel. Try resetting the LTFTs with the scanner and see if they stabilize or if they revert back to deleting fuel. If they don't correct, you may need to change the oil or at least get a good long warmup with the STFTs being correct.

The STFTs might be fighting the LTFTs. How old are the upstream O2 sensors?