[FIXED] P0300 Misfire on Bank 2

I'm working on my dad's 2007 Chevrolet Suburban 5.3 LC9 with 285,000 miles. This is a fun one so far, I've had no luck on fixing this so far. Symptoms are a misfire at idle on cylinders 2, 4, 6, 8, P0300 on the scan tool. B2 LTFT is around -20 , LTFT on Bank 1 is around 0. Checked compression on Cylinders 2 and 4, both were 175, so I didn't dig any deeper there. Inductive timing light shows it skipping firing every so often on Cylinder 2 wire. Front B1 and B2 02 sensors look like they are oscillating as they should when I can get it to stay in closed loop. It will not stay in closed loop due to the misfires (flashing check engine light, service stabilitrak, etc). Fuel pressure is 56 PSI.

New plugs and wires as I wasn't sure if the wires had ever been replaced and there were mismatched plugs on Bank2 (2 platinum plugs, 2 iridium plugs). I went ahead and replaced the fuel injectors (as they were leaking a bit based on leak down test on fuel rail). Checked ground connection G102 and G103, 102 was very crusty on the outside, but clean on the connections, the wires and ring connections are a bit green looking. Swapped coil rails from Bank 1 to Bank 2, no difference. I've run out of things I know how to check (not a technician here, just a backyard mechanic).

Video (assuming the link works!) is secondary ignition on the scope from B1 Cylinder 1, I have little experience with this, but it looks weird to me (shifting upwards after firing line).

Hey dbadge! I have a couple things I'd like to clarify. First, does the misfiring occur at idle only, or at all times? Second, does the misfiring occur during closed loop and open loop? Or closed only?

The first thing that comes to mind is an O2 problem, but you've stated that the O2's seem to switch normally. This is kinda why I wanted to clarify the closed/open loop part. If it misses in open loop, then there's no worries.

Second is a plugged cat on bank two. The fuel trims support this to an extent, which is why I was asking if it misses all the time. If it really has a plugged cat, then it'll also have a severe lack of power doing down the road. If it misses at idle only, then that's a different problem.

If you suspect a cat issue, you can attempt to remove the upstream O2 on that bank and see if the engine runs any better. You could also install a backpressure gauge, if you have one available. FYI, the O2 test isn't foolproof. If the cat is plugged solid, even removing the O2 may not allow enough flow to create a noticeable difference.

Thank you for responding. Misfire occurs at all times, with one exception. Pretty much for the first 30 seconds after startup from a cold engine. During both closed and open loop, but it does seem worse during closed loop. I'll start pursuing plugged cats, I do have a fuel pressure gauge and a vacuum gauge that I might be able to ghetto fit to the O2 port. Attached the front 02 sensor readings, they aren't completely normal, but it's hard to keep it in closed loop long enough for things to settle.

No problem! Thanks for clarifying those things for me, and the shot of the O2's. Bank two is definitely struggling. :silly:

Before you go to the trouble of pulling the O2, you can also try monitoring Engine Load on the scanner during wide open throttle. Run out first gear from a stop, and note the highest reading. If it's a cat, you likely won't see anything higher than 50 or 60%. These trucks normally go to 95% or better.

You may also notice the bank one O2 going flat lean, and the bank two O2 going flat rich during the WOT run. That's another way to ID a breathing imbalance, and that bank two is the issue. Let us know what you find.

Pulled o2, misfires stopped, pretty healthy puffs out of port, so I'm going to call that conclusive.

Sounds right to me,

dbadge wrote: Pulled o2, misfires stopped, pretty healthy puffs out of port, so I'm going to call that conclusive.

Nice. I mean, that sucks about the cat, but at least you know now!

If you end up repairing this one, let us know how it runs afterwards? Also, Paul has some excellent videos for restricted exhaust testing out there, for anyone else interested. This Premium series is right up your alley:

www.scannerdanner.com/scannerdanner-prem...ghlight=WyI1LjMiXQ==

Thank you for the link to the videos, I think I reviewed a couple of them earlier, I will be sure to check them out. I know I have learned a metric ton of knowledge on these modern engines in the last week working on this car. This site and all of Paul's videos are absolutely indispensable for learning. I ordered his book as well, but slow shipping times. I'm much more familiar with carbureted motorcycles than any of these computer controlled engines (somewhat ironic that I work in Information Technology on computers all day long).

I visited our local muffler shop in town, the mechanic on duty used a bore scope to look in the upstream O2 port and the cat looked pretty clean... I was bummed out, asked him to check from the downstream port, looked pretty plugged on that end. Is it typical to plug on downstream side? Honeycomb was intact, just plugged, not bad for 290K original miles. Didn't charge to look at it. I'm going to end up buying a new y-pipe with both cats and have him install it for trade on the old cats. Seems worth it to me to not have to use choice language on frozen bolts on the flanges.

So, the root cause of cat failure? I'm thinking the leaky injectors? On cold starts in the morning after sitting all night, we would have soot/fuel spit out the exhaust, so I'm thinking there was fuel in the cylinder that doesn't get burned and gets spit out? I haven't seen that since swapping them out, but we also haven't been driving it.

O2 Imbalance? B1 looks lean, B2 looks rich WOT from 190-230ish.

dbadge wrote: Thank you for the link to the videos, I think I reviewed a couple of them earlier, I will be sure to check them out. I know I have learned a metric ton of knowledge on these modern engines in the last week working on this car. This site and all of Paul's videos are absolutely indispensable for learning. I ordered his book as well, but slow shipping times. I'm much more familiar with carbureted motorcycles than any of these computer controlled engines (somewhat ironic that I work in Information Technology on computers all day long).

You'll love the book, no doubt. Interesting that you mention IT, because you may find that kind of experience will pay off when it comes to network diagnostics on cars! :lol: Application layers, network topology and the like matter when chasing faults on vehicles with multiple networks.

I visited our local muffler shop in town, the mechanic on duty used a bore scope to look in the upstream O2 port and the cat looked pretty clean... I was bummed out, asked him to check from the downstream port, looked pretty plugged on that end. Is it typical to plug on downstream side? Honeycomb was intact, just plugged, not bad for 290K original miles. Didn't charge to look at it. I'm going to end up buying a new y-pipe with both cats and have him install it for trade on the old cats. Seems worth it to me to not have to use choice language on frozen bolts on the flanges.

No substrate failure? I'd say you're looking at an oil consumption issue, which is pretty common for the LC9, especially at this mileage. Any history of eating spark plugs? Cylinders 1-4-6-7 are the main culprits, as they're the AFM cylinders, and most likely to have oil control ring problems.

Thanks for posting the scan data! Very valuable to see on a known problem vehicle.



The Load graph at WOT is the giveaway. I somewhat misspoke earlier - on a V-engine, you'll see the Load trace get super choppy due to 'stacking'. This is caused by airflow stacking up in the intake, because one bank is flowing while the other isn't. That causes irregular airflow over the MAF, and you get this kind of trace. For reference, this is from a '12 GMC with an LC9, known good:

Tyler wrote:
No substrate failure? I'd say you're looking at an oil consumption issue, which is pretty common for the LC9, especially at this mileage. Any history of eating spark plugs? Cylinders 1-4-6-7 are the main culprits, as they're the AFM cylinders, and most likely to have oil control ring problems.

Yes, it started consuming oil around 40k miles, and around 150k Chevy dealer replaced lifters and new valve cover if I'm remembering correctly. He opted out on the lower engine rebuild to replace rings as the Chevy dealer indicated that they didn't think it was the rings. He has been just changing mainly spark plug 6 every so often (6 months?) due to fouling since.

dbadge wrote: Yes, it started consuming oil around 40k miles, and around 150k Chevy dealer replaced lifters and new valve cover if I'm remembering correctly. He opted out on the lower engine rebuild to replace rings as the Chevy dealer indicated that they didn't think it was the rings. He has been just changing mainly spark plug 6 every so often (6 months?) due to fouling since.

Yep, that's probably why it plugged the cat. Unfortunately, that means the new cat will have a rough life.

Did they offer to attempt to unstick the oil control rings? I can't find the exact TSB now, but it involves soaking the cylinders in top end cleaner for several hours. May be worth attempting once you get the new cats on.

I can't recall if they offered, it was a number of years ago. Also unsure if the AFM shield was installed, valve cover I remember having been replaced. In case anyone is looking for the TSB:
10-06-01-008F

Confirmed restriction on bank 2 with a back pressure gauge, at Idle...

dbadge wrote: I can't recall if they offered, it was a number of years ago. Also unsure if the AFM shield was installed, valve cover I remember having been replaced. In case anyone is looking for the TSB:
10-06-01-008F

Thanks! I dunno why I couldn't find that earlier. :silly:

That's a fantastic picture. :lol: If it's that bad at idle, it'll probably bury the gauge with a throttle snap. Let us know what happens with this one?

Post-Repair. No more misfires, engine power is back (yay, I can go over 45MPH). I'm still running into negative fuel trims, but I didn't remember to reset the fuel trims, so could just be resetting yet. I'll clear it this evening and take it for another run.

Looking good! Glad to hear this one is back to running on all cylinders.

How rich are the trims? If this is Flex Fuel, you might have a look at the learned alcohol content. If it's high (and your Dad doesn't use E85), then you can reset it and watch the trims improve.

ECM says alcohol content is 3% (probably low, it's all E10 around here). It was having trouble calculating the content prior to the misfire episodes, reading around 50%, I reset it back then. He says he has never used E85. I advised him of the GM recommended fuel fill up the procedure of driving around at least 7 miles or so after fill-ups for the alcohol content relearn.

The LTFT are getting down to -18, it looks like it is while decelerating. Does this look normal? Might be when 4 cylinder mode kicks in?

dbadge wrote: ECM says alcohol content is 3% (probably low, it's all E10 around here). It was having trouble calculating the content prior to the misfire episodes, reading around 50%, I reset it back then. He says he has never used E85. I advised him of the GM recommended fuel fill up the procedure of driving around at least 7 miles or so after fill-ups for the alcohol content relearn.

:lol: WAY ahead of me. Sorry about that!

The LTFT are getting down to -18, it looks like it is while decelerating. Does this look normal? Might be when 4 cylinder mode kicks in?

Yeah, I'm OK with that. I *think* that's part of the PCM's fuel cut decel strategy. But, why doesn't it just go open loop, like around frame 1700? Dunno. :silly:

Just to make sure, I dug up an '08 LC9 for comparison:

Thanks for that, it doesn't look far off from that 08. If you look carefully, at 1730ish, it does go open loop.