[SOLVED!!] 2011 Chevy Silverado 5.3L Cyl 8 Misfire

I'm hoping this will be an easy one for someone who has more experience on these than me.

2011 Silverado 5.3L. Engine has about 50 miles on a reman from Autozone due to the all too common cyl 6 collapsed lifter. I didn't do the swap.

It came in with just a P0300 code and misfires showing on 2, 6, & 8 but mostly on 8 with only a few on 2 & 6. No misfires are felt. Checking fuel trims it showed -17 on the LT and +/-3 on the ST on both banks. WOT O2 sensor response is full rich and both upstream sensors appear to cycle normally. MAF showed around 6.5 - 7 g/s at idle which seems a little high but it has an aftermarket K&N cold air intake on it and my misfires are only on bank 2. The Fuel composition was running at 16%. After confirming no E85 was being used, I reset that which put it at 3% and the fuel trims were more normal after that but still negative most of the time. I don't know what kind of oil was put in it but it didn't smell like regular oil and might possibly have some fuel in it but since I didn't do the swap I didn't want to change the oil just yet.

A visual inspection found a twisted up B2S1 sensor and when I went to unplug and untwist it I found the sensor was loose. Aha, I thought. I fixed that and did another test drive. No change.

I decided to have a look at cyl 8 spark plug. When I went to pull it out I found it loose in the hole. I then checked the others and found every spark plug loose in their holes! Aha! That has to be it! I tightened every plug..they were all barely hand tight. I also swapped the #8 plug, coil, and wire with #1. Test drove the vehicle and still have a cyl 8 misfire. I didn't do a relative compression test but did do a clear flood crank which sounded perfect.

What I've found with this misfire is that it only happens once per ignition cycle and seems to occur on the first decel event of the key cycle while driving it. When it does it I see a bunch of misfires on cyl 8 and I get a flashing MIL. After a few seconds of that, and not being able to feel anything, it never misfires again until the ignition is cycled off and on again. I tried to do an injector balance test but my Autel DS708 doesn't give a very robust test for this. I can select a cylinder for test but it doesn't prime the pump between each event, nor is there an option to allow me to, and it only runs the injector for less than a second, dropping the fuel pressure by 1 psi or less, so that was inconclusive.

I did perform a CKP relearn, as well, which didn't change anything.

So, my question is what occurs during the first decel event of a key cycle that would cause this misfire that never happens again after that? I do suspect possibly some fuel in the oil, but that doesn't explain why this misfire happens only once during the first decel event of an ignition cycle. Fuel is cut during decel so I was thinking possibly leaking injector but no sign of a leaking injector when I was trying to do the injector balance test.

Yes, it should be covered by the shop that installed it but it was Thursday night when the guy picked it up and he went back to tell the guy,who was getting ready to leave for the long weekend, about the misfire and he told him it may just have a bad wire or coil or just need some seafoam. Really? A new engine needs Seafoam? So, he brought it to me, thinking if it needs a wire or coil I could easily find that. I spent a couple hours on it but the guy had to leave so I don't have the truck at the moment but it still has me perplexed.

Anyone have any thoughts on this one?

Thanks,
John

Fuel in oil would be rich on both banks.

A head gasket may be leaking on that side that is rich.

I think you’re right about the leaking injector though.

Fuel cut-off would happen during decel, a leaking injector could possibly be causing cylinder wash.

If you smell fuel in the oil, that could be a clue pointing you toward a leaking injector.

What does the spark plug look like?

Id start with a secondary ignition capture which should give some direction. Can you post a secondary capture and an injector capture? If you can along with current ramps.

I haven't looked at anything gm that new but on some earlier ones I've noticed a list of actions taken until next ignition cycle for several P codes. Maybe take a look to see if P0300 turns anything off which could be causing it. Or possibly it's switching loop status which turns the problem off?

Awwww man, I was SO ready to tell you to do the CKP Relearn. :silly:

What's the eighth digit of the VIN?

This might sound wild, but you can try removing the serpentine belt (the alternator/PS belt) and doing another decel event. I've seen worn belts and malfunctioning accessories cause changes in crankshaft speed that get interpreted by the PCM as a misfire. I know the A/C uses a stretch belt, so leave that alone.

Do you have an in-cylinder pressure transducer? At this point, it may be worth getting a waveform from #8 and a known good cylinder. Save your captures. If you find an intermittent valve sealing issue, you can pass that information along to the original shop, who can use that to get The Zone to pony up.

Thanks!
It's a VIN 3. I hadn't thought about something on the belt circuit. I don't have the truck and probably won't get it back before it goes back to the shop that did the engine swap.

I have a Vantage Pro and have a cheap pressure transducer. I haven't yet tried it for in-cylinder waveforms. I wanted to give it a try but the guy had to leave. I'm not sure it would be fast enough to give me a good waveform but I need to get on the stick and see what it will do.

I'd suspect a valve sealing issue but how could only the few seconds of misfire and blinking MIL go away and not come back during the key cycle? The MIL stays off after that, too.

Sitting here writing this post something hit me. I didn't think of it until now but this would make sense...if the engine is fully warmed up and there are no codes then the engine would likely go into cylinder deactivation on the first decel or cruise. If it sees a misfire on #8 during decel/cruise and during cylinder deactivation, sets a code and blinks the MIL, I assume it would stop cylinder deactivation after a few seconds. Is it possible it stops cylinder deactivation for that entire key cycle? I didn't think to check this with the scan tool.

Theory: Is it possible to mix up the #6 and #8 injector connectors? If they were mixed up it would probably run fine with all cylinders firing. When it goes into deactivation it would kill injector #6 but if injector #6 connector was plugged into injector 8, it would misfire on cylinder #8 and only during deactivation. Additionally, it would explain fuel in the oil as injector #8 connector would be firing injector #6 when it shouldn't be and likely contaminating the oil.

Now I wish I had the truck back to check!

Cylinders #7 and #8 are on opposite banks....the harnesses are to short to get messed up, I'd guess. If they are long enough to get crossed than you may be on to something.

Yeah, I mixed up my cylinder numbers and corrected my post above. I meant cylinders 6 & 8.

So....SOLVED!!! The guy lives just a couple miles from my house. I messaged him with my theory and he opted to come over and let me look, even though it was 10pm here. He has one of those AFM defeat devices so I told him to plug it in on the way over and let me know if it misfired. No misfires on his drive over...first time he's driven it with no misfires. When he got here I grabbed a diagram and, sure enough, cylinder 6 and 8 injector connectors were mixed up!

So, my theory above was proven and the truck is fixed! He drove away VERY happy with me!

Wish I could make videos. It would make a great case study. Having those wires mixed up goes right along with loose spark plugs and O2 sensors. Makes me wonder what else is loose in there.

Well done.

How far out of place did the injector connectors look while they were swapped? As in, were the wires stressed? Or, would you not have noticed unless you were looking for it?

(sorry, for some reason I'm not getting subscription emails from this thread)

The harness looks fairly trashed...like not much tape and the plastic wire loom is all broken and gone, likely from heat from the manifold. I don't know if that's how it was before the swap or not. It has one of those plastic zip tie style harness stays that presses onto a stud on the coil pack bracket. It was pressed on to the wrong stud. At first the cyl #8 connector wouldn't reach #8 injector but when I moved the stay to the rear stud then the harness seemed to fit correctly and the #8 connector reached to the #8 cylinder. It's an honest mistake but if you're unsure when putting it back together, how hard is it to pull a diagram and make sure they're all in the right place?

He's likely going to bring it back to me so I can clean up the harness and fasten it down properly so we don't end up with chafed wires in the future. I think the shop that did the work should do that but we all know that a shop that lets work go out looking like that, if pressed to do more, probably isn't going to do it many favors.

It was obviously a hurried job with not much care taken. I mean, every spark plug was loose in the hole--loose enough that I could back them all out by hand. Had I not found that it may have needed cylinder head(s) in the near future when a plug or plugs blew out.

The wires weren't stretched or tight when they were swapped. There was no way one would have noticed it other than the wire colors were wrong on those two cylinders.