Need help identifying source of cylinder misfires

[Mango357]

I have a 2011 Silverado V8 4.8L. Recently I was having issues with a rough idle, but it wasn't bad. Two days ago I started hearing an intermittent ticking while driving. When I used my OBD2 reader, it showed P0307 but there was not a CEL. Further system diagnostics showed several hundred misfires from the last 10 trips in cylinder 7.

Random side note, I get a puff of white smoke from the exhaust on cold starts. Previously had it diagnosed at a leaky valve seal, so adding Lucas oil treatment every other oil change seemed to largely fix it.

Today's tried to fix the misfire and ticking, so I put in new oil (5w-30 for high mileage engines) and a quart of Lucas oil treatment, which didn't fix anything. Ran a few errands and it acted the same as before. Next I replaced all of the spark plugs and swapped the spark plug wires for cylinders 1 and 7. The misfire stayed with cylinder 7, so I know it's not the plugs or the plug wires. After swapping the plugs, I did a short drive around the block and everything was the same as before. Had 50-100 misfires per the OBD2 and the ticking sound was still very intermittent.

After dinner, took the truck back around the block to show my dad the ticking sound. The sound came on as usual after a minute or so, but this time didn't go away. Stayed on the entire drive. Turned it off and back on and the took the attached video.

Afterwards I drove ~10 min home and had almost 7000 misfires in cylinder 7. Ticking sound occurred the entire drive, but was much reduced at 40+ mph.

My fear is that this is a bad lifter. I'm wondering if maybe there is crap in the lifter and the Lucas oil treatment just made it worse since it's so thick?

Any insight on path forward would be much appreciated! I'm thinking about trying to replace the oil tomorrow with full synthetic and some sea foam. Thoughts?

[shorod]

Welcome to the forum! There was no video link, but you would not be able to attach a video directly anyway. You need to host the video on a video sharing site, then link to the video here.

Since you feel you've ruled out spark plugs and wires, what about either coil or fuel injector? Before being too concerned about a lifter issue you'd want to do either a compression test or a cylinder leak down test.

Does your Silverado have an individual coil per cylinder or does it share one coil for multiple cylinders?

-Rod

[Mango357]

Thanks for the quick reply! Here is a link to the video:

https://youtu.be/zVKKXqDUJAs

My Silverado has an individual coil per cylinder.

I haven't done a compression test or leak down test. I'll work on doing that today.

[Mango357]

Ran a compression test. All cylinders except number 7 read normally at ~150psi. Cylinder 7 was reading 90-150 on the first compression test after the engine sat, and then would read basically 0 for the subsequent tests. Seemed odd.

I ran air from my compressor into cylinder 7 at about 60psi and traced the leak to coming out of the oil cap.

Since there clearly was an issue inside the cylinder, I took off the coils and the valve cover to get a better look. The valve springs for cylinder 7 both looked pretty good. Cranking the engine showed everything moving correctly, except the second to last rocker which was completely motionless. I pulled the rocker off and pulled the pushrod out. The rocker and pushrod both seem to be in good condition.

I reinserted the pushrod and held my finger on the top of it while someone cranked the engine. The rod didn't move at all. I'm guessing a bad lifter?

Next step seems to be to remove the head and get to the lifter. Was hoping to avoid this, but oh well.

[Ol'Jim]

Sounds like a bad (wiped) cam lobe, and/or lifter.
Not sure about these LS engines, I work on older BBC & SBC stuff, but can't you get to the lifters without pulling a head?

[harleyguy]

Shine a light in the hole and see if you can see the lifter moving when someone cranks it over. Sounds like a cam lob pull the intake and you'll be able to see the lifter.

[j cAT]

These engines require dexos GM spec oil. your comments indicate you used the wrong engine oil . remove the valve covers and inspect .. valve train failure from wrong oil .. sludge blocking the small oil passageways..

[maxwedge]

Hmm , roller cam is usually not an issue with lobe wear, I do believe there might have been roller lifter issues though, either way disassembly is in order.

[Ol'Jim]

Unfortunately, I agree with Max Wedge. If that pushrod is not moving, there is no magic additive that will correct the cause. Even if the lifter was collapsed due to a clogged oil passage, it should still move.
I suggest you pull the intake manifold, and remove the valley cover under it, which should allow you to see what is going on down there.
You can find many good videos on YouTube, showing step-by-step how to change intake gaskets, that same procedure will get you to where you can open the valley up. Takes about 1.5 hours, (working carefully and methodically), to get to that point.
Please keep us posted with your findings, and best wishes for a speedy and not-too-expensive repair.