2011 Suburban 5.3 cylinder misfire

[Thedortondad]

I didn’t plan on posting, but searched for about 30 mins to find answers to my questions without being able to find them. 
 

I recently bought a 2011 Suburban with 135k miles. Ran fine for about 3 weeks, and one morning my wife went to start the truck and it had a god awful miss. I changed plugs and wires, as I assumed they had probably only been changed once and could use a new set anyway, which seemed to make it run a little better,  but still has a miss. Tracked down the miss to cylinder #6, changed the coil, and nothing changed. Checked the compression and it has absolutely none. Checked cylinder #4 for compression and it’s fine. Pulled the valve cover off and checked to make sure all rockers were moving while trying to start it. 
 

At this point, I’m trying to put it in TDC to blow air into the plug hole and see if it’s a piston/ring issue or valve issue. What are the indicators on this 5.3 for TDC? Really just trying to narrow it down to what it might be and what my next move for it will be. 
 

Any help/suggestions is greatly appreciated. 
 

Alex

[CamGTP]

Sounds like a collapsed AFM lifter on cylinder 6.

 

Cylinder 1, 4, 6 and 7 are all AFM cylinders that shut off when the engine goes into V4 mode.

[swathdiver]

While rare for a 5.3, they sometimes break a valve spring, pull the valve cover on that side and have a look.

[Thedortondad]

I turned it over with the valve cover off and the rocker arms were moving for cylinder 6. Does that rule out the collapsed lifter diagnosis? 
 

I originally pulled the valve cover to check and see if a valve spring was broken, but they all looked to be intact and undamaged. 
 

Would a burnt valve cause these issues, and still allow the valve to move up and down, showing the rocker arms still functioning as normal? Used the “rag test” on the exhaust last night and it acts like it wants to suck the rag up into the muffler. 
 

Thanks for the replies. 

[Papajohns45]

Same issue here on 2007 Tahoe 5.3 LMG motor. Cylinder 7 but on mine the lifters were ticking for almost 2 years off and on. Then on highway it stopped ticking and started missing real bad. I changed plug and wires and coil then did fuel injector. Nothing really helped and its smoking after running for 5 or so minutes and smells like fuel real bad in exhaust. 

[Thedortondad]

4 hours ago, Papajohns45 said:

Same issue here on 2007 Tahoe 5.3 LMG motor. Cylinder 7 but on mine the lifters were ticking for almost 2 years off and on. Then on highway it stopped ticking and started missing real bad. I changed plug and wires and coil then did fuel injector. Nothing really helped and its smoking after running for 5 or so minutes and smells like fuel real bad in exhaust. 

Have you taken the valve cover off to check if the valves are moving the rocker arms?

[swathdiver]

On 11/12/2020 at 5:14 AM, Thedortondad said:

I turned it over with the valve cover off and the rocker arms were moving for cylinder 6. Does that rule out the collapsed lifter diagnosis? 
 

I originally pulled the valve cover to check and see if a valve spring was broken, but they all looked to be intact and undamaged. 
 

Would a burnt valve cause these issues, and still allow the valve to move up and down, showing the rocker arms still functioning as normal? Used the “rag test” on the exhaust last night and it acts like it wants to suck the rag up into the muffler. 
 

Thanks for the replies. 

Could, do you have one of those little cameras to stick down the spark plug hole?  Make sure the piston is intact.  If more fellas don't chime in here, check in over at the TahoeYukon forum.

[bg1988]

On 11/12/2020 at 2:14 AM, Thedortondad said:

I turned it over with the valve cover off and the rocker arms were moving for cylinder 6. Does that rule out the collapsed lifter diagnosis? 
 

I originally pulled the valve cover to check and see if a valve spring was broken, but they all looked to be intact and undamaged. 
 

Would a burnt valve cause these issues, and still allow the valve to move up and down, showing the rocker arms still functioning as normal? Used the “rag test” on the exhaust last night and it acts like it wants to suck the rag up into the muffler. 
 

Thanks for the replies. 

 injector? maybe

 

you need fuel spark and compression to make the fire

Edited February 20, 2021 by bg1988

[Jeffrey majeske]

I am having a similar issue with my 08 Sierra 5.3. started with p0307, misfire in 7th cylinder. did all the normal things still misfired. Took it to dealer, they said they think valves on 7 aren't seated well, needs to be machined but they can't do it. Took heads to machine shop, good guy, told me there's not enough material to machine, I needed new heads. Heads were $1600 but i didn't want to go through the trouble to have DOD problems down the line so I bought a remand motor which came with heads for $3200. Installed the motor, new knock sensors, crank position sensor, crank shaft sensor, already replaced all injectors, plugs, MAP and MAS sensors. After all that, I am having the EXACT same code p0307, now with a p0300 (random misfire) in-between. Its driving me crazy!!