Mystery Misfire #6

DJ Ragsdale · October 13, 2019

This is my first time posting so hopefully I'm in the right section. I have a 2012 Yukon with 6.2 and AFM. This is a long one but I'll try to be short. It all started with a loud knock coming from the passenger side of the engine. No check engine lights but the knock was there. I figured it was the lifter so I decided to go straight for it. Took off the valve cover and the #6 intake rocker was loose. I tore it down, replaced all 8 lifters (4AFM, 4 normal), replaced the lifter guides, cleaned the heads of the pistons with WD40+elbow grease and green scotch brite pads, cleaned the all the carbon off the heads and tops of valves, installed new felpro head gasket, intake manifold gasket, exhaust manifold gasket and put it all back together. Cranked her up and check engine light came on with misfire on #6. I also noticed some coolant coming from somewhere under the car near the back of the engine. So, I assumed I messed up the head gasket torque or something and decided to tear it all back down and triple check everything.

Got it all back off and decided to go ahead and do a valve job and replace the valve guides too. So, I pulled the valves, super cleaned the head since the valves were out, lapped the valves, installed new valve seals, another new head gasket and put it all back together. Cranked it up and same thing. rough idle and #6 misfire. I then checked spark on # 6 and it was good. Swapped coil pack #4 & #6 to make sure and same thing. Swapped both passenger and driver side coil packs to rule out wiring harness for #6 and still misfires. Installed new #6 fuel injector and cleaned the rest, still misfire. Swapped plugs and wires, still misfires (note everytime i've pulled the #6 plug it is wet with fuel). Checked compression on all cylinders on passenger side and all good (#2 180, #4 175, #6 175, #8 180).

Finally I was stumped so I decided to take it to a local mechanic shop that can diagnose better. They said that once the engine fired up the #6 valves stopped moving or moving very little and pinpointed it as the VLOM. I brought it home, ordered a new VLOM and Oil Pressure sensor and installed it today. Cranked her up and still misfire on #6. Checked spark again, compression again and still cannot for the life of me figure out what could be causing all this. I've removed the valve cover and turned over the engine (with all coils and injectors disconnected) and # 6 rockers are both moving perfectly. I have not started the engine up with the valve cover off due to fear of oil going every where in past experiences.

I'm about to just take it to GMC to see what they figure it to be, but wanted to post something here first.

Can anyone maybe shed some light on what I might be missing with all this?

Appreciate all the feedback.

silveradosid · October 13, 2019

do a compression leak down test, might find that number 6 wont hold compression

DJ Ragsdale · October 14, 2019

Did a leak down and all is good. no more than 10% loss. It appears the #6 lifters are unlocking only after start up. The only thing I did not do during the lifter replacements is blow out the oil passages from the VLOM. I didn't see the TB until yesterday when scratching my head. However, I still can't understand how any blockage in those port would cause the afm lifter pins to unlock. They require high oil pressure so if the port is blocked little oil pressure would reach them...

I'm about to either tear it all down again to blow out those ports or take it to GM. I just know if I take it to GM it's going to be $$$...

Mikebtte · October 14, 2019

You could disconnect the plug on the VLOM and start it, that will deactivate AFM (will have a CEL). If it runs good than you have an issue with AFM staying on and activating the AFM lifter.

You could also crank it with the valve cover off and see if that #6 is not responding.

DJ Ragsdale · October 15, 2019

I can't get to the VLOM connector with the intake manifold still on. I guess I'm just going to tear this thing down again for the 3rd time....

swathdiver · October 18, 2019

You've done a lot with limited information. You need a Tech-2 to see what's going on electronically. You can test the coils, injectors and AFM system and run graphs for misfires, knocking, etc. Replacing the VLOM is SOP when doing AFM work as it is the VLOM solenoid failure that usually kills the lifters.

Have you posted about this on the Performance Trucks and TahoeYukon forums?

tatedog · December 27, 2019

i have the same issue, did the same and have ordered a vlom cuz i didnt do it before!! Have you had any luck? I follow your logic exactly with the high pressure. I wouldnt unplug vlom plug, read elsewhere i causes truck to go v4 w unhappy results