I have had a real hard time figuring this one out and am hoping someone from here can give me more insight into what may be happening.
Here's the backstory: 2013 Silverado with 5.3 came in with cylinder 7 misfire. After several tests it was determined there was something wrong with the valve train and the lifters were replaced on that bank. Fast forward to this week. After putting the truck back together it runs nice and smooth until about 9 seconds then it has a dead miss on cylinder 7. If i take a standard compression test it is fine (~180psi). If I do a running compression test the compression drops to 0 in about 9 seconds. Once the compression drops to zero if I immediately turn the engine over the compression returns. The compression never drops while doing a standard compression test no matter how many times I let it crank over. One other aspect that was changed was when the VLOM was off plugs were pressed into the engine AFM towers to essentially delete them. In my opinion I think the cylinder 7 intake valve is staying closed, I need to run it with the valve cover off to test this.
Truck sat for a while since the last time and it seems to have the same issue. I ordered the oil plug for under the oil switch, cut the gaskets and still have an intermittent compression issue. I can do a static compression test with no issue. But as soon as the engine runs cylinder 7 loses compression. I am completely stumped.
Just an FYI- I pulled the plugs out of the AFM towers, cut the gaskets on the VLOM and the truck is back up and running smooth. I am planning on sending the computer out to be re-programmed to disable the AFM.
Thanks for the quick reply. I wasn't aware of that. Luckily the plugs are threaded although I am not optimistic in the small thread working to pull them out. One thing I can't explain is why it is only cylinder 7. I suppose it could be other cylinders but the scan tool shows 7 as a dead miss after the 9 seconds. I appreciate the support, I have purchased your lifter release tool and VLOM plug for my avalanche and they worked flawlessly. I orderd the tower plugs off amazon during the holiday break. Turns out that was a mistake...
You can't run the tower plugs while you have AFM lifters. When you do this any oil pressure that comes from the main oil galley below the lifters and goes around them ends up pressurizing that tower. That's why when I do the VLOM mod I remove the gasket above the oil tower. This ensures that it can never be pressurized. Now you either have to find a way to remove those plugs or just finish the full delete and replace all the AFM components with non-AFM components.