Cylinder 7 Misfire After 9 Seconds
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.
You might have a spun lifter from a bad plastic guide then. You will need to look down the oil tower hole after cleaning it out and see if you can see the hole in the side of the lifter. If you can't see the hole in the side of the lifter then it has spun from a bad plastic guide. When this happens there is no choice but to do a full delete where you mechanically delete all the components including camshaft, lifters, guides and all. I do now offer kits for it. This is because when a lifter rotates it damages the cam. So replacing a lifter and a guide doesn't fix the vehicle long-term. The damage that's done to the cam shaft will also damage the new lifter you put in.