Ryan Bokros

Thanks....that is kind of what I have been thinking. At this point, I only have the valve covers off, so its pretty easy to get back together and to the dealership. Since the lifter is engaging, I'm cautiously optimistic that it isn't the lifter and is the VLOM as you also suggested. I'll update moving forward. On another note....I ordered the Range AFM Delete module and will be shutting of the AFM once I figure this situation out!! What a pain this thing is and I'm baffled that GM has never truly addressed it....

Ok, new guy here! So I know that there is and has been a ridiculous ongoing issue with the AFM lifters sticking or collapsing entirely and all that mess. I have a 2007 GMC Sierra 5.3 Flex Fuel 4X4 - 160k miles. Pretty regular maintenance, I am not OCD, but I try to do my best at around 3500-400 miles - full synthetic Mobile 1 oil, AC Delco Filter, every other change gets a half quart of MMO. Here is my situation.... No ticking whatsoever (which I know is usually attributed to this), BUT recently a terribly bad misfire on Cyl 4 (one of the AFM cylinders) and throwing the typical 304 code with stabilitrak warning. Before pulling the valve covers, I checked EVERYTHING (well, except compression, should have done that) - I checked plugs (a little black, but not wet), coils, injectors, fuel and oil pressure, etc. That is what led me to go ahead and pull the valve covers to check for any slack in the rockers/valves due to a collapsed lifter (even though there is no noticeable tick). Initially pulling the covers, I was pleased with how clean everything was....and how tight everything was. No slack in the rockers whatsoever, from what I could tell the valve springs look good, etc!! Yay...but WTF is causing this? Well, I fired up the truck and found that the intake valve in cyl 4 was not moving. BUT...when my wife got home, I had her turn the engine over for me so I could see it all without doing it myself. Here is what I am confused about - on start up, the valve moves...once maybe twice, it goes through the whole compression cycle then stops once the engine is running...when the engine it turned off, it does the same thing - one or two cycles. If it was collapsed or busted, it wouldn't push the valve at all, would it? Could this be the VLOM and not the lifter? My oil pressure reads well, so I am not experiencing low pressure like others I am reading, but I am aware of the filter leading to the VLOM...maybe that is the next step. Thoughts? Thanks!!