Short Version: MY 2011 Sierra (specs below) is showing (very sudden) symptoms of a loose VLOM. I plan on having my son pull the manifold and correct this for me. I don't believe that I have had a collapsed lifter yet, but we will check deeper when he does the work. So the short version of my question is whether it would be any benefit to replacing the VLOM at this time, versus just retightening it to reseal better? (RockAuto Dorman $165 ) Longer Version: My 2011 5.3 Sierra SLT with 40k miles is a garage queen, grocery-getter that typically makes a couple of very short drives per week. Always changed with spec synthetic at 25% D.I.C interval. (DIC is currently above 50% remaining on this change). Oil pressure has always consistently been 42 to 45 gauge-psi cold and 38-40 hot. A few weeks ago (that's a 1/4 tank of gas for you guys) I noticed an abrupt change in my oil pressure readings. It was a hot day, the AC was max, and I drove 10+ miles when I noticed the oil pressure near idle (city streets) dropped to around 20 psi. I freaked out, kept the rpm up, and got to my destination a few blocks later. I did an immediate restart and the oil pressure wasn't too bad (not perfect, but not scary). Two hours later on the drive back home, the oil pressure started out closer to 50 psi (slightly high for this truck) and settled down to 40 for the rest of the drive, until I got a couple blocks from home (10 miles) and the pressure again dropped into the 20's. Since then, the oil pressure has been starting at 50 cold (again higher than normal for this truck) and staying above 38 for these short trips, as long as it isn't approaching a 10 mile distance. By the way, this truck reaches normal gauge temperature (210) within 3 to 5 miles, winter or summer. Secondary symptom: Oops, I forgot about a secondary symptom. The only times I have ever seen the oil pressure drop is ALSO the same time I see a drop in charge voltage, including the ones above. The first time this happened was last year when my 11 year old battery finally gave up the ghost on a 20-mile trip. Battery voltage dropped to 9 volts just as I was reaching my destination, and oil pressure tanked. Needed a jump start to return home, and the whole way back, I noticed the oil pressure coincided with the charge voltage. Until the battery was replaced, cold pressure started at 38 and dropped to 30 when at op-temp. (I installed a temporary volt meter in the cab to monitor voltage during this time. It was always lower than spec even when charging.) But as soon as the battery was replaced, pressure went to 45 psi cold, and barely a droop at all when hot. I don't know how this relates to the current issue, but before someone say it, yes, it's a 3-wire, 5-volt pressure sensor. So if the 5-volt bus out of the ECU can't maintain voltage, then the sensor will show lower pressure.