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.
If your voltage is still dropping with fans your alternator may not have enough output anymore. That battery voltage shouldn't effect oil pressure because of the above mentioned 5v reference that is regulated by the ECM in theory. One could watch the 5v reference to see if it's changing but I bet that's unrelated in this situation. The more likely culprit is your oil pressure is actually dropping. (Confirm it first of course) if it is actually dropping it sounds to me like you may have some sludge in the pan that as you drive sloshes about and is plugging up the sump. (Could be heat related but my gut tells me sludge) Once you park it, it settles on the bottom of the pan again. If that's the case you could try cleaning out the pan by flushing with diesel fuel through the drain or pull the pan and clean it out. A test you could do before hand though, start it up and just let it sit and idle to prevent sloshing. If pressure never drops it may confirm my theory. Then I would while it's hot, shut it off, open a door, and use the door to aggressively rock vehicle. This will move sludge. Quickly drain that oil into a clean pan. As soon as it starts draining get up and rock the vehicle while it drains. Then check pan for sludge/chunks. Don't forget to change filter and cut old one for inspection. Oh, and hopefully your catch pan is big enough to prevent massive oil mess... Let me know your findings.