Oil Pump failure?
How many miles?

 

I would have expected the oil pump to fail over time and not from being parked.
264K all original miles.

 

Puzzling indeed. I would change filters and try it again. Stick with a Bosch, mobil 1, Wix... don't cheap out. Then put a direct reading gauge on it. How long did you run it with the valve cover off?

 

PLEASE keep us updated on what you find is the problem.

 

Puzzling indeed. I would change filters and try it again. Stick with a Bosch, mobil 1, Wix... don't cheap out. Then put a direct reading gauge on it. How long did you run it with the valve cover off?

Add Wix XP or Baldwin to that list of Filters

 

Oil Pump.

If it's not pumping to the lifters and, you say you visually verified that, it has to either be the pump or o-ring. To change the o-ring you're already at the pump. I just changed mine due to low pressure. 235,000+ miles on a 2012. Don't chance it. New sending unit and oil pump is what I'd plan on.

 

Update time, pulled the pan, pickup, and pump out and there's nothing obvious causing the loss of pressure.

The relief valve is lightly scored but moved relatively easily and was at the top of its bore when I got to it, the only place it got hung up was at the very bottom but the spring could have easily pushed it if that were the case.
Pickup O-ring was still pliable, but obviously squared and old, I have my replacement waiting to go in. On the L59 I'm working on the O-ring was heavily plasticized and just broke apart, so this one is in remarkably good shape. I think it still may be at fault though, I noticed that there wasn't a drop of oil in the pump, or oil feed for the engine, which either means the truck has sat long enough for the oil to leak down through the pump, or the oil was never making it to the pump in the first place.
I do not cheap out on oil filters, I tried that once with an STP oil filter and lost 10PSI, picked up a Mobil1 filter and it instantly came back, that's all any LS I have worked on or own has gotten since.
I did two 20 second runs without the valve cover, it's all I could bear to stand with how loud the clattering got. Confirmed no oil making it up the rockers, during one of those runs I did notice that the oil pressure gauge was not sitting at its minimum point, it jumped up when the engine started but I wouldn't have known if I wasn't watching it. The pump was worn but nothing major, perhaps a little bit more scoring on the georotor than I'm used to, but nothing severe enough to indicate failure.
With the pan off I also took the liberty to check my rods for any abnormal play, there was some front to rear play as expected and none side to side or up and down, so I do not think it's possible for this truck to have spun a rod.

Anyway, the plan for this truck is to put the pump on from the L59 since it was in excellent shape, and the L59 is getting a new pump, I can't justify spending money on a new pump for this truck while a perfectly good pump will be sitting in a box in my garage for who knows how long. The L59 will be going into my new daily so I care more about the longevity of that motor, and the pump going in this truck will hopefully last it long enough for it to be put out to pasture, whether that be another 260K or just another 50k. Pictures coming later tonight, it's wet and cold out and all in all an unpleasant time to replace an oil pump when the truck is parked in the street.

 

Maybe consider shimming the pump for a little more pressure?

 

Maybe consider shimming the pump for a little more pressure?

Already done, wanted to play it as safe as I can using a used oil pump.

Oil Pump Picture Time.

Scoring on the relief valve, something definitely got into the bore and was doing scratched it up, this is the only place where the piston is scored.

Some normal wear, there are a couple of scratches that I can catch my nail on, the bottom right is where it is worst.

There was some scoring within the bore of the relief valve.

No signs of cavitation or wear in the rotor.

Some of the grooves are deep enough to where I can feel them with a nail but it doesn't catch.

There has been some lifter noise for the last few thousand miles, it always quieted down around oil changes but never went away. The oil pressure has slowly been decreasing over the 2 years as well(was at 30-35PSI cold start with 5W30), I chalked it up to bearing wear, but the sudden loss of oil pressure makes me wonder if it's possible that the bearings are ok and that was just the first signs of something going wrong in the lube system.

 

The new pump is in and the pan is back on, gave her a whirl with the injector fuses and fuel pump relay removed and saw 35PSI after letting go of the key. It also started super clacky and then quieted down once the oil started getting around. If I'm doing 35PSI cranking that makes me wonder what it will run at now, considering it used to idle around that, shimmed it so that the spring sat just under the lip of the cap.
I've read lots of threads on oil pump replacements and everyone has their own opinions on priming the pump, I didn't pack the pump with vaseline and I didn't soak it in oil, I took the pump apart, and used a synthetic grease made with PTFE to lube all the contact surfaces between the gear set, housing, and lid, and added some to the teeth, any that squirted out, I used to lube the next tooth so on and so forth. Oil pressure came up within ten or so seconds of cranking so it must have worked a charm.
The new pickup tube O-ring was MUCH tighter on the tube than the old green one, and it took some careful persuasion to get it to go into the pump, had to loosen the bolts that hold the tube to the block, install the pump and then slowly tighten the tube back into place before using the pump side bolt to pull it home. I am sure that this contributed to the pressure issue before but I don't think it is the sole cause of the loss of pressure. The newer pumps relief valve operated much smoother than the one I removed, the old one required more pressure to get it moving, but it didn't necessarily catch on anything.
It started raining just as I started cranking so that's as far as I made it, a final update tomorrow with cold and hot idle pressures, as well as if the valvetrain noise went away.

 

It started raining just as I started cranking so that's as far as I made it...

Sissy.

 

LOL, I've been working in the snow for the last couple of days, but the trucks parked on a storm drain and it was 37* out, didn't want to be drowned in the torrent that was heading my way. Snow and mud have been bad enough, working on the street and being splashed is on my nono list.

 

Brrr! Too cold for me.

 

And me to be honest.
Final update, ran the truck today for a minute or so, couldn't get the balancer back on so didn't want to run it for too long without coolant. I saw 70PSI immediately after it lit up and then it slowly dropped to 60, I think I'll be idling around 50-55PSI once everything is warmed up. Almost all valvetrain noise is gone now and I had to strain to hear what little was left, hopefully, that too disappears with a little run time and a temp cycle.