The fluctuating oil pressure topic again

I'm looking for some information on how the oil pressure sending unit works.

I have a 2008 LTZ 5.3 with 164,000 miles.  I have never had to add a drop of oil during its life.

Yesterday, the oil pressure indication started fluctuating wildly, to the point that the low pressure/shut off engine warning came on.

When I checked the dip stick, there was NO oil visible!

No signs of oil on the engine, on the ground, no other problems evident.

Added a quart, still no reading on the dip stick.  That couldn't be right, so I just stopped and started reading.

This morning I went out, and now the dip stick is reading too much oil....of course.  Like I said, it doesn't use oil and I added a quart.

Drove it around a little, and now oil pressure is higher than normal....about ~50 psi.  Slight fluctuations, more than usual, but only over a range of about 5 - 10psi.

But...check the dipstick again....no oil on the dipstick.  

It was getting close to time to change the oil anyway, so I did that to get the oil level back to normal.  So right now, I KNOW there is the right amount of oil in the vehicle.

Did some errands, still slightly more fluctuation that normal for this truck, but seems relatively normal.

After searching/reading, seems like the most likely culprit is the pressure sending unit, with another possibility a stuck relief valve, but now it's operating ok.

My question:

When this happens again (it will), is it a problem with indication, or is it a really a problem with pressure?  If it's just indication, and I'm on the highway, do I limp it home or do I stop.  I don't know how the oil pump and the sending unit interact.  I also have NO idea why sometimes I get no level on the dip stick.

Anybody like to take a shot at enlightening me?

I apologize if this has been asked/answered before.  I looked, and while I found this problem, I didn't find details on how the system actually works.

Thanks

Disappearing and reappearing oil is a case for concern and likely has nothing to do with faulty oil pump/relief valve or sending unit operation.

Sounds more like the amazing Avy-dini's disappearing and reappearing oil magic trick. Assuming the two piece dipstick is still riveted together properly and inserted all the way into the standpipe, oil pump and pressure gauge OK,  no leakage from the engine on the ground AND the oil magically reappears upon sitting.......only one place for Avy-dini to have hidden it up his sleeve......accumulating in valve covers on top of the heads......and the longer you run the engine the lower the oil level in the sump.

It is possible to partially block the oil drain holes in the heads which channel the oil constantly squirting from the lifters into the covers, lubing the rockers then draining back to the sump are partially blocked, especially if the engine was poorly maintained at some point, has had a mechanical stop leak added, or has ever overheated at some point and the baked crud from the rocker arms sloughs off or maybe even detached pieces foil seals from oil bottles blocks the return drain holes......since it is gravity fed to the drains all crud and any crap entering the valve cover through the oil fill pipe heads right for the drain holes.

If so, the engine is dumping the oil into the valve cover(s) faster than it can drain back to the sump....the longer you run, the lower the oil level gets...... then oil pump starves, pressure gauge goes nuts and then the low level light goes on.......let it sit for a while with the engine off, oil drains slowly back into the sump and the dipstick reads the correct amount of oil in the engine including any extra added in the interim, pressure returns, low level light goes out....until the next long run.


When the low oil light comes on and you continue to run to where the stick shows no oil and the oil pump starves...if you don't stop the engine it will very soon stop itself....no oil in the sump means scored bearings, no splash lubrication of the pistons/cylinder walls and rings, collapsed lifters, overheating and eventual seizure of the engine.

Next time your Avy performs its disappearing oil trick tap a key or piece of metal against the valve covers (especially the one with the oil filler) and see if is sounds different from the cover when the oil reappears in the sump.......a change in sound is a good indicator that  the missing oil has backed up inside the cover..

Ugh. Not what I wanted to hear at all. Yeah, dipstick is intact.

The truck's been meticulously maintained.

Looks like it's going in. Thanks for the information.

Hopefully, the problem really is a clogged drain hole(s)....a lot easier and cheaper to fix than a bad oil pump and hopefully no damage from oil starvation...remove the valve cover clean/unblock the hole/channel and reinstall the covers, maybe with new valve cover gaskets if they get damaged in removal..
....my '59 Impala pulled that magic trick when the wire mesh insert in the breather cap got sucked into the cover and blocked the forward drain hole in the head......mine was easy to diagnose because it had a leaky valve cover gasket..

X2. What Thomcat said.