I have a 1994 suburban w/7.4 L.A couple of days ago on a coolish morning,mid 50s I started the truck and oil pressure
went to 80psi instead of the normal 60 psi.While driving I noticed the pressure dropping until it was at 20 psi,fortunately
I was at my destination.I checked my oil level and it was full,I started the engine and it went to the normal 40psi,so I drove home and the pressure was erratic.I replaced the sending unit and when the engine got warmed up the pressure
was dropping to 0psi at idle.I installed a mechanical gauge,when warm it showed approx.10psi.Where do I go from here,
what else do I need to check?Thanks for any help.

 

Any leaks?
Was the mechanical Guage erratic, or can you watch it driving?
I'm not familiar with the entire system, but oil pump could be picking up some sludge built up in the pan and causing blockages (just thinking here,) and it could be the pump itself spring worn (blockage for a while it drops clears out goes to normal blocks and clears operating is the thoughts along with it a age) I don't know if it would work but if you had some skinny maybe clear tubing and a syringe (stick syringe on the end your holding) you could stick tubing down the dip stick tube shooting for bottom of pan and see what you could draw in for sample. Just a thought or pull valve cover if it's easy and see if it looks like there's some dry lifters etc. Could need a flush/clean. I did have a Yukon and there was some screen that can get plugged off if I find the info I'll post it here.

 

https://www.chevytalk.org/fusionbb/s...hp?tid/292000/

https://bryansgarage.com/oil-pressure-fluctuating/

 

No leaks.I can watch the gauge,when warmed up it drops to 0 at a stop sign and up to about 20 while driving.I changed the oil today and used 10w-40 instead of 10w-30.I cut the oil filter open to check for any metal,no iron or bearing material found.Pressure is now at 10 at stop sign and 35 at higher rpms,so the thicker oil helped.
I guess I'll drive it awhile and maybe try some 20w-50 oil and see how it does.But it bugs me not knowing for sure what caused it to happen all of a sudden.Fortunately
it only gets driven a couple of thousand miles a year,mainly during hunting season.

 

Could you have a faulty oil pump? The oil pump is mechanical, and hangs down in the oil pan, attached with a single bolt to the rear main bearing cover. It is driven by a shaft up into the engine.

I think I would be checking and probably replacing the oil pump before I drove the truck much at all.

 

Could be a pump problem,I think there is a relief valve in the pump from what I've read and the valve may have stuck open or the spring may have broken.
From looking at pictures of the pump this maybe repairable without removing the pump.I need to get back under the truck and see how hard it will be to remove the
pan.