I have a 2000 Explorer with a 5.0L engine. I just did a rebuild on the engine. When I start it it runs great for about 2 1/2 to 3 minutes at 1100 RPM Oil pressure is about 30 PSI at idle. Then as the engine warms up it drops to about 900 RPM at that point the Oil pressure guage starts to flicker and continues flickering until it just drops to Zero if I keep the RPMs up it will stay steady but I have to keep raising it to keep it steady.
I tried a new oil pressure sending unit with no change.
During the rebuild I put a new oil pump in it from Melling. Thought maybe it was pumping all of the oil to the top of the engine, so I checked the oil level as soon as the flickering starts with engine off and it is reading full. I thought maybe it was bad so I tore the engine apart and put the old pump back in. It does the same thing. When it did that I realized after putting it back together I had put the drive shaft in upside down thought that was it. tore it apart and flipped it over, no change.
Installed a new camshaft synchronizer and found no change. Purchased a new drive shaft but see no difference between it and the old one and it works great if the engine is cold until it warms up so I have yet to replace it as I am tired of tearing the engine out only to find no change.

Any help would be appreciated

Thanks

UPDATE: I was using a Fram TG 2 Oil filter so I tried a Mobil 1 Oil filter and New Oil first brand was Castrol GTX tried Valvoline with no change in performance as a matter of fact it only went 2 minutes before it acted up this time.

Still hoping for some assistance.

Thanks again

 

Well the oil pump driveshaft can be ruled out, as it will run the pump if it is present, and if it is broken or missing it won't. I can't see how it can partly work.

Since this is a new rebuild, I suspect assembly errors.
I would check the main and rod bearing clearances. Perhaps there was a mixup in the bearing sizes or the crankshaft was ground too far undersize.

Also, an oil gallery plug could have been loosely installed and then popped out.

I would temporarily install a mechanical gauge to read the oil pressure directly to eliminate another possible source of error.

 

Stupid question: Sure you have enough oil in it?

 

did you replace the cam bearings, and if so, what way did you face the holes?