1985 ford e-350 351 cid. 4 bbl loss of oil pressure

A few questions to get a background:
Does this happen all of the time?
When was the oil last changed?
How much oil is in the engine?
How many miles does the engine have on it?

Check the oil pressure guage (has a guage?), oil pressure sending unit and the wiring in between to make sure everything is in good working order and the readings you are getting are indeed correct. You can also double check oil pressure (as best you can without the vehicle moving) with a mechanical oil pressure guage in place of your electronic one. I hope this helps.

it actually just started today,the van itself has been sitting for a few months after it failed emissions.so i put a new carburetor also a full tune up.including oil oil filter.i also had some leaking valve cover gaskets that i replaced.the guage it self and everything between is working it seems.because when the guage drops to no pressure you can hear a lifter start to tap.

You might have a bad or weak oil pump.