1998 /4.6 oil pressure will not stay up .. runs 3/4 gauge then drops to 0 then back up.. could sludge be causing prob.? is there anything i can add to cranckcase like diesel and run parked and then change oil n filter..

 

When it drops to zero can you hear any engine sounds? If not it may just be a bad sending unit.

 

+1, I think it's a bad sending unit (switch). Remember this is an idiot light type of gauge that either reads ZERO, or about 1/2 scale on the gauge when the sending unit (switch) closes. This normally happens
with about 7psi of oil pressure.

 

How many miles on the truck? Could possibly be a bad oil pump or worn bearings.

 

I'm leaning on the bad sending unit wagon as well. When your oil psi drops to nothing, in the case of loss of oil or pump; your engine will be making enough noise to concern you.

 

when it zeros it starts multiple tapping from lifters or rocker arms or combo of both...i know it has sludge build up..i paid for new pump n pick screen bout 25,000 ago for same prob...it has round 170,000 on it other than that runs as smooth as a new one

 

If your hearing tapping when the gauge reads near zero, then it's not the oil sender unit. That's real oil pressure drop. If you replaced the oil pump and are still having the same problem, then the pump isn't the problem. Have you tried running seafoam threw the oil?

 

It sounds like the rod and main bearing tolerances have opened up too much to keep the oil pressure up. Its probably time for an overhaul.

 

what is seafoam?

 

It's a engine cleaner. If you have sludge it will clean that out. But if you have bearings going out, it wont help any.