My 1999 4.0 has pegged the oil pressure gauge since I got it running after replacing #1 & #2 pistons, rings, etc., and I know it don't have 80psi all the time. I bought it with a "blown" engine after a major overheat. I don't recall what it read before I redid the engine, only ran it enough to pull into the garage. Either a bad sender, or someone replaced it in the past with one for an idiot light? I've hear some Fords are built with a switch sender even for models with a gauge, just reads mid-range if there's any pressure at all. Seems like fraud to me... (not altogether out of character for FMC)

 

More then likely your oil pressure sending unit is bad. They usually read max when they fail.

 

This is a fact. Couple weeks ago I replaced the oil pressure switch above oil filter and now gauge read correctly.

 

thanks! that's what I suspected.

 

Best part is, it's so damned easy to fix. Just five minutes it ll take you