Erratic oil pressure

[Paulos]

I have an '87 2.9L 4X4 Ranger. Any ideas why my oil pressure would be erratic? It starts out around 50-60lbs when cold, after warming up it runs about 40lbs while driving, drops to about 15-18lbs at idle, but the whole time the needle on the gauge bounces around a couple lbs in either direction. The odd thing is that the oil pressure gauge acts quite similar to the vacuum gauge, which indicates that I've got a stuck or burned valve. The oil pump has 300K+ miles on it, so I figure it's time for a new one, but I've never seen an oil pressure gauge act like this.

 

[Uncle Gump]

A 300k mile engine with those oil pressure numbers... Not shabby... people cared for it well.

Electric? Mechanical?

Doubtful the oil and vacuum gauge bounce is related

 

[Paulos]

It's an old Stewart Warner mechanical gauge, with a braided steel line from the port on the block straight to the gauge. It doesn't matter if the engine is cold or hot, the needle still bounces.

 

[Uncle Gump]

I would swap in a mechanics gauge and see if it does the same thing.