1995 Ford F-150 no oil pressure

Engine Mechanical problem
1995 Ford F-150 6 cyl Two Wheel Drive Automatic

I have a ford F150, 6 cylinder, oil pressure is not marked and much heated engine, starting pressure marks, but in one or two minutes go to 0, has oil, new filter. This problem started after washing precion engine and has sound at the top of the engine. Tank you for helpmy, turn on once more my pickup to prove it, but when you turn on have ressure oil, but after a few minutes precion needle starts moving from one side aother, sometimes mark a quarter of pressure but then down completely to 0, and not becomes amover