I bought a 2002 f150 4x4 with a 4.6l. It has 250k miles on it. a couple days ago. It had no problems at all during the test drive or for the 150 miles home. Yesterday after it warmed up, the oil pressure dropped to 0 while at a red light and had a rattling noise. Not a knock but more of a rattle. I can't isolate where the rattle is coming from. Oil pressure comes back with just the slightest of increase Of RPM and seems To have plenty of power while driving. I had read to check the pressure manually but with the noise I sense there truely is a problem. the oil appears clear and is full. Any ideas of what I should be checking on this.

 

the pressure gauge is basically an on- off sensor that turns on at something like 7 psi... If pressure goes below that, the gauge drops to zero... You cant tell if you have 7 psi, 15 psi or 25 psi... gauge always reads the same. You would need to put a real gauge into the tap if you really want to know the pressure. ......... Since you say it does " rattle" we can assume the sensor is not broke, but you actually are dropping to very low pressure... I think the first thing I would do is drain the oil ( lets assume it has 5 w 20 oil) and put something much thicker. You could go to 10 w 30, but I might be tempted to use a 15w 40 or something like that.. That should not hurt and you will have a 40 wt viscosity oil when the motor is hot instead of a 20 wt oil...That should add several psi of pressure to an OLD motor.