I'm new at this so please forgive my mistakes. i own a 97 f250 351, 5 spd, 2wd with 111,000 miles. a few years ago i noticed my oil pressure gauge would start jumping eradically when i came to a stop while i had the a/c or heat on and/or had been driving for a while. it has recently gotten worse. the gauge starts jumping almost instantly after starting and it now makes a "knocking" (the best i can explain it) sound but if i rev it just a little, the gauge stops jumping and the knocking goes away. i cant figure it out. any ideas?
First, your oil pressure gauge isn't a real gauge. It's hooked up to an oil pressure "switch" which will register the gauge to read "normal" when oil pressure is above a certain PSI. If it drops below a certain PSI, then you will see that difference in the gauge and probably get an idiot light.
My guess is either the connection at the oil pressure switch is loose or you've got a bad ground in the circuit.
As for the knocking sound... you're saying the gauge is making the knocking sound, or the motor?
by the sounds of it i would say that the oil pressure gauge is dropping at idle which would be caused by a clogged or failing oil pump, under higher rpm's it is able to build the pressure high enough to keep the sending unit happy, but at idle it is dropping under the 7? ( i believe thats where the gauge drops off at ) psi the gauge drops.. also the knock at idle could be the top end rattling from lack of oil if it is a dying or clogged oil pump
I had this same issue with my truck when I first bought it. I tried the sending unit and gauge first but to no avail. I wound up dropping the oil pan and installing new pump and pickup tube/screen. You might try installing a manual or electric gauge first to verify the reading. The pans on these trucks suck to remove so good luck!