I have a problem with the oil pressure gauge dropping, it will bottom out then go back to normal, this had been going on about a week, no oil light visible, although it does come on for a few seconds when you start the truck (1997 f150 w/rebuilt engine, 4.2L 5 speed manual tranny. 30,000 miles on rebuilt), so I changed the oil (10W30, I have used this oil since I purchased the truck new) and filter a couple of days after it started happening and for a couple of days everything was fine. It only drops after it is warmed up, usually within 15-20 minutes of driving the gauge starts dropping, almost like a fishing cork when you have a nibble, then goes back to normal. I took it yesterday and had a pressure test done on the engine block, they ran the test for 45 minutes, it tested fine, no pressure drop at all, the engine oil on the dipstick is fine. Last night on the way home the gauge dropped and for the first time the engine oil light came on and went back off. There was a slight chatter coming from the passenger side of the engine. The mechanic did tell me that he pulled the codes and the EGR valve and the catalytic converters should be changed out, however those were the only things that showed up. He suggested changing out the sending unit, so I am going to do that today. The truck seems to run fine other than what is mentioned above. There is a clatter coming from the cat. converter on the front drivers side (there are 4 total), so I'm thinking about putting dual mufflers on her. Is there something I'm missing here on the oil pressure, could there be something else wrong? The mechanic told me if I wanted him to change the oil pump it would be $698.00. I thought that was a little on the high side, but would pay it to get her running right. Any suggestions would be much appreciated. Thank you!

 

The oil pressure gauge is an "on/off" gauge. It doesn't give a good indication of what the pressure really is since it is no better than a light. The oil light will come on when you are low on oil.

That said you could have a faulty gauge causing it to bobble or it could be an intermittant drop off from the oil pump. $700 is pretty high for a pump swap but nothing is really cheap these days. You could save yourself some money and do-it-yourself if you are handy with tools.