My 1997 F150 4X4 with 4.2L has recently been doing this. After warm up driving at idle in drive pressure gauge goes low and bounces. Some times oil light comes on, sometimes not. Just changed oil and yes it is full with new filter. Always used Castrol 5W30. Used Castrol Synthetic years ago but have not for long time.

Sending unit for light...gauge...what?? Don't want to trash engine at this point. I will consider all information here as well as visit my mechanic soon.

Thanks for help.

 

There can be several issues that you MIGHT have. I'd suggest replacing the sending unit first. If that doesn't get it, put a REAL oil pressure gauge on it. If you are actually having oil pressure that is fluctuating like you say, the screen on the oil pick up can be sludged over. I would suggest Auto-Rx for that. It will slowly clean all of the internals of the engine. You can also have a stuck/sludged oil pump relief valve. But your probably going to find that the sending unit is bad. I'm assuming the engine is original and has over 100,000 miles on the clock.

 

yea those early 4.2's had some issues with the oil pump pickup screen getting sludged up and the relief valves had some issues as well. but check to make sure if the gauge is correct or not.

 

Thanks guys. Yes it has 116K miles on it. I have tried to take very good care of it since I got it with 30K.