Wednesday morning I started my 4.2 to take it to the shop for some work. As I drove away from my home the oil pressure suddenly dropped to near zero and the oil lamp came on. There was no additional engine noise and everything seemed ok. The shop I use specializes in European makes and the have a Jag mechanic, so I continued on, it's only a mile. When I got there I told them what happened. We decided to install new senders for the lamp and gauge. When I picked the car up, they asked what oil I was using, and I had just three weeks ago changed oil and filter with 10w30 high performance 100% synthetic oil with zink additive. The mechanic said I needed a higher viscosity oil. I consulted my manual and he was right, although the oil had been there three weeks with no oil pressure issue(20 lbs @ idle,40 lbs @ speed). I immediately changed my oil and filter again, this time with 20w50 high performance 100% full synthetic. There was only a slight increase in pressure. This engine supposedly had a new oil pump installed about a year ago.
My engine has under 100k miles and occaisionally on a cold start, will have bearing rattle.
For it to do this so suddenly is puzzeling to me. I figure if the pickup tube fell off the pump, there wouldn't be any pressure at all.
Anybody got any ideas? HHHHHHEEEEELLLLPPPP!

Submitted by lorr3@att.net on Mon, 11/28/2011 - 12:02

To John Testrake et all.
The bypass valve was indeed stuck. A small sliver of metal, smaller than a staple, was jammed in the top corner of the valve cylinder. I dissassembled everything, cleaned it all up, disslodged the sliver of metal with a scratch awl, polished the surfaces with scotch brite, reassembled all, fired the engine and oil pressure is better than ever.
I am so gratefull to you for confirming my suspicions of it being possibly the oil pressure bypass valve.
Larry.