2.7 low oil pressure only when driving
 

Hello,
I need some advise. I have just rebuilt my sisters 2.7 in a 2001 Concord. The reason for the rebuild is that the water pump went out and dumped antifreeze in the oil pan and #1 crank bearing started to spin, car has 117000 miles on it. After new head gaskets, head bolts, complete timing kit, rod and main bearings,I am having this problem, I can start the car and let warm up to operating tempeture, all is fine. Oil pressure is 45 lbs at idle at startup. I did not leave the oil pressure guage in after I saw this. Took the car out for a test drive and the oil light comes on at idle or any speed. Pull in the driveway and the light stays on. Shut the car off for 10 minutes, start it up and the light stays out. Drive it and it comes back on. When the light is on I don't hear any noise from down below like you would expect to hear with low oil pressure. I know that I need to hook up the oil pressure guage and see what the oil pressurer is doing when the car is driven to verify thatoil pressure is good. Do any of you have any ideas??? I did not replace the oil pump. We did use plastic guage on all the bearings to verify that the had the correct oilclearance and all did.

Thanks for any help!!
Craig