Deere 1650 oil pressure problem

[eme]

Have a 1984 John Deere 1650. On start up oil pressure will run 40lbs plus at 800rpm & stay there until engine warms up. As you are using tractor in the field oil pressure will drop to 25lbs wide open, my tech manual says it should be 50lbs. Idle pressure will go to 10lbs and red light will flicker. The engine temp never goes over 185 . The tractor had just water in the cooling system when I got it! Have put new core in radiator(the old one was badly plugged), thermostat , fan belt and run cooling system cleaner & flushed numerous times. Recently I drained the system to put in mechanical temp gauge & noticed dark stuff settling out of the antifreeze. The tractor doesn't lose water or oil. When I refill cooling system I only get 8qts in, the book says 11qts plus. Have had water pump off & passage to intercooler looks ok & you can flush water through to thermostat passage. I have had oil bypass valve off & apart & saw no problem.
I think the intercooler is plugged up. You have to take the injection pump off to get to intercooler. I got to that point once but chickened out. Have the tech manual that tells how to take pump off. Just wondered what other people thought. Thanks

 

[Tx Jim]

Check oil pressure with a manual gauge