I have been working to get My 1986 F350 four door running reliably. It is a two wheel drive 6.9l diesel. It seems when I get one problem solved another showes.... I made the mistake(?) of changing the oil and replacing it with Amsoil without first doing the recommended flush... I have only used gassers before this and simply changed the first batch of Az early. I have wondered if it could possibly release a huge amount of sludge and plug the Amsoil filter... The Oil enabled the engine to continue starting cold with that tired old Delco starter... but 1200 miles later I started noticing lower oil pressure reading at the dash.
I would start with "normal" reading and drop very low 15 miles down the road. Shortly I was hearing an unusual acompanyment to the engine sound. I could stop the engine and restart it after a couple minutes and the sound would go away like the My chevy's knock. It returned in the same manner. This morning I took it in to get an oil filter the pressure didn't come back as high. I changed the filter. No pressure. took the sender out and tested the gauge and sender, fine; screwed a dial gauge in the sender hole: 5 psi idle with an occasional spike to 10. 7psi with an occasional effort to 15 at about 2000 rpm. A tracter diesel mechanic suggested that the pressure relief valve is stuck open and or that the sump screen is plugged. I would really appreciate any info..JNL

 

jayenelee,
Welcome to FTE and the IDI diesel forum.

First the dash gauge is about worthless, get a mechanical gauge.
Your idle pressure is not that far out of line, but your 2000 RPM pressure is way low.
My first guess would be the pickup screen is blocked.
If the pressure relief valve was stuck open the idle pressure would be even lower.

How many miles on the engine?

 

Thanks, You confirmed my suspicion. The truck and engine apparently have 143some origional miles on. Any tips on solving the problem?

 

I have read you can pull the oil pan with the engine still in the truck with the motor mounts unbolted.

I have never tried it though.

That oil pan is huge and the cross member is real tight to it.

 

I decided to try one thing before tackling that oil pan: motor flush. I very gently drove the truck three miles in to town and grabbed a change of oil and two quarts of 'flush. I returned home, shut the truck off and added 1 quart of 'flush. I started it up and observed the pressure needle return to it's former position. The truck started sounding my friend's old carburated C60 on a cold morning. After a couple minutes I shut it off and drained only three quarts of real cruddy fluid. After completing the underside part of the change, added two and a half gallons of shell rotella and started the truck with the dial screwed in; it showed 40 psi idle and 60 elevated. This time I studied the dip stick much more carefully. (I am neerly positive it is not the right one.) At about 1600 miles that is about 2.5 gallons of oil consumed. It only smokes a little blue during a cold start and when you give it Nellie on a steep hill. PTL and Thanks guys! Mayby I now understand why the truck's fuel bill is only half of the Chevy 350 gasser I drove before it.(?)