anyone have a clue???please help

i have a 1985 f250(351)..it starts great,runs good..i will drive it about 15 miles and it starts knocking bad when i hit about 55...the oil pressure will drop while idling and go back up while driving...does anybody know whats going on or anything i can do?:shakehead

The drop in oil pressure would cause your engine's internal parts of loose lubrication between the parts, if this is a high mileage engine this will cause enough space to appear between the parts to cause them to start banging against each other which is probably the knocking that you are hearing.

I would have your oil pressure tested at idle and at whatever RPMs your engine is usually at when you are doing 55mph, it is probably below specifications at idle and gets worse 15 miles later when the engine has warmed up and the parts have expanded apart from each other.

first of all thanks for the info...so what do you think would be the fix?..i dont know if it could be oil pump or what...i hate to put alot of money in it cause its an old truck....do ya think maybe some of that high dollar engine lube would help a little....thanks again

Have it tested if the oil pressure is too low you will have to replace the oil pump, to do this you will have to drain the oil and remove the oil pan.

I don't know if you can get it now,, but they used to make an oil thickener called STP and Oil Honey. I think a couple of cans in an oil change would make a big difference. This stuff is bad to put into a normal engine. It causes drag which hurts the fuel mileage.
But, in your case it is an approprate response to the problem addressed. Your oil pressure will stabilize and it will fill in all those gaps that are knocking. lol

Have it tested if the oil pressure is too low you will have to replace the oil pump, to do this you will have to drain the oil and remove the oil pan. Don't put it off. Things can ugly and expensive quick!:nono:

Another possibility is that your engine has so much wear on the internal parts that too much oil is leaking from between them causing your oil pressure to be too low, unfortunately this isn't easy to figure out without trying to replace the oil pump first and seeing if your oil pressure went back up to where it should be.

How many miles does your truck have on the odometer?

the odometer is reading 107000....is the oil pump hard to replace?i think im gonna try the oil thickener or oil treatment before i replace the pump.i have been told that it was a big job.is it?how much is a reasonable price to pay a repair shop to fix..anyone?

Basically you need to unbolt the motor mounts and jack up the engine about 4 inches. Put a 4x4 under each mount. I would look for a plugged strainer. If its clean I would pull the rod bearing caps next then the main bearing caps.
Some people have gone to a .005 oversize bearing. Just make sure its not destroyed on one bearing.
Ask a local machine shop for help before you decide what to do.
They also make high capacity pumps but they don't generally work.