the oil pressure is low like 25psi while driving and the truck sounds like a diesel ratalling and knocking do you think it could be the main bairing that are bad and thats why I have low oil presure

 

sounds more like a timing issue, 3.9's have an issue with this

 

Main could be the problem, but start with the simple (and cheap) things first. How's your oil level, what condition is the oil in, is the oil pump working properly, is the pickup partially clogged, and I've never heard of the timing affecting the oil pressure, but it's worth checking. With a leaky main you'll lose oil pretty quick, especially with pressure that low. Definitely need to find the problem before driving it anymore, you'll make the problem a lot worse if you don't.

 

try addind a bottle of Lucas to the oil. I had to retard my timing ever so slightly to stop the knock sound when going up hill.

 

Well i replace the oil pomp in mine to, and it is the same as yours. a '92 3.9 liters the oil pomp its only about $28 . But it's aall day work. How many miles your truck has?. You may find out that replaicing the oil pomp it wont hurt any, and you'll have less thinks to worry about.Or you may want to spent a $100 to take your truck to a mechanicand see if it is not a bookend head of your motor the one its causing the noise

 

i forgot to ask you for a pic of the left side of your throttle body in particular i will like to see were it goes the air house that is under the PVC valve house. I been trying to figure it out with the tag under the hod but i dont understand it. if you do that; I will really appreciate it.