i got this truck back in june 2009, and ever since i changed the oil, it has never had the oil pressure it had when i got it. i have tried different filter brands to different grade oil and all with no luck. when it starts up and idles cold it has right at 58-59 psi. but once you drive for say 10 miles and come to a stop, the oil pressure stays around 3-5psi and the rocker arms chatter a little bit. once you take back off its back up to around 50psi going down the road. what i dont understand is how changing the oil affected this. ran great and never had a problem before. start up on cold idle would be about 60psi, then when warm it was around 38-39 psi. anyone have a clue whats goin on?

since i have got the truck i replaced the cam and lifters, timing chain set and the oil pump to a stock style melling oil pump. none of which did any improvement.

 

Mike, Does it have manual or elec. oil gauge?

Better to keep oil in old FEs than worry about pressure. The more wear on the crank bearings, the less idle pressure there will be.




John

 

it has an auto gage mechanical oil pressure gauge in it. the oil level has always been full. its just weird that an oil change would make all this happen. with 10/30 oil it looses oil pressure quicker. if i use 20/50 it still loses pressure, but not enough that the rockers start to chatter. very faint if any noise is heard with the thicker oil at idle.

 

Sounds like the bearings are shot, and the heavier oil covers it up- a little. Remember that the oil pressure guage pickup point is at the filter mount, and doesn't really reflect the pressure inside the system very well- if the pump's good, it will always show some pressure at the stock guage point when cold, before the oil goes into the main system, but if the bearings are shot, what's supposed to be controlled leakage through the bearings is more of a gusher when the oil's hot- so no pressure on the guage. A high-volume pump and/or heavier oil will get a few more miles out of it, as it can keep up with the leakage better, but the real cure is probably going to be a rebuild. The noise you're hearing is probably the lifters, as they're running out of oil

 

I agree with the guys above. Biggest cause of low oil pressure (most noticeable at an idle) is worn main crank bearings. I run my bearings quite loose (0.0025") and have a little under 15 psi at a hot idle, yet it's up to 45 psi at 2000 rpm (high volume pump).

I've tried different filters and even some of the higher flow rate filters only increase the hot idle oil pressure about 3 psi. Never did see much difference by changing oil viscosity, however thicker oil should do something. So why not try a can of STP oil treatment? That stuff's like molasses...in fact, sitting the open can in some warm water for a few minutes makes it easier to pour. If the oil pressure at hot idle goes up with the STP, then I'd say that pretty much confirms bearing issues.

Thes engines have a lot of press-in plugs in the oil galleries, especially along the center oil gallery on top, but under the intake manifold. Old oil is usually very thin, and an old filter could be partially plugged up. It's possible when you put new oil in it, the increase in pressure popped out a plug. However, I'd say if one of those popped out, your oil pressure at higher rpms would be a lot lower than you are finding it. The thing here, is that if you decide to take the engine out, you may want to remove the intake with the enigne in the vehicle first, just in case it's an easy fix like a popped out plug. External plugs (in the back of the engine) would cause a major mess in your driveway, so you can rule them out.

I can't remember how the distributor gets oil...it may be from the end of the driver side lifter gallery, but I seem to think it is right off one of the first oil galleries. You may want to pull the distributor and look for wear on the end of the shaft as well as wear in the hole in the block it goes in to. Doubtful, but possible.

Another "doubtful but possible" thing is one of the rocekr shaft stand bolts coming loose. Each side has 1 bolt that the oil flows around to get to the rockers. If one of those loosens up, you'll have a major oil pressure drop.

I sort of mention these "doubtful but possible" things as it may prevent you from major engine removal and rebuild work, and those parts have to come apart anyway if you do decide on a rebuild.

Yeah... I know.. a lot I metnioned is pretty lame...just trying to help

 

oh man i didnt really want to hear those things lol. but thanks for the pointers guys. soon as it gets warmer out i will eventually pull the motor out to see whats really going on with it. thanks for the help again guys, was really appreciated

 

You never mentioned what weight oil your are running. In some of my higher mileage engines I've run straight 50 weight to make them last long until I was ready to rebuild or swap the motor out.

 

right now i am running 10/30 castrol gtx.

 

I use cen-pe-co straight 30 w/ no problems.

 

supposedly this 390 was rebuilt a few years back and "should" have around 65K on it. im thinking they did just a backyard build and stuck new bearings in it without having the crank turned. i was going to rebuild it later on down the road but not this quick lol. i'd like to keep it alive for atleast a year or 2 till i get all my 460 swap parts for the f250 in my sig. this 390 will be going in my other 79 f150 project. i did notice it had fairly decent pressure with the 20/50 castrol so i might just use that or switch to straight 30 or 50 weight for the summer. truck doesnt get driven very much, mainly in town. but i do take the occasional joyride for about 20 miles one way.

 

It sounds like the engine was doctored before you bought it, probably because of oil pressure issues.
It's too bad you didn't check the rods and mains when you put the new oil pump in. If it's not knocking you could just drop the pan and change them to get some more time before a rebuild is required. If the engine is knocking it may be too late. Also, in case you don't already know, if the crank is already undersize the bearings will be marked on the back side so you can just get the same size for the freshening up.

 

My ol' 390 does the same thing, at first it bothered me. I straight 30 in it and make sure it is full, I actually run 6 quarts in it to make sure it doesn't run low on road trips. It works fine and has not gotten any worse. I even put a new sending unit on it "just-in-case" but that made marginal improvement. I am rebuilding an old Merc 410 for it, so I am not too worried about it. It is my workhorse, haul the sleds and the camper and the firewood. It does not get babied.

 

i also had the same problem when I got mine,but got lucky w/ the fix.I started running lucas treatment in it,1-4 ratio. That helped a little, but I changed the pump and my problem disappeared.It has had no problems for about 3 years,runs a steady 35 or so lb. hot,60 or more cold.

 

So the pump could be shot then? I never thought about that being part of the problem.

 

Possibly,but like I said,I got lucky. More likely than not it's crank bearings.