390 with oiling problem
Freshman User
Joined: Oct 2006
Posts: 43
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There's quite a few possibilities obviously but...
.. there's a gallery plug behind the distributor (I believe) that can easily get left out and from what I've read is a chronic cause of low oil pressure in FE's. Worn valvetrain (rockers and shafts) can be a culprit too.
Tuned
Joined: Mar 2009
Posts: 360
Likes: 3
From: Napa
Since the pressure dropped at the same time you first changed the oil, I'd be willing to put $20 on it didn't have any oil pressure, and the PO loaded it up with STP or similar to phony up the readings so he could get rid of it 
- when you put normal oil in it, the "magic" oil pressure was gone 
- when you put normal oil in it, the "magic" oil pressure was gone
Freshman User
Joined: Oct 2006
Posts: 43
Likes: 0
Yeah...
Probably had some 60W racing oil in there or some other heavier oil (50W, 40W, 30W, 20W50, 15W40 etc.) with a bunch of STP/Lucas or some other oil treatment/supplement. Unless you got the truck pretty cheap, I'd probably be calling or driving to whoever sold you that truck and be for having a talk with that person (allthough on the other hand it probably won't do anything other than maybe make ya feel better).
Elder User
Joined: Sep 2009
Posts: 503
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From: nor cal
it might not be as bad as you think.. i'd change the oil and let the oil run threw a paint filter if there is no fragments your bearings are fine, i would check the rods for blockage..is there and oil leaks? maybe when they rebuilt the block they used gasket maker on everything lol i seen it done...
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Senior User
Joined: Apr 2006
Posts: 332
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From: horton, ks
thanks for all that info guys. it was really appreciated. a couple days after i started this thread i ended up selling the truck. but i will keep this info in mind incase i get another FE motor with the same problem lol
thanks again
thanks again
Postmaster
Joined: Jan 2009
Posts: 4,584
Likes: 3
From: Chesapeake, Va.
Here's something more to remember if you get another FE or any pushrod engined truck. If and when you change the cam you should always look at the cam bearings. you can't see much through the lifter bores but if you see that the bearings are wiped or if you see copper you will have lower oil pressure than if the bearings are in good shape and they should be replaced at that time. You also have to be very careful to not let the cam lobes scrape against the bearings while installing as just a little scrape will help to reduce your pressure.
Good/better luck with your next truck.
Senior User
Joined: Jan 2010
Posts: 115
Likes: 0
From: Texas
if i have one of these problems with my 390 will the oil pressure change when driving or just randomly. some times it shows fine oil pressure and then others low oil pressure.
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Postmaster
Joined: Jan 2009
Posts: 4,584
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From: Chesapeake, Va.
Rod bearings going Bad will often have good oil pressure for a while and then fluctuate a bit.
Rod bearings that are about to spin or sieze will often have decent oil pressure but the guage will flicker from good to bad or low and then back to good.
All of the above warnings should prompt you to get ready for an overhaul or a complete rebuild of the engine real soon.
Postmaster
Joined: Jan 2009
Posts: 4,584
Likes: 3
From: Chesapeake, Va.
There is no way to guesstimate that. Your best bet would be to park it and fix it asap. If you keep driving it you will cause severe damage to the crankshaft. At that point you will be faced with a complete rebuild. From what you have said I think you could get away with just replacing the rod and main bearings while the engine is in the truck.
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