351 low oil pressure
 

I just did a head job on my 90 F150, had a blown headgasket and cracked head. It had good oil pressure before the job but I didnt run it long due to the coolant in the oil. Now it starts and has about 40-45psi (sunpro gauge) and factory gauge is in the normal. After it gets hot the pressure starts dropping, goes to about 10psi at idle then will fall to 0 and the light comes on. I changed the oil thinking that it was thin oil from all the crap in the oil passages and some gas from running rich. It got better so I went out on the road, after about 1 mile I got on it hard to see what it would do and right away the gauge went to 0 and the light came on so I got home, shut it down and restarted it: the light was still on and 0 on the gauge. After it cooled down the gauge went back to normal. I think I have a bearing problem, but theres no metal in the oil and when the gauge is showing 0 the engine isnt rattleing like theres no oil pressure. I had a Chevy 350 do this to me years ago when we installed the wrong bearings, it was fine cold but at operating temps the light would come on...
Could it be anything else? Maybe I'm overlooking something...

are you going off of the stock gauge or the mechanical oil gauge?

what oil filter are you using? if its a FRAM remove it and put a Mobil1, Purolator,Wix.

Clogged oil pick up screen ?? Comon problem in these trucks... Any oil build up on the heads when you took them off ?? Lew

Both gauges are showing the same. The engine had lots of gunk in it, I flushed the system with clean oil a couple times before starting it. It sounds like I'm gonna have to pull the pan and see whats going on, it would be great if its just a cloged screen.

Thats probabley it then.. We have seen it so many times...Good luck ..Lew

Bet ya the screen is clogged like one said. I just did my oil pan drop and replaced pump and cleaned pick up screen. It was a job with the engine in place. Best thing about the job was the one piece felpro blue neoprene gasket. It went into place nice. Would reccomend removing the oil pan because most likely there is sluge in there that wont come out with changing oil change. Once removed you can go ahead and check out the bottom end for wear.
Dont expect to perform this quickly. I took my time and it took 1 day and 4 afternoons. I went over everything at the bottom end though.

but would a clogged screen have good startup pressure and drop off once warm?...when i first got my truck it did just like his so i went ahead a nd did my bearings and pump and that fixed it....not sure which, but my bearings were worn from the looks and my oilpump and screen were clean

IMO, a pick-up screen cogged 85-90 % could draw in just enough oil for a pressure on start of engine but soon afterwards could be picking up "other" loose sluge/debri from oil pan and then restricting/plugging it 100% dropping oil pressure to zero

alrighgt, i wasnt sure if that could be an issue or not

A friend on here is having the same problem with his 400. We changed the oil pump and the screen was clean, and we cleaned it again to make sure. After putting it all back together, it's still the same. He also rebuilt the top end when he got the truck.
I had a thought when I saw the pick up tube wasn't the one that came with the engine, that the pick up isn't close enough to the bottom of the pan. It sits 3/4" from the bottom, shouldn't it be closer? has anyone measured theirs or know where it's supposed to be sitting.
Oh I didn't mean to hijack your thread, but maybe it's relevent to your situation.

3/4 sounds about right, your friend sounds like he needs bearings

Just found this thread and it matches my problem exactly.Bought my 1992 4WD F350 with 351 engine cheap (1500) because owner said engine was "tired".I ran it several times around the island where I live and no problems.It was overheating when I got it due to low coolant and a leaking heater hose repaired with electric tape.I replaced hoses and ran it for hours with no problems.Compression was 150 lbs on all 8 cylinders.I took the truck into town today and after going on highway for a few miles oil press dropped right off.Sounded like a diesel with all the lifter noise.If I turn it off for two minutes the oil press comes right back then soon drops off.
My theory so far is that the oil is not getting back to the pan from the heads or the oil gallery between the lifters.Either a build up of sludge or a poorly installed gasket.Any thoughts.

sounding like a diesel......is it knocking your hearing?......thatd be worn bearings...whats the miles on it?

No that noise was lifters with no oil.

Have to say mileage on this truck is unknown.Odometer reads 150000 KM that's 90000 miles but was replaced, I assume, due to malfunctioning speedo.Problem with speedometer turned out to be differential speed sensor.I know instrument cluster was replaced because of ink on it from wreckers.Everything on this truck that the previous owner worked on is a mess.He is one of these people who thinks he is a mechanic but knows only enough to get into trouble.