Ok, this is the quick and dirty of it.

Just bought a 1994 f150. 300-I6, e40d, 2wd, 250k. Guy I bought it from didn't drive it much because the pushrod cover leaked really bad and didn't want to mess with it. I have had it a month or so and hadn't driven it. Yesterday I drove it to school (sophomore in college) and had some stuff to do that I needed a truck for. On the way there the oil pressure gauge went haywire. Since it was flickering and bouncing around like crazy, I decided to ignore it, but turned down the radio to listen just in case. A mile later it started clacking. Limped it to where I needed to be, had a test, and parked it. About 5 hours later I went back to it, and was waiting on my brother and a truck to tow it home. I decided to start it to just see what happened. It had oil pressure! For about 3 minutes anyway. Then same dancing by the gauge followed by clacking. Let it sit 5 minutes or so, started it up, oil pressure for about 3 minutes again.

So after messing with it, here is what it does exactly. No matter how long it sits, once the pressure drops off, kill it, re start it, and have 3-4 min of pressure. Doesn't matter if it sits 30sec or 30 min, same thing. And it is loosing pressure, engine noise confirms it. It runs fine till the pressure drops. I haven't changed the oil yet, but if it were a clogged screen, I don't think it would be so regular. Oil level is full on stick.

Any ideas or help? Thanks a bunch

 

It is not an oil pressure gauge system. It is an OFF and ON oil switch system.
Just put new main and connecting rod bearings in it and you will be good to go.

 

Oh great,so my F150 has the same crummy "suicide" oil pressure bogus gauge set up as my Marauder? The MM set up shows an arbitrary PSI pressure on the dummy gauge but all it actually means is in reality you have as little as FIVE (5) PSI oil pressure or more.Hence,suicide,by the time you notice it doesn't show pressure you killed your engine.
At mercurymarauder.net the guys have come up with a real replacement gauge set up and I saw somewhere on here a partial fix on our gauges..Maybe I'll have to see about doing the partial fix and put a set of real gauges in.

 

Ok, so what your saying is that the mains and/or rod bearings are spun causing oil pressure loss thru a restriction? As for the systems, I understand how they work. I realize that the oil pressure sender is nothing more than a pressure switch that supplies resistance to the circuit based on the pressure it sees, and the gauge in the dash just reads that resistance as a pressure based on its calibration. I also know they layout of the oiling system. I have read that a spun bearing can cause oil pressure problems. But this thing is damn near like clockwork which is where my confusion lies.

 

I was just saying the bearings are wore out and need to be replace. In other words you oil pressure is at about at 5psi when you start the engine and then the oil thins as it get warmer and drops below 5psi and the engine starts to rattle.

 

I was hoping to avoid that. But I'll pull it anyway. Didn't know if this was something common. My last truck was a toyota and its gauge actually read the pressure. I assumed this one worked similarly.

As for the warming up part, not so sure. I spent 45 min messing with it, starting it, waiting for the pressure to drop, kill it, wait 30 sec start it again for 3-4 min and repeat. It had warmed up, well according to the gauge anyway.

 

I agree with Subford. But the only true test is to put a mechanical gauge on it and get a true reading.

The 300 is an easy oil pan drop compared to the 302 and 351. I would pull the pan and check the mains and rods. For what it's worth, the oil pump may be part of the problem. Its $35 and new oil pan gasket is $26. Either way, take a weekend to drop the pan and inspect. I have done mine inas little as a day and reassembled in 2. Ensure that it has a FL1-A filter, no Fram oil filter. 300's have been reported to eat oil pumps, but that is rumored, since some averaging over 300,000 miles. If you do drop the pan, report back to us on your find and we'll try to help you from there. At the risk of being flamed, if the rods and mains are worn, but crank is still in good shape, may just be able to roll in new mains and rod bearings.

 

I don't use fram period. Purolater or motorcraft usually. Has a fl1-a on it so the last guy wasn't that bad. It may be a while before I get to drop the pan. I may just pull the whole thing. Most every gasket looks leaky. Check the mains then, if they are shot, it's already out. Thanks guys

 

Ouch... Mine is doing same thing...only difference is, it takes about 20 miles then the oil gauge flickers and few miles later drops to zero.... I'm not as mechanically inclined as u guys and being the truck needs tires a water pump and brakes, as much as I love my 91 F150 Shortbed may be time to retire her

 

It could also just be that there's trash in the sump at the bottom of the oil pan. After startup it gets sucked into the pump strainer and kills your oil pressure. Just a hopeful thought, but since you have to pull the pan to check that anyway... might as well do spot checks on the bearings and toss a new pump at it while you're in there.

 

I wouldn't jump straight to the bearings, although it's possible with 250k on it. Check out the pickup, oil pump, and oil filter first. Sometimes oil filters can get clogged and not bypass and cause some strange issues. Just trying to be hopeful. Could be that you'll have to do new bearings, but I'd check the other stuff first before you go through a bit of trouble to replace them.