1990 Ford Bronco, 4X4, 302, auto. It all started with a trip to the mechanic to seal up some oil leaks. First the rear main seal was replaced, then a second trip to the garage(same place) and the intake manifold gasket was replaced.
The truck has 116,000 miles and always had rock solid oil pressure. After the intake manifold gasket was replaced, the oil pressure was erratic. At idle after it was started, it would be normal, but when it warmed up, the pressure would drop to nothing and would increase only when engine rpm increased. When the truck was driven, the gauge acted like a tach, the pressure would come up when given the gas but would drop when engine rpm dropped after going into a higher gear. A third trip back to the same mechanic had him scratching his head, he replaced the oil pump, (no charge for labor)I was there when he pulled it out, the pump was clean and a new one was installed but no joy, pressure was below 12 psi in gear and 46 psi in park. The mechanic informed me that my main bearings are probably shot and I need a short block, but I don't understand how the main bearings could affect overall oil pressure in the system. I took the truck home and changed the oil and filter to see if it might have been a clog in the filter but no go. The truck now loses oil pressure as soon as it warms up in park and has a definite knock after it's been running awhile, pressure will come up if the engine is revved but doesn't maintain. I'm wondering if there is something on the distributor that I can check that drives the oil pump and a friend suggested that the oil bypass relief valve might be faulty. This truck was running good, it took a nosedive after 2 trips to the mechanic (that's another issue) I'm having a hard time believing this engine is shot. Any suggestions out there?

 

if you can afford a second opinion from another mechanic (engine type), he can check the relief, and the oil pump gear drive off the camshaft..he aloso see if the pick-up tube was bumped off or damaged during the rear seal change.

 

Not obvious from your post if you checked the pressure with a mechanical gauge. If not, need to do so. If your pressure readings are correct - you have classic signs of worn cam/main bearings where the pressure drops as the engine warms up and the oil thins out. Pressures of 12 at idle and 45+ at 2000 rpm and above is acceptable in an older engine. The dash warning light typically comes on at 7 psi and below (if equiped). Electrical oil pressure gauges are really symbolic and cannot be trusted.

Good Luck

 

I don't know the deal about the 2 trips to the mechanic thing
but I can tell you that I have a 90 bronco with a 351 and
I had the exact same problem to the tee.
I had all the same parts changed but no go. Believe it or not
it turned out to be spun main bearings ............
I have pictures on my hard drive but dont know how to post them
So it sounds kind of similar JC

 

Thanks for the replies, I watched the mechanic pull a main bearing cap off when the oil pump was being replaced and examined it. He pointed out what appeared to be a large amount (over 50% of the surface) of brass or copper showing through the bearing and that it indicated a worn bearing. Being distrustful of the mechanic, I figured it was time for a second opinion, at any rate it sits in front of my house for the time being. When you say a spun bearing, what exactly does that mean? Again thanks for everyones advice and opinions. PS, is it feasible to just replace the crankshaft bearings without pulling the engine?

 

If you are showing copper through the bearing it's about had it!
My bearings were showing like 75% through

It's hard to explain a spun bearing you almost have to see it
but the bearing in two pieces the top and the bottom bolth
being u shaped one have is on top in the block and the second
one is in the bottom piece that bolts the crank into the block.
And when they dont have enough oil they get hot and the metal
starts rubbing together causing the copper to show.

And instead of the two bearing haves staying in place they
begin to spin with the crank and instead of them being up and
down from each other you will find them spun in any position

Possibly like () Send me your e-mail address I have pictures
Jeff jdawg373@prodigy.net