Help needed with oil pressure issues

[Timebomb]

I put a a 351 motor in my 92 ford truck about a month ago because it decided to eat a couple of pistons out of the old 302 that came from the factory. The 351 is out of a 88 truck and everything was pretty much a staright swap, the issue I'm having is with the oil pressure. When cold the truck has great oil pressure at idle and above, when it gets warm or after about 5 minutes of driving the pressure drops very quickly. If your cruising on the highway at about 2000 rpm the oil pressure reads very low but if you let off the gas it will climb back up for a minute then drop back off. I've replaced the stock sending unit and got the same results. The 351 came out of a 60K 1988 truck and looked spotless before I put it in...the lifter valley was very clean and so where the heads. I did not pull the oil pan so I don't know what the bottom end looked like. The truck does run well its just the oil pressure has me a little worried. Anyone have any thoughts or suggestions on what might be causing this?

Here are a couple of things that have ran across my mind:
1) Could the motor has a high volume pump on it and may be sucking the pan dry over time and causing the pressure to drop
2) Could the oil pump shaft be bound up somehow when the motor gets warm?

Thanks for any info you might have and what I can try to solve the problem.

Brent

[maveRick]

Brent,

You sure have some peculiar stuff happen to you (ie,. stuck distributor :confused: ).

Do you know if the oil-pan is the one that came with the engine (ie., are there any dips and valleys along the bottom contour that look unecessary)? The oil pan and pick-up may not match.

Just a thought.

Oh yeah, check your oil level (tee hee :D).

Good luck.

[Neighbors]

I'm going to go by what I think, not what I know, but I think I know, you know... :D

If it has a high volume pump, chances are low that it's sucking the pan dry at 5K or below, and , if the pump shaft is bound up, it would turn itself into a pretzel quickly...

If I were to hazard a guess, I say the main bearings are worn. However, before I tore into it, I'd put a 'real' gauge on it. The rule of thumb is 10psi for every 1000 RPM...

Did you, or can you, check endplay? I wonder if the thrust washer is worn, allowing a reduction in pressure?

The fluctuating pressure when you let off may be rod bearing related, but WTH do I know, LOL! (...and that's pure conjecture; seems load related...)

[Blue91]

That's what I suggested to him, worn bearings. Looser clearances = less psi. My 86 was doing the same thing, good oil press on startup, and after it was warm, the mofo would go down to like 10psi (checked with real gauge) on the highway! Since then, as most of you know, I've rebuilt that motor and with new bearings, the oil pressure is great.