oil pressure drops at high rpm
 

Cant figure this one out i have a 77 400 and at idle hot pressure is 25 psi at 2500 rpm 40 psi 3000 35psi and at 4000 25 psi. I have replaced the crank mains and rod bearings, new heads, pickup screen and hi vol. oil pump, mechanical pressure gauge and new valve lifters. When its cold pressure is 60psi. Also when you are doing about 2500 to 3000 rpm or higher and let off gas and coast pressure goes up about 5 psi. I am running straight 30 oil. This has been driving me nuts tried everything i could think of the oil pump dont appear to be sucking air and the clearance between the pan and pick up screen is ok. i'm getting real tired of taking off the oil pan. Any help would be greatly appreciated.

What type of lifters?

sealed power hydraulic also checked all returns they were all clear ran engine at 3000 rpm in park and pulled dipstick it read full with no foaming or air bubbles. I tried running it with 5 qts and 6 qts of oil with no differance.

Did you do the cam or put cam bearings in it. Also when you put the rods and mains in it, did you use plasi-gage and check tolorances on the bearings. I have a Mack engine that lowers the oil pressure under load, Mack says it is normal.
One other thing, it is obvious you are using a mechanical Gauge, did you have it on before the rebuild, if so what were the pressures.

The oil pressure unit works off a shaft that runs from your distribuator down through the engine to your oil pump. I had this problem with a 78 400M 4x4 that served well for 21 years. The shaft started to round off causing it to slip at the higher RPMs. Drove me nuts until I mentioned it to a friend who pulled the distrib cap and rotor, using a plastic straw inserted into the rotor hole he was able to extract the worn shaft so that he could drop a new shaft in. The process took less time than the time it took the parts guy to get the shaft off the shelf and ring it up.

Good luck!

Thanks for the help after working on it all day i think i solved the problem. I dont know which thing i did that solved it but this is what i did i raised the pickup screen up off the bottom of the pan to 1" from 3/4" removed gasket between oil pump and block and used halomar sealer and changed oil filter which was a new fram (which was a mistake using fram) to napa gold. Its now 30 psi hot idling and 45 to 50 psi running down highway about 3000 rpm. Still have a problem when letting off gas and coasting pressure goes up 5 to 10 psi but i think that might be the after market oil cooler doing that. As far as plastic guaging the bearings i didnt do that i bought the crank and bearings all together in a kit and they were supposed to be matched to crank.

That is what my Mack does, it is normal for the diesel to do, under a load it will push down the crank and increase the gap on the top, and it will also have a bigger gap on the bottom. Lowers the oil pressure a little bit but like I said on my Mack it is normal.
I would almost guess that anyoone that runs a mechanical oil pressure gage would have the same thing happening