rebuilt 351 cleveland, oil pressure reads 50 psi untill the motor warms up and drops to 12 in drive help !!!

Question

351 clevelan rebuilt has an oiling problem were the oil pressure is 50 psi until the motor warms up to 195 degrees. Then the pressure slowly goes down to 25 psi then in drive the pressure drops further to 12 psi. As soon as I accelerate the presure goes to 25 psi only to drop down after you stop accelerating. have no idea what the problem could be please help! I also changed the oil from 10-40 yo straight 40 with no change in oil pressure.

Answer 1

Well it sounds very much like there is too much gap in the bearings, which of course is the worst case scenario. Still there are three other things less troublesome things to check and one can hope it is one of them.

1. The oil pressure control valve could have a weak spring in it that is allowing pressure to drop and becomes more noticable as the oil thins out.

2. The oil pump could be bad.

3. The pickup screen in the oil pan could be partly clogged or the tube itself may be bent a bit, which is not uncommon in a backyard rebuild project.

I am sorry you are having this problem, as I know first hand how much work it is to rebuild an engine, and when a problem shows up it is really vexing.

Answer 2

As I suspect you already know, there are numerous problems which could cause this.

During the rebuild: Was the oil pump replaced? New Screen? Were the cam bearings replaced? Were all of the oil galley plugs reinstalled if the block was "tanked?"
And, obviously: Were the bearing clearances carefully checked during assembly?

And, most importantly, was this a concern BEFORE the engine was rebuilt?

I'm not much help here, but there's just too many possibilities. If that's the best pressure you can get with 40 W oil - there's definitely a concern there. Remember, a pump doesn't create the pressure, the restriction to the flow does - so any internal leakage will result in lower than normal pressure.

Answer 3

You said the oil pressure drops after the engine warms up. That sounds a little more serious than a bad sending unit. You probably need to replace the oil pump. If you hear pinging when the oil pressure drops it's the pump, if no pinging it probably is the sending unit.

Answer 4

If I remember right, the oil pressure should be around 6lbs hot and at idle. You can change the idle oil pressure by making the oil pressure regulator spring longer, about one inch longer. If you are running stock lifters high oil pressure will damage them. Do not plug the relief pressure hole in the pump housing, doing so will cause oil pressure above 300Ibs.

Answer 5

THIS IS NORMAL AS THE MOTOR WARMS UP THE OIL GETS THIN AT IDLE YOU WILL NOT HAVE MUCH PRESSURE THE REASON THE PRESSURE DROPS WHILE DRIVING IS THE RPMS ARE DOWN WHEN YOU GIVE IT GAS THR RPMS GO UP SO DOES THE PRESSURE THERE IS NO DANGER WITH THIS BUT IF YOU WANT MORE PRESSURE GO WITH A HEAVEIR WEIGHT OIL OR GO WITH A HIGH VOLUME OIL PUMP BUT YOU DONT REALLY NED EITHER ONE IT IS O.K

Answer 6

Sounds like you have an internal leak loosing your pressure. Are you sure everything was torqued right when it was rebuilt? Check your water for signs of oil, and your oil for signs of water. Also check your exhaust for signs of burning oil. or water steam. Check the torque
on the head bolts

Answer 7

You've got a weak oil pump. Buy a high volume pump for 90 bucks and it will make a big improvement. Good luck.

Answer 8

Can try to change the oil pump to a high volume one. May need more flow to get what your looking for.

Answer 9

To much tolerances in the bearing clearances .