hard stop oil pressure drops to zero

callbald

callbald

Senior Member
39 pickup had engine rebuild and now experiences pressure drops on stop. Prior to rebuild there was no pressure issue. After rebuild oil pressure ranged from 15# at idle to 30# at 40 mph. I just went back and replaced the pressure plunger, spring and cap - results pressure jumped from idle 15 to 40+ at 40 mph.

On hard stop pressure drops to zero and then jumps right back up to 15#. Thought oil pickup getting starved but oil level is good with 5 quarts. Can't figure it out.
 
If it´s the right oilpickup on the oilpump so it´s not higher up then intended i would check the gasket between oilpickup and pump, if you overtighten it the flange on the pickup bends and sucks air.
 
There is not much for baffles in those old oil pans. A hard turn or stop can allow all the oil to go forward or side to side. If the pick up screen loses the oil, it will draw air and that could be a problem for a bit. At least till it gets a shot of oil again and pushes the air bubble out.

Changing out an early pump for an later type can require a change in the oil pick up assembly if anything like that has taken place.
 
I have a wet (direct feed) copper line gauge in addition to the OEM electric gauge. There is no oil filter.

The oil pressure drop occurs on firm/hard stops and turns have no effect. The oil pump was good and not replaced in the rebuild. Pressure immediately comes back at 5+mph.
 
rotorwrench said:
There is not much for baffles in those old oil pans. A hard turn or stop can allow all the oil to go forward or side to side. If the pick up screen loses the oil, it will draw air and that could be a problem for a bit. At least till it gets a shot of oil again and pushes the air bubble out.

Changing out an early pump for an later type can require a change in the oil pick up assembly if anything like that has taken place.
Click to expand...

Rotor, You have nailed this one. I would bet heavily this is exactly the issue - that is, the baffle has been removed. I have seen this first hand a number of times. That baffle was in the pan for a reason - not simply to add manufacturing cost.
 
Maybe your baffle didn't get put back during rebuild. It would seem all indications are the pump is working while seeing oil. Hard stops without baffle as oil pan is much longer than wide could let enough oil move forward to expose pickup. Possibly turns let oil pickup remain in enough oil to get reading. Not sure but worth a try to get a flex scope cam to look inside oil pan to see if you can see a baffle. Or the painful obvious is to remove oil pan.
 
on hard stops the oil surges forward away from the oil pick up tube screen leaving no oil to be picked up, when you stop the oil flows back to the pick up tube, as was said a baffle is missing or the pick up tube is to tight in the oil pump and not floating with the oil level change
 
An easy attempt at a possible fix is to replace the oil pressure spring if it was not done during your rebuild. I had the same thing and chased my tail trying to fix it until I replaced the spring .....
 
Just so everybody knows . . . many of the flathead oil pans had no baffles in them at all. And the baffles that were in some of them were not to keep the oil on the pickup during hard stops - more along the line of keeping the oil on the pickup during fast starts.

Side Note: Many folks will replace a 32-48 oil pump with a later 49-53 pump - and replace the oil pickup as well (or modify one as I used to do). Some vendors sell the "correct pickup" for using the 49-53 pump in a 39-48 pan - only problem is that I've seen the pickup screen being 1" off the bottom - not good - should be about 1/4" off the bottom. I recently had to rework a pickup screen from Speedway for exactly this reason.

Here is a picture of a typical 'stock' baffle - notice it is behind where the pump screen goes:

IMG_4577 copy.jpg

Here is my modified version - to keep oil on the pickup during acceleration (I've done nothing for stopping):

IMG_4578 copy.jpg

IMG_4595 copy.jpg
 
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