39 pickup oil pressure fluctuation

[callbald]

the flathead was rebuilt 300 miles ago and they reinstalled the old oil pump saying it was good. I started to notice pressure fluctuation from 30 to 20 at 40 mph. To double check I installed a mechanical oil gauge and got confirming readings. I have been really paying attention to the pressure. Today twice the oil pressure dropped to 5 pounds at 30 mph and bounced right back to 30 pounds. I changed from 30 to 40wt oil today and saw the bounce again. I have been driving in 40-50 degree weather.

[petehoovie]

You mean you installed a mechanical oil pressure gauge???

[41ford1]

Was the pressure relief valve under the manifold checked? The spring that holds the valve closed weakens over time or exposure to high heat.

[trainguy]

Sure sounds like a pressure relief valve problem.like 41 ford said on up to 48 is under intake manifold 49-53 is on oil pump.The valves under the manifold have a tendency to bind causing the problem you are experiencing.i freed one up by adding a quart of mm oil and fixed the problem within 5 miles! And the problem never returned.Phil

[Willit Stop]

I had a similar problem.My pump was assembled incorrectly and there was a gap between the tin can and the pump body.An easy way to check if this is your problem is to park on a steep incline(front end up) and check the gauge.You should have steady pressure.Next,turn it around(rear end up)and check the gauge again.If there is no,or low,pressure,you might have the same problem.Hopefully,it's something simple because pulling the pan isn't fun.
Good luck.

[Tinker]

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Question, isn't 40w a bit thick for 40-50 degrees? Thinking straight weight as it was not mentioned.

[callbald]

Got same pressure change with both 30 and 40 wt oils and read the same across both gauges. The re-builder is looking into the problem. The immediate thinking is there could be debris moving around in the sensor area and yes the check valve could be it. At least, they are going handle it.

[cas3]

hind sight is 20-20, but they should have switched your old long pump to the later 8ba short pump weather it looked good or not. better pump, higher pressure, most folks do not use the old long pumps. hope its an easy fix in the manifold as others have guessed, but if you have to pull it out, get rid of that pump

[G.M.]

Originally Posted by cas3

hind sight is 20-20, but they should have switched your old long pump to the later 8ba short pump weather it looked good or not. better pump, higher pressure, most folks do not use the old long pumps. hope its an easy fix in the manifold as others have guessed, but if you have to pull it out, get rid of that pump

I wouldn't be in a hurry to get rid of what may be a good pump. Just
yesterday I was checking an old unknown engine for oil pressure.
Mechanical gauge installed, plugs out, lubricated cylinders and
spinning with 12 volts. 17 lbs oil pressure. This method would
duplicate pressure at 35 MPH. Remove front bypass valve and
installed 3/8"spacer on top of spring which normally boosts oil
pressure. Still 17 lbs with spacer. Pulled oil pump, removed drive
gear, installed my test fixture on pump. Running with a 1/2" drill
had well over 75 lbs. Could have been 90 I didn't look real good
when it went past 75. This tells me engine bearings and or cam
bearings worn out. Needs completerebuild. G.M.