Oil pressure - falls away

[344739lv]

I have a 1961 3.8 MOD. The engine was rebuilt by a specialist 10,000 miles ago and all has been well until this weekend. After an "energetic" run on the motorway where the revs got up to 4000 to 4500 maximum very briefly and stayed around 3000 most of the time, I noticed the oil pressure as I slowed down. It fell away as the revs dropped all the way down to 0.

Since then the pressure runs up to around 40 while accelerting or under load but once cruising it gradually tails off to around 15 and drops to 0 at idle.

Typically the car used to run at 60 when fast running and never less than 40 at idle.

I am thinking either sender unit, or pump which is odd as this was replaced at the rebuild.

The car runs well, makes no odd noises and doesn't smoke.

Am I likely to have binned the engine? Is there anything else I haven't thought of?

[ecosselynx]

Hi

Agreed it doesn't sound like your pump. You should try changing out the sender or get a mechanical gauge system to check it out the pressure over the range.

[piman]

Hello 344

if a pressure check (or new sender) doesn't give a different reading I would look at the pressure relief valve in case that's causing the problem. From memory, the recommended presure is 40 - 45 psi over 2000 rpm hot.
Another thought is what oil are you using, if a modern oil this could contribute, my choice would be a good quality 20 - 50 multigrade non synthetic? Not that that explains the sudden drop on reflection.
Alec

[344739lv]

Jon and Alec - looks like it was the sender. Did a mechanical test today and it ranges between 40 and 45 psi at 2000 rpm.
New sender installed seems to have fixed the initial problem. Now I notice the gauge reads almost off the scale at 65 psi under load at around 2500 revs.

This is the opposite end of the scale to the initial problem! I suspect a lazy pressure release valve given the gauge reads the same as the mechanical gauge at Rickover and up to 2000 revs.

Mechanical gauge pipe isn't long enough to use while driving so I have had to do it whilst stationary.

Cheap fix so will give it a go.

Thanks both

[darryl1959371]

Yes, I was aware of this ' Problem ' around 40 years ago with the 3.8 engine and have driven 3.8's 100 of Thousands of Miles..!!
Solution...
Fit Mechanical Oil gauge + Oil Temperature Gauge
Only use Good Quality Paper Filter....Felt ones tend to disintegrate etc... or Fit ' Spin-On ' Filter
Use Good Quality ....for over 40 years I have only used Mobil 1 5w-50 ( Amazon often Cheapest )...Oil Change up to 10k Miles and New Filters only changed every @ 5k miles

ALWAYS CRANK ENGINE UNTIL MECHANICAL OIL GAUGE SHOWS PRESSURE THEN TURN ON IGNITION

Happy Miles...!!!

[peterheywood52573]

I fitted a splitter and a mechanical gauge. For me it is enough to know that the one on the dash reads half what it should do. It would seem a bit dodgy to fir a mechanical gauge with its capillary tube in the fold down dash arrangement. Or am I misunderstanding something?
I am also not sure about cranking the engine before switching on the ignition. It may drag unburnt fuel into the bores. Not sure about that either. On mine the starter button is fed from the ignition switch anyway.
Peter