Oil pressure drop

I finally got my mark ix our on a successful family trip through the mountains. I do have one question regarding the oil pressure.

The oil pressure is right where it should be almost all the time. The only issue is when going up a long, steep mountain hill which climbs 4000 feet of elevation over a very short distance. The car had no problem maintaining a steady 65 mph up the hill, but I did notice a drop from 40 to just below 35 on my oil pressure gauge during the steepest section. It immediately returned to normal when I began the descent on the other side of the summit. Is the drop in oil pressure something to concern myself with? Why would it do this? The rpms were steady.

Thanks for everyone’s help in getting me this far!



Did you check the oil level?
Looks like that’s to low causing drop of pressure.

Peter Jan

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I have an oil temperature gauge on my 73 XJ6. When under heavy load, the oil temp will go up, and there is a concomitant drop in OP. But it takes several minutes to come back down after the load is decreased. If by “immediately” you mean just that, then oil temp wouldn’t explain it.

I’m tipping same. When I do an oil change on an unknown car I put the exact quantity per the manual in an “calibrate” the dipstick after running. They can be out a bit as they can be bent out of shape. Can be the difference between add and full. Paul

I’ll check how much oil I put in after my next oil change, but it was well topped up before I left. (Probably one quart extra…)

The oil thinning with heat makes some sense to me. I believe that the temperature of the engine rose somewhat during the climb, and likely the oil pressure went down at the same time. It is likely just a bit thinner and drops the pressure.

How low is concerning? At 60mph (3000rpm) what oil psi should I start to ease off the gas? Is 35 or even psi ok for short periods?



Jaguar wanted 40lbs @ 3000 rpm. 35 is probably still fine.
If you’d never exceed 35, gauge, sender or relief spring are faulty. But you tend to have 40lbs.
If the pressure at idle, hot, is okay I wouldn’t be too concerned. I can imagine that the oil pressure drops uphill quickly, but it will also take a few minutes to come back up as the oil won’t cool instantly. Longer to cool than to heat surely.
On the E-Type and XJ sumps, low oil level shows first when you brake and corner, never uphill, especially with the XJ.

What could also be is that the sender or wiring changes resistance when the car is “tilted”. If the slightly low oil pressure worries you, change to 20W50 if you don’t run that already.


Oh, and another possibility - my gauges change when tilted around. You could try and stop somewhere steep when the engine is still cold and see if you get 35. Maybe the gauge only ever goes to 40 and when the oil is hot and the gauge is tilted it will go down a little.

And another strange phenomena I have experienced with my 3.8S (stock electric gauge) is that when the instrument lights are on the oil pressure reads about 5 lbs lower than when they are off. I believe that is due to heat from the bulb affecting the dash instrument. The situation went away when I converted to LEDs for instrument lighting.