Puzzling oil pressure question

[snarf]

Can anyone offer an explanation to this change in oil pressure behaviour?

My C2S has just had its four year service and on the 50 min, 25 mile, drive back home, and after the oil temperature was above 90C, I noticed that even at tick-over speeds the oil pressure never dropped below 4.5 on the gauge; at engine speeds greater than 1500 rpm it was on the maximum. My initial thought was that the gauge had broken but there are very slight fluctuations in reading between 800 and 1500 rpm. I supplied my own oil "" Mobil 0W 40 - so a change in viscosity is an unlikely explanation.

Previously after, and between, oil changes I've never noticed any significant difference in behaviour; at tick-over speeds the pressure would read between 2 and 3, and at engine speeds greater than 3000 rpm it would be at or near to the maximum. I know high pressure is generally considered a good thing but I don't like unexplained changes.

 

[stomper]

That's interesting, I've been seeing the opposite, since I last added oil to my c2s. Previously, the pressure gauge never went below 4, but now it's exhibiting the behaviour you describe, as soon as the oil temp goes above 90 and the engine idles the pressure drops to 2. Coincidentaly, it was Mobil 0w 40 that I put in, I was thinking that this was due to viscosity and the change in behaviour happened right after topping up which caused some concern as it wasn't a full oil change, simply a top up.

 

[snarf]

Cleaned the car and took it out for a run this afternoon. Initially the oil pressure was exactly as described earlier but while stationary in a queue of bank holiday traffic I noticed the gauge drop from 5 to the normal 2 to 3 level. When the traffic moved a few feet and I increased the revs slightly the gauge leapt back to 5 and stayed there even when the engine speed returned to idle. After about a further 30min driving the gauge behaviour became what I consider to be normal with the pressure increasing from 3 to 5 in proportion to engine speed in the range of 2000 to 3000 rpm.

Still not sure why a service might have bought about this behaviour but it was rather like there was a partial blockage in the pressure measuring system acting like a non-return valve. Do cars get DVT?

 

[garyw]

When warm mine has always read 5 if the engine is working, at tick over it hovers around the 2 mark..

garyw

 

[snarf]

ORIGINAL: garyw

When warm mine has always read 5 if the engine is working, at tick over it hovers around the 2 mark..

garyw

That's how mine had been since new until this last service. Any way things had returned to normal by the time I put it back in the garage earlier today - still puzzled as to what the explanation is but the more I think about it the more I like the idea of a partial blockage acting as a non-return valve.