low pressure after oil change

[gary stoddard]

I have a 1984 w123 280e which i bought recently. It has done just over 130k miles. I've just changed the oil filter and oil with 10w40 and when it warms up the pressure drops to 1/3 at idle.When cold it idles at 3. It goes back to 3 under acceleration and stays there above 1500 rpm. Before the oil change it would not drop below two. My question is what could be wrong and what should I do next. I've followed the manual and made sure that the compression spring is in what looks like the right way.

 

[Parrot of Doom]

That sounds normal to me, the pressure on my TVR dropped down quite low at idle, and quite high when I was booting it

Obviously I don't know what pressure should exist on your engine, but have you considered checking the oil pressure sender?

 

[gary stoddard]

I was concerned because it used to idle at 2 before the change. I would have thought if the sender was faulty it wouldn't register the proper pressure when cold. Plus I've not been anywhere near the sender

 

[Mat_b]

My 300TE is the same, according to handbook, pressure can be as low as 1 at tickover but must rise quickly to 3 by 1500rpm, dont worry

 

[television]

It sounds fine to me as well, idle pressure is not that important, and there is nothing on the engine that can change it if it is 3 when driving

 

[shirubaby]

Would thinner oil make it drop more than usual? I've been suspecting that this might be the case with mine. BUT, if the pressure reaches 3 when booting it there is no problem.

 

[Hibbo]

Aye man, dinna worry. As long as it shoots up when you hit the pedal. (I'm sure I've read in some manual somewhere that the minimum idle pressure is as low as 0.3 bar)

PS. On my w123 the oil pressure gauge is direct reading - that is an oil pipe goes up to the gauge. Don't start the engine with the cluster out

 

[wireman]

They only fit an oil gauge to worry you when this sort of thing happens, my 124 handbook says just what Hibbo said min 0.3 bar, its OK if it goes back up when you are on the move.
The oil pressure on my car falls to 1.5 when hot at tickover and is up to 3 at 1000 RPM, I have found that 10-40 oil behaves this way and with 15-40 it stays at 2.5 at tickover.
Oil viscosity plays an important part in holding up the oil pressure at low engine speeds, The oil pump moves a volume of oil which is proportional to speed and the oil sprays out of all the bits inside the engine at a more or less constant rate (under constant pressure) so it may well fall when the engine is turning slowly.
Because of the high volume of oil being pumped at higher speeds there is a relief vave that lets the surplus volume of oil go back to the sump, this valve is set at 3.0 bar.