I have a 1988 300SE and the oil pressure gauge works when the pressure is about 1.5 or above but when I am at idle the gauge sits at 0. As the revs drop off it drops off normally but when it gets to about 1.5 on the gauge it falls to 0 in an instant. Is this a faulty gauge or sender? I have compared it to the gauge on my 260E and above 1.5 they act the same. It is almost like the gauge just doesn't read on the low end. Oil pumps either work or they don't right? Sorry if I have confused anyone. Thanks.

Sounds like a bad sender. You aren't hearing any tick-tick-tick noises from the engine at idle, are you?

The sender is probably the easiest to replace as opposed to the gauge.

On my '92 300se, there's a diagnostic program for the instrument panel that allows you to check the gauges in the 4 quadrants of swing - would be handy if you had a similar test on your earlier model.

a quick test to dertimine if gauge works fully is to pull plug from sender unit at oil filter area turn on ignition switch gauge should go to full pressure if gauge works properly replace sender unit

Thanks, I will check it out.