Bonkers Oil Pressure Gauge- W124

[chazola]

The oil pressure gauge on my W124 has started acting up when the engine is warm (after 1/2hr- 1 hour or so). At idle, in drive or in R, 3 or 2, the pressure bounces around 1. Doesn't drop below 1 though. B4 today it was always around 2. If i shift into N or P it goes back up to 2. the RPM also drops to around 500 when the pressure is at 1, then back up to around 7-800 in N or P.

At normal driving the meter is pegged back on 3.

oddly, it started this just after i'd just filled up with gas. I was getting the surging idle a couple of days ago, again when the engine was warm, which seem to stop today.

Last oil change was @ 2000 miles ago, at dealership.
I have pulled the R16 resistor trick yesterday, gonna put it back it when it stops raining.

91 300E 192k with leaky front cover.

any thoughts?

[G-Benz]

I'm not sure how long you've owned your car or when you started paying attention to the oil pressure gauge, but what you've just described the oil pressure gauge functioning properly.

The surging idle is a different problem altogether, and is usually the result of a vacuum leak. The oil pressure gauge just responds in kind to what the engine is doing. Mine has the surge problem, and the needle bounces along.

Unlike some other makes where the gauge needles just leap to some stasis point and remain there, MB gauges actually "tell" you something.

Here is what you should see (on average).

When car is cold (and oil is cold and thick), pressure gauge should reach "2" or above.

After warming up, it will go down to as low as "1" on idle.

Pressure reads between "2" and "3" during easy cruising and pegs to "3" during acceleration.

If you have a REAL oil pressure problem, your readings would be "1" or lower ALL of the time.

BTW, the measurement is in bar...

[chazola]

thanks for the speedy reply.
I've owned the car for only a few weeks and I pay (almost!) constant attention to my pressure gauge. This is my first Merc though, so it's hard to know what to expect or to relate to, as the manual doesn't give you many pointers.
The reason I posted this thread is that this was the first time i'd noticed it going down to 1, probably because I've done only short trips/highway driving with it up until now. i.e., from what you say it would either be at 2 with the cold oil, or pegged at 3 on the highway.
I believe the numbers correspond to 15 psi or so steps?

[G-Benz]

According to Google:

1 bar = 14.5037738 pounds per square inch

And watch your temp gauge needle too. It will surprise you as well (especially during the hot summer months)...