Thoughts on the oil pressure guage

[Mistel]

I seems odd to me that the oil pressure is pegged at the limit when the car is driving. Is the idea that it does not matter how high the pressure is when driving, it is only important that you have pressure? Everything on these cars seems well thought out but it seems odd that a gauge gets pegged in normal use?

I always thought that was a little strange too. From a design point of view, they must not consider the actual pressure at high rpm to be critical (as long as it's greater than 3 bars), instead they ranged the gauge so the normal hot idle pressure is mid-range. It does seem to have some logic, although it does look funny.

[TX76513]

It probably falls off to about 2 bar at idle, when hot, right? I have one 126 that just started going beyond the 3 bar when cold or driving - don't know what that's all about.

[Number_Cruncher]

>>they must not consider the actual pressure at high rpm to be critical

For the hydrodynamic bearings in the crank, this is definitely true - once the bearings are turning quickly, you only need oil flow into the bearing, the pressure at the closest point between shaft and bearing is created solely by the rotation, and is much higher than the bearing inlet pressure.

For the piston/bore lubrication, you only need enough pressure for any upwards pointing jets to hit the pison underside, and for the intermittent loading of the cam, you just need to have plenty of oil being swept along by the cam.

That the pressure does drop a bit at idle when the engine is hot suggests that MB have sized the oil pump absolutely correctly - there's still enough pressure at hot tickover. Under all other conditions, lots of oil will flow straight back to the sump from the pressure relief valve, and this represents wasted engine power.

[Chas H]

If the actual oil pressure were displayed, drivers would be constantly worrying and bothering Benz about it.

[bustedbenz]

It also would have made more sense to me to use a gauge that displayed the correct reading... even if the correct reading was NEAR the top of the gauge, it does not make any sense at all to me from a design point of view that the gauge in fact tops out very low on the actual pressure range.

I consider it analogous to designing a fuel gauge that goes from E to 1/4.

Originally Posted by bustedbenz

It also would have made more sense to me to use a gauge that displayed the correct reading... even if the correct reading was NEAR the top of the gauge, it does not make any sense at all to me from a design point of view that the gauge in fact tops out very low on the actual pressure range.

I consider it analogous to designing a fuel gauge that goes from E to 1/4.

Well, it's just a design choice. You normally specify analog instruments to keep the parameter of interest near the center and still have sufficient resolution to read that parameter accurately. IMO, a full range instrument (maybe 8 or 10 bars) would not have enough resolution to see degradation at idle. If this was an industrial application there would probably be two gauges, one full range and one narrow range.

[toomany MBZ]

This info is from the boys at Click and Clack, as I recall. The pressure at high rpm itself is not as big a deal as how quickly the pressure builds after the engine is warm, and the gauge drops at idle, then goes to full when you hit the go pedal.
I also understand if you are seeing the gauge is dropping, you're too late to save the engine.

[babymog]

BMW had this problem in the late '80s, they had a full-range oil pressure gauge.

Once guys in the clubs started comparing oil pressures, the dealers started to get lots of complaints from people below the top 10% of oil pressure that their cars were somehow not built properly. BMW removed the numbers from the scale to solve it.

The M-B approach avoids this, all of their cars read full-scale, no problem.

It is IMO a good idea, even though many of us would prefer the full-scale calibrated gauge, it saves M-B a lot of hassles.

Since many carmakers have "gauges" that are driven by an on/off sender, and either read in the high end of the scale or off, at least M-B gives us a real gauge.

[Jeremy5848]

It also means that, as long as the gauge is pinned high (and therefore the needle is not moving) you're OK. Should you see the needle move (other than hot idle) you want to pay attention and react quickly. A blinky red light and/or buzzer would be even better (we've discussed such a mod before).

[Chas H]

Benz also gives many/most/all of us a low oil level light, which to me is just as important, maybe more, than a pressure gauge. If an engine suffers a quick loss of oil pressure, the gauge indicates it's too late to save the engine. That loss of oil pressure normally comes from a lack of oil in the pan.

Originally Posted by Chas H

Benz also gives many/most/all of us a low oil level light, which to me is just as important, maybe more, than a pressure gauge. If an engine suffers a quick loss of oil pressure, the gauge indicates it's too late to save the engine. That loss of oil pressure normally comes from a lack of oil in the pan.

My gauge saved me from a failing oil pump a few months ago. It was pegged at 3 bars for about 300 miles of highway driving. When I pulled of an exit and stopped, the gauge dropped to 0, instead of 1.5 or 2. I shut it off and called a tow truck.

[Chas H]

Originally Posted by Craig

My gauge saved me from a failing oil pump a few months ago. It was pegged at 3 bars for about 300 miles of highway driving. When I pulled of an exit and stopped, the gauge dropped to 0, instead of 1.5 or 2. I shut it off and called a tow truck.

And then what? Was the pump failed?

Originally Posted by Chas H

And then what? Was the pump failed?

Yup, both the pump and the pump chain were in very tough shape, I'm lucky that it didn't fail completely at highway speed.

[Chas H]

You are very lucky. Benz pumps are usually trouble free. What model was it?