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The joy of the oil pressure gauge

Posted by Jose Pinto 
September 02, 2014 08:59AM
Hi all!

My daily driver, the Mercedes Benz w124 250d has oil pressure gauge as standard.

The last oil service I got Castrol Edge Turbodiesel Synthetic 5w40 in it, about 5000km ago.
Before I bought it, the usual stuff was some 15w40 every once in a while... tongue sticking out smiley
Then I started to put 10w40 semi-synthetic and now this one, I found for sale at a good price. Overkill, I know.
There are no signs of oil usage, the drips and leaks below the engine are the usual, and I am quite happy with the engine sound and power.


The thing is: when I am cruising at low revs. the pressure gauge drops to just above 1Bar (the top of the scale is 3 Bar), and when at idle the needle jumps up and down like crazy.
My question is: the thinner oil, requires less pressure to complete it's path and passages along the engine or is the lower pressure preventing the oil from reaching some furthermost areas inside the engine? I am not familiar with the oil circuit inside the engine, and I'm not worried, just wondering.
The manual says 1 Bar is normal at idle.

I have no expensive turbo, EGR or soot filter to damage, or warranty to void, so no worries in this car.
:dance:

If I had no oil pressure gauge, i would never think about all this.
September 02, 2014 10:37PM
Since the oil pump is likely somewhat worn, it will have less ability to pump the oil as the thinner oil can leak back to inlet past the gears of the pump. On the good side, the thinner oil can flow through the passages easier but then it can pass through the bearings easier as well. Also the load carrying capacity of the light oil is less.

On an older engine I would prefer the heavier oil even though the higher viscosity does slightly lower the fuel economy.

Bob in Everett
September 03, 2014 12:45AM
But this oil is mostly thinner when it is cold, which is a good thing. At operating temperature, there is not all that much difference between them.
September 03, 2014 03:13PM
If I'm right, 1 bar is OK for these engines but should raise to 3 bar as soon as you blip the throttle. Oil pressure is important! Bearings and crank journals that have too little pressurised oil on them could push through the oil film and cause real damage. On a high mileage engine making longer runs at the time (I assume you mainly do longer runs) I wouldn't bother to put in low viscosity oil in, it won't bring you much advantage. When starting at really cold temperatures, just keep the load low for the first minute, so the oil gets pumped around before you slowly bring up the load while the engine heats up.
September 04, 2014 10:53AM
Quote
Michiel 318iS
If I'm right, 1 bar is OK for these engines but should raise to 3 bar as soon as you blip the throttle. Oil pressure is important! Bearings and crank journals that have too little pressurised oil on them could push through the oil film and cause real damage. On a high mileage engine making longer runs at the time (I assume you mainly do longer runs) I wouldn't bother to put in low viscosity oil in, it won't bring you much advantage. When starting at really cold temperatures, just keep the load low for the first minute, so the oil gets pumped around before you slowly bring up the load while the engine heats up.

That was my idea when I first bought the oil.
The advantage was having the Castrol Edge on sale, cheaper than Castrol Magnatec I was looking for.:eyes:
Like everything in life, the cheaper option seldom is the most adequate.
I don't get very low temperatures over here, and I drive 60km straight to work in the morning, that gets the engine to be warm for most of the abuse I give it.

I began to wonder about this when cruising at low revs and the gas floored, and the pressure started to creep down from the 3bar mark.
This engine was transplanted into this car few years ago, and little is known of it's history until that moment, except it was running in another ex-taxi. It pulls strong and sounds good, so I'm assuming all is fine. But the extra cleaning properties of this oil exquisite oil may upset old things, I never know.
Oil is more about religion than a science.
:boohoo:
Of course if I had no oil pressure oil gauge, I would never give this a thought.
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