What is normal Idle speed/Oil pressure?

[hansr433]

Here is a photo of my oil pressure and tach.  Engine is warmed up and at idle.  1000rpm and pressure at roughly 40 psi (not sure what scale is used)  Does this look normal for a M187 engine?

[Benz Dr.]

You're fine.

[hansr433]

Thank you Doc, good to know. 

[wwheeler]

My '68 280SE has full oil PSI when cold and at idle. Only when it is fully warmed up, does the oil psi start dropping a smidge on the gauge when at idle. How many miles do you have on the engine?

I agree with the Dr., I don't think it is anything to worry about.

[lpeterssen]

Dear Wallace

That behaviour of dropping oil pressure when engine is idling and warm up at the same time is according to my mechanic due to low oil viscosity.  And that from my engineer point of view has some true on it. Oils as we all know, drop their viscosity when hot, and for that reason their formulas incorporate viscosity correctors to keep viscosity under certain parameters.  When engine oil gets old viscosity corrector additives loose their properties, and oils start to behave has they had no oil viscosity correctors at all.  For example an oil 10w 30 has a viscosity like an oil with SAE10 viscosity at 0 degrees Celsius.  And 10 SAE graded oil at 90 degrees Celsius has  very low viscosity which is not good for classic MB engines.  The viscosity correctors increase viscosity and make the oil behave like an SAE 30 oil when hot (90 degrees Celsius).  That is why when oil is old and degraded we hav the tendency to see a very low oil pressure on the instruments cluster gauge, since viscosity correction is gone and the oil becomes very very light, at a point that it does not lubricate at all this old engines.

These MB engines FROM 60’ are designed to handle very thick engine oil.   In the past there were no Multitrade oils as we have today. Nowadays there is the tendency to use oils as 15w-40 or thinner (which is a SAE15 oil with viscosity modifiers to make it behave like a SAE40 when hot).

 My mechanic likes to use monograde oil viscosity SAE50 on my Mercedes. With this thick engine oils, engine oil pressure tends to behave much better when car has warmed up completely. And as we do not have freezing temperatures down here in Venezuela it is not important to have a low viscosity at cold.

Regards
L.peterrssen

[wwheeler]

I agree with hot oil losing viscosity and therefore pressure. I think it is common for these engines though to go below pegged on the oil gauge when fully warm. I run 20W-50 VR1 which is about as thick as I dare run. In colder months, it takes longer for heavier oil to flow. That can create more of a wear issue while the engine is running cold. There is no perfect made-made situation. It is always a compromise to achieve what you think is best for your situation.

The reason I asked about the mileage is because older engines will have increased main bearing wear and therefore lower oil pressure at all temps. 

[Desertpagoda]

For  an interesting read, go to Bob is the oil guy.com and read all there is about oil/visco/pressure and their relationships. Counter intuitively to me I learned that higher oil pressure indicated a resistance to flow and reduced/slowed lubrication not an improvement. This is why modern oils are often 5 or even zero! Also, check out the section on   dyno vs synth oil and about filter performance. You could send hours there!