oil pressure question

[Joe]

My oil pressure has always pegged at idle, about 600 RPM. When I changed the oil from Quaker State 10-30 to Mobil-1 10-30, the oil pressure at idle changed to a point between 2 and 3. It pegs quickly with any revving, but I'm wondering just why the pressure is lower at idle.
An ancillary question: What is the relationship between oil weight and oil pressure? I.e., does thicker oil result in higher (or lower) oil pressure?
Joe Alexander is probably salivating at the chance to talk about this!
Joe Melton

[Cees Klumper]

I was noticing yesterday, changing my oil from regular to (a lower W value) semi-synthetic, that it is much thinner. Someone else commented before how the oil pressure will decrease at idle when going to synthetic, because it is thinner. It does make sense to me: letting water run through a hose is much easier than molasses. After this charge of semi-synthetic oil I will at the next oil change be moving to 100% synthetic, so it will become even thinner then. One advantage, besides better lubrication properties, is less friction so a more fuel-efficient engine (we can all use that).
Even on my newly rebuilt engine, oil pressure is not pegged at idle, and my idle is around 750-800. I don't recall anyone ever reporting that their oil pressure is pegged at idle, so if I were you I would not worry about it; so long as it pegs quickly at higher revs.
I suppose we could all check to see at what revs exactly (warm engine) the pressure pegs, by slowly increasing revs from idle.
If pressure is too low, it can be from a worn engine / oil pump, or a faulty pressure release valve?

Cees Klumper in Amsterdam
'69 white 280 SL automatic

[ja17]

Hello Joe,
Cees' observations are correct. Higher viscosity will increase pressure but not necessarily improve lubrication.

Most experts agree that most engine wear occures during cold starts. Thinner oils  will travel and lubricate much quicker when cold.

Following manufacturers specs on oil viscosity is safe. Become familiar with your oil pressure readings at different times. If you notice a steady decline in pressure over a period of time this may be your first warning of upcoming problems. If you jump around between oil types brands and viscosity you may never get a consistent reading or feeling for what is normal for your car.



Joe Alexander
Blacklick, Ohio

[knirk]

Cees, you wrote:

"If pressure is too low, it can be from a worn engine / oil pump, or a faulty pressure release valve?"

I think that the pressure release valve's purpose is to limit the maximum pressure. If you mean that due to a fault it opens at a very low pressure I don't think you would see a pressure rise with increased rpm.


Per G. Birkeland
69 280 sl aut 834
Norway

[Cees Klumper]

Well Per ... yes I was thinking exactly that, that a faulty pressure release valve could open too soon. That could still allow the pressure to rise from idle, but only to the point where the valve opens up - so allowing the pressure to build up, but not to where it's supposed to go. But all this is just theroy / conjecture, maybe in reality this valve could not really be the culprit. I would be interested in what others with more experience would think about this possibility.

Cees Klumper in Amsterdam
'69 white 280 SL automatic

[Benz Dr.]

Not really.
 Oil is measured at two different temperatures - 0 degrees C and 100 degrees C
Viscosity range is a little more complex than all that and you might be surprised to know that 20W50 is about the same viscosity as 80W90 gear lube. Thinner or lighter viscosity oil doesn't always translate into less oil pressure at normal operating temperatures. You really don't need a lot of oil pressure at idle anyway and as long as your gauge pegs out by 1,500 RPM you have enough pressure.
The pressure relief valve is a compression type spring and only opens when the oil is cold so the oil pump shaft isn't sheared off ( which they can and will do) Lighter or synthectic oils help in this regard by having lower pour points. The days of heavy thick oils is pretty much over with the advancements in oil chemistry available today. Many of the NASCAR guys are running 5w30 synthectics and these engines get a BIT harder use than a 113 does.

Daniel G Caron

[Cees Klumper]

Update - my oil pressure pegs at just about 1,100 rpm's - that's with sem-synthetic oil, 10 W 40, rebuilt engine including new oil pump.

Cees Klumper in Amsterdam
'69 white 280 SL automatic

[Ben]

My oil pressure pegs on the starter motor before it fires, using 15W40 Castrol mineral oil !!

It never drops, warm/hot blah blah ! !

Regards,
Ben in Ireland.
'64 230SL 4sp.
'03 CLK Kompressor

[Cees Klumper]

Addendum - let's report with the engine warm, because when the engine is cold it should peg even at idle, 750-800 RPM. But with the engine at operating temperature, the pressure is lower.

Cees Klumper in Amsterdam
'69 white 280 SL automatic

[knirk]

My oil pump must be the same batch as Ben's. The meter is always pegged even when the idle sometimes drops to 600 rpm. My engine is just rebuildt, but I didn't touch the oil pump. I use Carlube Tripple R Fully Synthetic 5w-40 oil. By the way, it was pegged before the rebuild as well.

Per G. Birkeland
69 280 sl aut 834
Norway

[Joe]

I've been thinking about oil pressure, and here is my understanding of what is happening. I hope any misperceptions will be corrected.

Oil travels through passageways inside the engine, providing lubrication to all moving metal parts. The oil pump pushes the oil through these "tunnels", and the oil pressure is a function of how difficult it is for the pump to push the oil.
Thicker oil (higher viscosity) will not flow as easily. This means the oil pump pushes harder, and this results in higher oil pressure. Thinner oil obviously flows easier, and less pressure is required to push it.
Once the oil gets to the metal interfaces, it fills spaces between the metal parts, so rather than metal contacting metal, the oil provides a film between the metal parts, thus preventing wear. If there is any grit in the oil, it will act as sandpaper at this point, resulting in the metal parts wearing.
If the oil is very thick, it will circulate slower, and this results in higher engine temperature. Lubrication at higher RPMs may be inadequate.
If the oil it too thin, the engine will run cooler, but the lubricating film may break down at higher temperatures.
This is a good web site to read if you want to know everything about engine oils. Some of the information is pretty technical, but the tables of data comparing major oils are interesting and helpful.
http://www.repairfaq.org/filipg/AUTO/F_oil_facts.html#OILFACTS_002
Joe

[pembash]

[/font=Arial
This topic has been going on endlessly without any conclusion. Is there no way to ask or refer to Daimler Benz as to what was the standard value of Oil pressure in 230-250 and 280SLs of the period!
Conflicting and divergent views are unnecessarily confusing!
Best
Bob

[Benz Dr.]

As long as you have at least .5 bar at hot idle and the gauge goes to full peg and stays there as you throttle up you're OK.
All oil gets thin once it warms up and can actually get thicker if you get it too hot. Oil pressure and viscosity aren't as much related as you might think. Once you have your engine fully warmed up the pressure guage will read about the same using 20W50 or 10W30. The oil pump is actually producing hydrolic pressure because you can't compress a fluid. When oil is cold is produces a lot of drag on all moving parts but as the engine warms up it's main function is to carry away heat. Lubrication is the smaller part of what oil does as nearly half of engine cooling is done by oil. A litre would be more than enough but would soon over heat and become burnt.


Daniel G Caron

[gwuisman]

This weekend I had a lot of driving with my 280sl: 600 km’s. A nice opportunity to monitor the behaviour of the oil pressure gauge at certain stages of warming up of the engine. I use Castrol protection 15W40. After starting the car cold the gauge pegs fully at idle. This did not change the first 10 km’s or so. Then the needle slowly drops between 35 and 45 (I have a US gauge) after 25-30 km’s. When the engine is totally warmed up after 100 km freeway at 120 km/hour the needle sinks a bit below 35 at idle and stayed there the next 100 km of the ride. Like the car of Cees the oil pressure is at all times at the max at around 1100 revs. These observation confirm that the pressure at idle differs according to the temperature of the oil and stabilises at the end when the engine is totally warmed up.  

Gerard Wuisman
1970 280sl man

[pembash]

I have the greatest respect for Dr Benz's expertise and am extremely happy to note that at hot idle .5 bar is ok as long as the guage hits the top on revving up. But what is required to have a clear cut position. Whether the earlier 230-250 SLs were manufactured to have their oil pressure pegged at idle and if subsequently the 280SLs (1968-71)were expected to have a hot idle below the mid-point of the guage!! I vividly recall my father's 1955 220, 1958 220S and 1959 220S Coupe --all having their oil pressure pegged full at hot idle. Is there something I need to be filled in for getting my perception cleared on it? Or may be it is an indicator of a worn out engine of my 1970 280SL with 140000 miles on it. Or is it a week oil pump or worn out main bearings!! Last year, I drove some 2300 miles in one go without any problem.
Will appreciate receiving advice.
Many thanks
Best
Bob

quote:Originally posted by Benz Dr.

As long as you have at least .5 bar at hot idle and the gauge goes to full peg and stays there as you throttle up you're OK.
All oil gets thin once it warms up and can actually get thicker if you get it too hot. Oil pressure and viscosity aren't as much related as you might think. Once you have your engine fully warmed up the pressure guage will read about the same using 20W50 or 10W30. The oil pump is actually producing hydrolic pressure because you can't compress a fluid. When oil is cold is produces a lot of drag on all moving parts but as the engine warms up it's main function is to carry away heat. Lubrication is the smaller part of what oil does as nearly half of engine cooling is done by oil. A litre would be more than enough but would soon over heat and become burnt.


Daniel G Caron

[Benz Dr.]

Let me try and clear this up a bit.

Full peg at cold idle.
More than .5 bar oil pressure at hot idle.
Full peg before 1,500 RPM with oil hot.
ALL GOOD

No oil pressure at idle.
Oil pressure stays low on throttle up.
2 bar pressure or less at 100KMH.
NOT GOOD!!!!!!

Possible causes are worn bearings, oil pump and plugged oil pump strainer. I found a piece of chain rail facing stuck in John Lieswyn's oil pump strainer and it pretty much destroyed the whole bottom end. At 140,000 miles you can expect a certain amount of wear and lower oil prssure readings.



Daniel G Caron

[Joe]

How reliable are the oil pressure guages in our cars?
Joe

[Cees Klumper]

That's a very good question. Is there a way to easily check with, say, a low-cost 'Pep Boys' external oil pressure gauge?

Cees Klumper in Amsterdam
'69 white 280 SL automatic

[pembash]

Many thanks, Doc.
Best Bob

quote:Originally posted by Benz Dr.

Let me try and clear this up a bit.

Full peg at cold idle.
More than .5 bar oil pressure at hot idle.
Full peg before 1,500 RPM with oil hot.
ALL GOOD

No oil pressure at idle.
Oil pressure stays low on throttle up.
2 bar pressure or less at 100KMH.
NOT GOOD!!!!!!

Possible causes are worn bearings, oil pump and plugged oil pump strainer. I found a piece of chain rail facing stuck in John Lieswyn's oil pump strainer and it pretty much destroyed the whole bottom end. At 140,000 miles you can expect a certain amount of wear and lower oil prssure readings.



Daniel G Caron