Oil Pressure Fluxuates--Should I Change Viscosity?

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Originally Posted By: Brit33
Maybe we need to be concerned with Mercedes_220 oil pressure at higher revs.

Can you tell us what the readings are at high revs?

If I'm not mistaken, the Merc' gauge is pegged at about 3 bar (45 psi) which is the maximum OP our new member mentioned seeing when the engine is cold. So the gauge is really only useful for measuring idle OP.
 
Typical Mercedes behavior is about 1 BAR (15psi) at hot idle and pegged at 3 BAR (45 psi) while driving. It's normal and expected to see gauge movement during operation. The lower limit is specific to engine type (some are as low as .5 Bar) and I don't know what it is for your engine.

If you see a 0 reading on the gauge with the engine running, shut it down immediately. If the gauge needle drops off the upper limit with engine RPM above about 1500, you're heading for a problem.

This car has a direct pressure reading gauge. There's an oil line running from the engine compartment to a fitting on the back of the gauge and the gauge is pure mechanical in operation.
If it says you have pressure, you do. If not, either the line is disconnected (with oil spraying everywhere) or there really is no pressure.
 
Thanks mahansm.
So my memory isn't failing me yet about the 3 bar limit.
If 1 bar is typical then it sounds like the engine is in good shape dispite it's age with 2 bar of OP.
The low VI heavy RL is undoubtedly boosting his OP.
I'd suggest RL 0W-30 next time.
 
Originally Posted By: Brit33
Quote:
Yes, but manufacturers do not recommend an oil pressure -- they only recommend an oil viscosity


totally untrue.

Alfa Romeo specify oil pressure for idle and max revs.


Your statement and Gokhan's are both true -- and at the same time. You are correct: nearly all manufacturers specify a minimum and sometimes a maximum oil pressure at one or multiple points in the engine speed range. And Gokhan is correct: the owner's manual will almost never say, "ensure you use an oil that results in a hot oil pressure of 10 psi at 1,000 rpm".

I said "almost never" because I suppose there may be an outlier somewhere, but in the vast majority of situations, the manufacturer recommends a GRADE of oil. They do also specify the oil PRESSURE in service-related information so technicians can diagnose the lubrication system if there is a problem, but this is rarely, if ever, a recommendation made to an owner.

There is actually one correction to what Gokhan said: manufacturers don't recommend oil viscosities, they recommend oil grades. A viscosity is "10.5 cSt", and often given at a certain temperature, such as "at 100 deg C". A grade is "10W-30".
 
Originally Posted By: Hokiefyd
Originally Posted By: Brit33
Quote:
Yes, but manufacturers do not recommend an oil pressure -- they only recommend an oil viscosity


totally untrue.

Alfa Romeo specify oil pressure for idle and max revs.


Your statement and Gokhan's are both true -- and at the same time. You are correct: nearly all manufacturers specify a minimum and sometimes a maximum oil pressure at one or multiple points in the engine speed range. And Gokhan is correct: the owner's manual will almost never say, "ensure you use an oil that results in a hot oil pressure of 10 psi at 1,000 rpm".

I said "almost never" because I suppose there may be an outlier somewhere, but in the vast majority of situations, the manufacturer recommends a GRADE of oil. They do also specify the oil PRESSURE in service-related information so technicians can diagnose the lubrication system if there is a problem, but this is rarely, if ever, a recommendation made to an owner.

There is actually one correction to what Gokhan said: manufacturers don't recommend oil viscosities, they recommend oil grades. A viscosity is "10.5 cSt", and often given at a certain temperature, such as "at 100 deg C". A grade is "10W-30".

Thanks, that's exactly what I was trying to say!

Certainly, I meant SAE viscosity grade by the viscosity recommendation.

SAE viscosity grade specifies a range for the 0 C and 40 C kinematic (low-shear) viscosities and imposes a lower limit on the HTHS (high-temperature [150 C], high-shear) viscosity, where the oil flows through microscopic gaps between sliding parts (high shear) at high temperature. The low-shear (kinematic) viscosity, where the oil flows through macroscopic openings, is the pumping viscosity, which determines the oil flow and pressure. However, these days some manufacturers make an implicit yet more restrictive recommendation on the HTHS viscosity as well, by specifying a specific ACEA Ax/By category or the API Cx category, which impose additional lower and upper limits on the HTHS viscosity.

The two SAE viscosity numbers xW-y dictate oil flow and pressure in cold and hot engine, respectively, whereas the HTHS viscosity dictates the minimum oil-film thickness (MOFT), which in turn dictates bulk of the fuel economy (inversely proportional to HTHS viscosity) and protection against wear caused by high load (high vehicle speed, towing, or climbing), low RPM, and/or dirty oil (directly proportional to HTHS viscosity).
 
Hi Everyone,

Thanks for these great responses. You guys put my worries at ease, for the most part. I suspect the previous oil was a higher viscosity; the oil pressure gauge never left the 45psi mark until I changed it to 10w30.

I swapped the oil filter, so I don't think I have any resistance there, it's installed correctly.

One thing I will mention, though I doubt it's of much concern: when I am idling the gauge bounces back and forth between about a 5psi range fairly rhythmically (30-35 or so). I'm not sure if this could tell me anything about the oil pump or gauge sensor?

So does everyone pretty much agree that 10w-30 should be fine to run in this car? Older engine but still very tight.

I'll check the manual tonight for the specs on oil pressure and will post whatever I find out.

Thanks again for the knowledgeable advice!
 
Quote:
It could bust your seals or oil pump.

How? I didn't know oil pressure was directed at the seals?
My Honda DOHC water cooled 16v Honda had 130 PSI @2000 RPM and didn't loose any gaskets or seals in 20 years and 300K.
Now why would Honda have such high oil pressure?

Run 30 PSI in that engine and you you wont have an engine very long.
Quote:
Actually the analogy between the blood circulatory system and the engine lubrication system is quite good in many ways.

Actually it sorta sucks. You cant compare fragile flexible veins and arteries to oil passages out of metal or a human muscle to a metal pump.
Lighter oil is not going to clean better either just because its thinner, its the add pack that does that.
Seals and gaskets are more likely to be damaged because of excessive crankcase vacuum created by a poorly functioning PCV system.

Low oil pressure can certainly have a very negative effect on engines. Hydraulic tensioners, cam phasers, lash adjusters/lifters depend on pressure to function properly and can actually seal better with higher pressures.

Tell me which seals are under full oil pressure?
It's true some crank mounted oil pumps use o-rings but the Viton 90 material is rated at 1,000 psi and the ring is captured.

I asked for documentation not an explanation or your opinion.
 
I forgot to add. The Honda engine has a 13,000 redline so that falls in line with 10 psi for every 1,000 RPM.
I agree you wouldn't need 130 psi in a 6,000 RPM redline grocery getter but 60psi isn't anywhere near over pressure either.

I rather have 60 psi than 30 psi any day.
 
From the service manual:

“At operating temperature, oil pressure may drop to 0.5 atü at idle speed. Upon acceleration, the oil pressure should increase immediately and should attain at least 3 atü at 3000/min.”
Atü= Atmosphäre Überdruck (atm.)

The oil gauge pegs at about 3.0bar, the actual pressure at high revs is higher…

Two types of oil pressure relief valves are used, depending on the variant of M115 engine(your 220/8 has the M115.920):
-5 bar opening pressure (screwed on the engine block front side)

-or 8 bar, on the oil pump



…about the oil viscosity, SAE xw-40 HDEO or ACEA A3/B3/B4 would be more appropriate.
M115recommendations.jpg


The bouncing of the gauge is not normal IMHO...the arrow on mine was always "calm"(om621,w110),at hot idle it would not drop below 1,2 bar with HDEO 15w-40.I vaguely remember a service note from MB about installing some kind of a damper in the oil line to the gauge in cases of "bouncing" gauges,but cannot remember which models were concerned...Have it checked with an oil pressure tester...
 
Last edited:
Rollins,

Thank you. This is very helpful. My manual has a similar chart that just shows SAE 30 for temps above freezing. And it sounds like Redline's 30s are on the thick side. Maybe I'll see how it does over the winter and move it up to 40s in spring. Yesterday was probably the last warm day we'll have for a while.

Thanks for the tip on the jumping gauge. It's more of an eyesore than anything. It responds quickly anytime I hit the throttle, so I'm assuming it's just a benign symptom of the mechanical gauge.
 
Yes, a mechanical gauge that is not damped will show minor fluctuations in pressure if the idle is not perfectly smooth. The mechanical gauge that I installed in my WRX would bounce a few lbs. in time to the lope of the flat four idle.

Ed
 
Cold the gauge is pegged or almost and drops to about 1.5 bar hot? That seems normal for these engines. Some old diesels went way down but i don't remember that with the gas engines.

Are there any coils in the metal oil pressure gauge line? I remember seeing some with 3 or 4 coils to dampen the gauge.
 
I usually think of coils in the line providing protection from breakage due to vibration. A fitting with a small orifice will act as a damper.

It's been a lot of years since I owned a MB(280SE 4.5), so the details are a bit fuzzy.

Ed
 
Trav,

Cold the bar is pegged and never moves. Only after being thoroughly warmed up does it drop to 1.5 or 2 bars at idle.

My concern is that this never used to occur with the car. I've only owned it for a month though. I assume it had a summer oil in it prior--probably 20w-50 after speaking to the last owner.

My main concern was whether I should be aiming to have the gauge pegged all the time, idle or not. The last owner, who seems to really know the car inside and out, seems to be of this opinion.
 
Originally Posted By: Mercedes_220_8
Trav,

Cold the bar is pegged and never moves. Only after being thoroughly warmed up does it drop to 1.5 or 2 bars at idle.

My concern is that this never used to occur with the car. I've only owned it for a month though. I assume it had a summer oil in it prior--probably 20w-50 after speaking to the last owner.

My main concern was whether I should be aiming to have the gauge pegged all the time, idle or not. The last owner, who seems to really know the car inside and out, seems to be of this opinion.

I am not really sure what your concern is. According to Rollins' post, the hot-idle oil gauge pressure could drop to as low as 7 psi (half the atmospheric pressure) in your engine. Oil pressure is directly proportional to oil viscosity and it will be much more with cold oil. It will also be much lower at idle, especially when you are idling with the transmission in drive.

The pressure fluctuation could simply be caused by rough idle.

According to Rollins' post, xW-40 is the recommended SAE viscosity grade for your engine. If you don't have oil consumption, Mobil 1 0W-40 would be an excellent choice. If you have oil consumption, mainly for cost reasons, go with a conventional 15W-40 or a fully synthetic 5W-40 in cold months. 5W-40 and 15W-40 HDEOs are more tailored toward large truck engines but will still work in your gas engine. M1 0W-40 is more tailored toward gas and light-duty diesel engines.
 
Originally Posted By: edhackett
usually think of coils in the line providing protection from breakage due to vibration

That true but it also reduces "minor" fluctuations without impeding fast gauge responce.
Your right if its really bouncing like a carb synchronizer you have to either use a restrictor or valve to control it.
Originally Posted By: Mercedes_220_8
Cold the bar is pegged and never moves. Only after being thoroughly warmed up does it drop to 1.5 or 2 bars at idle.

From what i have seen this is totally normal. I have worked on a lot of these over the years and never saw one stay pegged at idle hot.
If you think about it if that gauge was supposed to stay pegged all the time then there would be no point in having one.
They could just use a Ford style on/off gauge or idiot light.
1.5-2.0 is good pressure at hot idle.
I wouldn't sweat this engine or the oil pressure, it sounds fine.
 
Originally Posted By: Trav

Tell me which seals are under full oil pressure?
It's true some crank mounted oil pumps use o-rings but the Viton 90 material is rated at 1,000 psi and the ring is captured.

I asked for documentation not an explanation or your opinion.


Yeah I want to hear this one as well...
 
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