190e oil pressure - is this normal?

This is a '91 190e 2.6 - I'm new to the car.

The oil pressure gauge goes all the way to 3 and stays there when accelerating hard or at highway speeds. At idle it floats around 1. Is this normal behavior for these cars? I'm always curious when I see a gauge maxed out like that.

Thx
DMS

[G-Benz]

MB gauges are more "dynamic" than you would expect for other vehicles. Same for the temp gauge.

When your engine is cold or when you are accelerating, you should see your needle peg to the "3" position. When idling warm or under very low loads, generally under "3" or "2".

"1" or lower may mean that your oil level is a bit low. If that's not the case, you may need to investigate further as to what is causing your low oil pressure condition.

[Mel Weaver]

Yes-check oil.
Mine does this if I neglect oil change schedule. Try changing it (and filter) if you're over due.
What weight are you using? Not to start a post on weights- I've been using 10/40 every 2500-3000 miles.
-M

[bobbyv]

oil pressure is proportional to engine speed (not "acceleration") and oil viscosity. Oil viscosity depends on the viscosity rating (e.g. 15W40), and which decreases with temperature.

therefore, the oil pressure rises when you rev the engine. The higher oil pressures at higher rpm are necessary to support the increased loads on the engine's journal bearings - hence the observation of a maxed-out reading during elevated rpm. If oil pressure remained low at high engine load, you could have metal-to-metal contact within the journal bearings.

this is also one reason not to "lug" the engine, i.e., run it in a high gear at a low rpm (this applies to those with manual transmissions), since this is a high-load, low oil-pressure condition, which may result in accelerated engine wear.

the oil pressure gage will also display a higher reading at idle in winter than in summer, because of the higher viscosity of the cooler oil in winter.

oil viscosity degrades as it is used, and you can expect lower than normal readings at idle.

the oil pressure gage is calibrated in "bar", where 1 bar = 1 atmosphere = 14.7psi.

the 190e2.6 owner's manual states that a reading of 0.3bar (4.4psi) at idle when the engine is at operating temperature, "will not jeopardize its operational reliability".

the oil pressure gage is your friend ...

[Duke2.6]

Normal idle oil pressure is usually in the range of 1.0 to 1.5 bar. As the revs increase to the 1500 to 2000 range it should peg at 3 bar and stay there until the engine returns to idle speed. Idle oil pressure is a function of oil viscosity and oil temperature. As stated, the owner's manual says as little as 0.3 bar is acceptable, but I've never seen much below 1.5 at idle with 20W-50 oil in mild ambient temperatures, maybe 1.2 in hot weather.

Automotive oil pumps are known as "constant volume pumps" and if their output is dumped into a bucket the delivery volume would be approximately proportional to engine speed. Since the bearings and other metered oil flow inside the engine constitute a restiction, pressure will rise with delivery volume, so all oil pumps have pressure relieve valves that open at a predeterminied pressure, and as this point the oil flow into the main gallery remains fairly constant as it is driven by a constant pressure against a fairly fixed restriction. The extra delivery volume is usually routed back to the suction side of the pump.

If there was no pressure relief valve, the pressure would continue to rise until the oil pump became overstressed at which point something would break!

Actual hydrodynamic pressure in bearings peaks in the range of several thousand psi. The feed pressure is there to ensure that the bearings receive a flow of fresh oil when the hydro pressure at the oil feed is zero, which always occurs at some point in the rotational cycle.

Duke

P.S. One bar is approximately equal to 14.5 psi, not 14.7. The "bar" is not defined as standard sea level pressure.

[blackmercedes]

According to MB, normal hot idle oil pressure can easily be as low as .3 bar. Our now-sold 2.6 often had around .5 bar at idle (hot) and at nearly 400K it's engine is still tight and strong. (I still service it as my Dad has it)

If I understand correctly then, the oil pressure gauge readings I'm experiencing are within the norm. I'll double check the oil level - tho it was fine last I looked. The PO said its using 10W-40, which is acceptable.