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I have a 1982 Mercedes W123 with a 2L gas engine. I have noticed some weirdness with the oil pressure and want to know if this is an indication of a problem.

Let's say I am driving at 60km/h and the engine is hot. The oil pressure sits somewhere between 2 and 3. If I step on the gas, the oil pressure immediately decreases a bit (a noticeable move of the needle, though since I am driving at the time I cannot look at it for longer to determine how much exactly it moved) and then increases as the engine RPM increases. If I release the gas pedal, the oil pressure immediately increases, then decreases as the car slows down.

I I not think I have noticed it before. Is this an indication of some problem or just normal?

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  • I thought it might have been something up with the pickup not being in the oil at all time. This would be something like the pickup had dropped from the oil pump, with the pump only pumping as the oil level was actually high enough for the pump to suck oil (and not air). The problem with this theory is the pressure increasing as the engine speed increases. Usually in this scenario, as the engine speed increases, oil pressure will drop off almost completely. I'm wondering if there could actually be a problem with the pressure sensor itself?
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Apr 16, 2015 at 15:12
  • The pressure seems to drop when the engine is working harder to speed up and increase when the engine is being turned by the wheels. The pressure gauge (there is no separate sensor, just an oil hose to the gauge) could be faulty, but why would it act like that?
    – Pentium100
    Commented Apr 16, 2015 at 19:46
  • You are saying there is a tap into the oil galley which feeds pressure directly to the gauge in the dash? How long has it been since your last oil change? Did you by chance use a different oil or oil filter than you normally would use on your last oil change? How many miles are on the vehicle?
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Apr 16, 2015 at 20:22
  • Yes, and if the hose isn't attached correctly, oil drips near the pedals. I actually started noticing this after the last oil change (which included a filter change), I trust the mechanic used appropriate oil and filter. The odometer has been replaced several times so nobody knows the true distance the car has traveled, but it is most likely a lot.
    – Pentium100
    Commented Apr 16, 2015 at 20:36
  • Could the increased force on the main bearings cause this perhaps? It's an interesting question =)
    – Allman
    Commented Jun 8, 2015 at 13:56

1 Answer 1

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I think this is because of the main bearings. The oil enters from above into the bearings. A worn bearing will cause the gap between the axle and the bearing to be larger, and more oil will escape, which leads to lower pressure.

Main bearing oil canal

This effect is bigger when you press the accelerator because the crankshaft will then be affected by a downward force from the pistons. This force will make the gap bigger on the upper side of the bearing where the oil enters and more oil will escape. This should be valid on inline and V-engines. I suppose boxers would not show the same symptoms.

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  • I think you're on the right track here. Generally rod up/down play is associated with a knocking sound (rod knock) and will quickly develop to catastrophic failure. I think this is thrust bearing wear and the crank is moving beyond tolerances. Under acceleration a crankshaft moves forward, and when coasting it moves backward. Excessive thrust bearing wear will cause it to move beyond spec and oil spills out the sides of main bearings. If the oil pressure change is noticeable then it's likely that insufficient oil is reaching the cam or upper engine components.
    – elmerfud
    Commented Jul 9, 2015 at 14:26

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