Sudden loss of oil pressure - PeachParts Mercedes-Benz Forum

JCSC2 · #1 04-05-2009, 09:13 PM

I just got home after a 90 mile drive. I noticed once I got off the highway that my oil pressure was low. When I got to a stop light it was at 1.5 bars. Which some may consider normal but not for this car, it is usually at 2 bars at idle when warmed up. But when I accelrate it should be a full 3 bars pegged but it is not. It is right under 3 bars. I pulled over and looked underneath. I did not see any oil pouring out or anything abnormal. I drove another mile home. I checked the oil and it is full. So I am not sure what is wrong? Any ideas? I am deeply disturbed by these change of events!

Jeremy5848 · #2 04-05-2009, 09:19 PM

I can think of four possibilities:
1) failed oil sender (SD has electric gauge, right?)
2) failed gauge or wiring (voltage or ground)
3) internal failure bleeding oil back into the pan so the pressure cannot build up
4) partial oil pump failure
I would confirm that the oil pressure sender and gauge are telling the truth before doing anything serious

Palangi · #3 04-05-2009, 09:21 PM

5. Oil was hot after a long highway run. Pressure will be normal tomorrow.

JCSC2 · #4 04-05-2009, 09:36 PM

Thank you both for your input. I could only hope it returns to normal tomorrow. That has never happened before though. But could be. I will certainly take it slow before I do anything drastic as far as repairs go. The engine seems to be running fine but I don't want to keep running it, ya know if something is really wrong. Just spent lots of money on it recently!

jt20 · #5 04-05-2009, 09:46 PM

replace the O-rings at the bottom of the Oil filter cap tube

Skippy · #6 04-05-2009, 09:58 PM

Quote:

Originally Posted by jt20

replace the O-rings at the bottom of the Oil filter cap tube

Yes. I did mine on the 240 recently and the old ones were so brittle they broke when I took them off. Running oil pressure didn't change for me, but the buildup of oil pressure on startup improved a little.

tangofox007 · #7 04-05-2009, 11:49 PM

Quote:

Originally Posted by Skippy

Running oil pressure didn't change for me...

You would not know if it did. The actual oil pressure is easily twice what the gauge will indicate.

the heap · #8 04-06-2009, 01:01 AM

Normal operating pressure of these engines is ~5 bar at highway speeds. I've re-clocked the needle on the oil pressure gauge on my 617 NA engine so I can see the oil pressure at cruising speeds, I don't care what is says at idle (at warm idle, the needle indicates 0, zero). Cold idle is 5.5 bar, and it drops to slightly less than 5 bar at highway speeds after it warms up. So, there's lots more oil pressure in the engine than what you can see on the guage if you're running the stock set-up.

'77 300D 355kmi

yellit · #9 04-06-2009, 01:11 AM

You should test it with a real high quality oil pressure gauge for knowing true stats...or just install one for keeps....I did for peace of mind and as a backup gauge....
I always wondered why Benz put these little cheap gauges on its fine machines....

JCSC2 · #10 04-06-2009, 02:08 PM

Thank you all for your inputs. Hopefully it is nothing serious. The car didn't act strange or anything and it was full of oil. I wouldn't have even known there was a possible problem if the car didn't have a gauge. It may be something in the dash(or at least something minor) My glow plug light doesn't always function even though all my glow plugs are good. My mechanic says it could be a fault in the dash. I will mess with the car more when I get home tonight. I just had the car looked over inside and out and everything was good. But I guess shiza happens!

JonL · #11 04-06-2009, 02:43 PM

Plugged oil filter also a possibility.

JonL · #12 04-06-2009, 02:54 PM

Why does MB use the type of oil pressure gauge they do? I have a theory. I developed this theory when I owned a Peugeot 504D. The Peugeot uses a "thermal" voltmeter instead of a normal voltmeter. The gauge responds very slowly as it depends on the change in voltage across a heating element to move the needle. So... my theory:

MB and Peugeot do not want the owners to sweat every little aberration in the gauge reading. MB know that the actual operating pressure will vary depending on RPM, oil temperature, oil viscosity grade, time since last oil change, filter type, etc. They also know that none of that really matters as long as the pressure is over 3 bar at normal driving RPM. They also want the gauge to be sensitive enough to show that it is not zero at idle. So they pick a gauge that has a range that eliminates the inconsequential high pressure fluctuations (don't worry, be happy, you've got at least 3 bar) and shows the idle pressure (hey, it's almost halfway up the gauge... it's fine).

Peugeot didn't want the owner to sweat that with all the accessories on and the car at idle the voltage drops like a stone to 12V. Or that the needle bounces when you hit the horn.

I believe this is a sound approach. Most owners don't want to be hyper aware of what their oil pressure is at 3200 RPM on a 87 degree day after running at a steady 73 MPH for 47.5 minutes. Only us readers of this forum care about stuff like that! The gauge still shows if there is a real problem -- at idle or at speed.

nhdoc · #13 04-06-2009, 02:55 PM

Also, if this is your first drive in warmer weather that will thin oil and reduce pressures at idle too. You didn't say if you owned this car last summer or not, but if your only driving experience with it was during the winter you will now notice that in the summer the pressure at idle will be lower than it was in the winter and it is perfectly normal. 1.5 Bar is well above the spec for minimum oil pressure so I would not worry about it unless you see it dropping further.

Skippy · #14 04-06-2009, 03:10 PM

Quote:

Originally Posted by tangofox007

You would not know if it did. The actual oil pressure is easily twice what the gauge will indicate.

Touche. I was thinking of oil pressure at idle when I typed that, but as far as pressure at speed, you're right.

JCSC2 · #15 04-06-2009, 03:51 PM

Quote:

Originally Posted by nhdoc

Also, if this is your first drive in warmer weather that will thin oil and reduce pressures at idle too. You didn't say if you owned this car last summer or not, but if your only driving experience with it was during the winter you will now notice that in the summer the pressure at idle will be lower than it was in the winter and it is perfectly normal. 1.5 Bar is well above the spec for minimum oil pressure so I would not worry about it unless you see it dropping further.

Yeah, I've owned this car for a few years and it has been in my family since 91. I have never seen the gauge act like that in any type of weather, time driven, etc. So we shall see if it acts up again. I changed the oil less than a 1,000 miles ago. Using Rotella 15w40.