Hello!

A few days after my idle came back and became fine (thread found here ) I've experienced a Sudden loss in oil pressure at idle (appr 700 rpm), the oil pressure at warm engine idle (83 C) is about 1 Bar (1½ week ago it was 2 bar)... Could this be due to oil dilution by gasoline? I mean this is possible if the mixture is to rich before closed cycle - right??? Or might there be another problem? The EHA current was changing around -3mA the last time i checked it (indicating slightly rich mixture).

The oil pressure readings are fine at warm up and at higher rpm. the oil was exchanged this winter. The viskosity of the oil is 10W-40.
The car is a 300E -88 with 3 litre m103.983-engine with (most probably) 250000km on it, the CO readings at warm engine was 0.0 last thursday HC was approx 100 (ppm?). The low oil pressure started at thursay or friday, Im not sure exactly...

Thats what I know. Thanks in advance! :-)

I have to bring this up again, in the last days the idle oil pressure have decreased slowly and today it was 0 at the instrument, and i had to go to 200rpm to have 3 bar.... I'm getting seriously worried. The things happen so fast and I don't know what it is that is happening (!).

1. Can the oil pump have gone bad?
2. Can the measuring device have gone bad?
3. Can it be that the oil (changed it in the late winter of 2004) is going bad already.
4. Something else.


Please give me an advice, or shall I just hand the car over to somebody that could measure the "real" pressure??

If it were mine I would do exactly that - but make sure the somebody is a MB specialist.
It may be a sensor/instrument fault but it is not worth taking the risk.
However, at your mileage you will be unlucky if you have bottom end problems.
Good luck and let us know how it pans out.

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NormanB
230 TE (W124) 1989 with 153,000 miles on the clock - hoping for at least another 100K

Have it checked with a mechanical gauge. Our 190E did the exact same thing, where the pressure would show zero at idle and then rise 3bar by 2000rpm. It was the sending unit.

If you really have zero pressure at idle, you can do a couple "at home" checks until you get the car to a shop...

1. Listen to it running. If there is zero pressure, you'll probably have valve lifter clatter. MB's lifters don't do well with no pressure and will likely be noisy.

2. With it running, check under the valve cover. See if there is a good supply of oil coming up into the valve train. The valve train is the fist place starved by low pressure.

With our 190E, there was no lifter noise and oil was in good supply in the top end, so I drove it to the shop for a pressure check. Turns out the pressure was fine. I haven't replaced the sedning unit yet, but will get to it this summer sometime.

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John Shellenberg
1998 C230 "Black Betty" 240K

http://img31.exs.cx/img31/4050/tophat6.gif

Yes, I've listed to the enigne and it idles very fine with no signs of "oilstarving" valves... Iä've talked to a mechanic, and he would try to fit e in on thursday, if he says the pressure is ok then I'll wait until next month and then change the sending unit.... Thanks for all of your advice! :-)

If you had gas in your oil you should be able to smell it on the dip-stick. Fluctuating at -3ma would indicate that you are at lambda and slightly rich in open loop which should be perfect.

As others have suggested, get it checked out against a good mechanical oil pressure guage immediately. Low oil pressure should make your lifters clatter but even if they didn't I wouldn't take a chance on driving it until I had the pressure checked out.

Sundman: I just had the same problem, and replaced the oil pressure sensor and went to 20w 50w and the pressure is now at 1.3 in D AND 2.5 In N after engine is running at full run. temp.

Oh boy,,,,, this subject has been beaten to death here. I went through the same thing exactly. At idle on an auxillary gauge, oil pressure was around 5 lbs at idle. After putting a new oil pump in, nothing really changed. I put in a new sending unit and amazingly the problem went away,,,, never below 2 bar at idle when warm. This is what I and my mechanic think,,,, Mercedes found a way with the new sending units to keep the oil pressure looking like it's high at idle just to help your peace of mind. Nothing is really different, but I do feel better now.

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95 SL500 Smoke Silver, Parchment 64K
07 E350 4matic Station Wagon White 34K
02 E320 4Matic Silver/grey 80K
05 F150 Silver 44K

Actually, 5 psi at hot idle is quite acceptable oil pressure. MB specs call for down to .3bar, which is just over 4psi.

I always thought the pressure sending unit was either broken or not. The fact that my pressure went right to 3bar by 2K-rpm made me think the sending unit was fine. It was (is) not. It's common for the sending unit to break in a fashion where idle pressure reads dead zero, but otherwise it seems mostly normal.

BUT! I wouldn't jump to the conclusion that the oil pressure sending unit is dead until you confirm. Check it, and as long as hot idel pressure is 4psi or better and it pegs at 45psi or better at above 2000rpm, you're good to go.

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John Shellenberg
1998 C230 "Black Betty" 240K

http://img31.exs.cx/img31/4050/tophat6.gif

The problem is almost always the sending unit. Yours is most is likely original and 16 years old. They are cheap ($40?) and easy to put in. To me that is the first step - this is one of those things that easier to replace than to diagnose - especially when you don't have the equipment to diagnose.

I just noticed today after replacing the oil sensor that at idel the needel on the guage moves from 1 to 1.5 back and forth. Don't know what that is. RPMs remain steady.

Hello everybody.

I'm getting slowly into a better mood now, since I've learned that the problem is most probably not the oil pump but the sender.

I have found a shop that actually could check the pressure (had the right equipment and TIME) for me tomorrow (thursday) so after that I will know wheter I need a new oil pump (expensive with all the work involved) or if i just need to replace the sender unit...

Again thanks for all the tips, I'll let you guys know what the problem was as soon as they checked the pressure for me! :-)

Yes, now I'm back! ;-) the pressure was checked at a shop and then the sender to the gauge was replaced, now the readings are ok! :-)

I just had mine replaced on my 1991 300TE.
Same exact symptoms: zero oil pressure at idle, would begin to rise back up to 3 bar above 1400 rpm or so.
It really bothered me that my oil pressure guage didn't work.
The sending unit cost $68.31 from Partsshop.
Now I have about 2 bar at idle when cold, after the engine warms up I'm still above 1 bar all the time at idle. (Using Mobil 1 15W50; was running 0W40, so it "could've been the oil" but I really don't think so.)

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Bob Roe
Lehigh Valley PA USA
1973 Olds 88, 1972 MB 280SE, 1978 Datsun 280Z, 1971 Ford T-Bird, 1972 Olds 88, 1983 Nissan Sentra, 1985 Sentra, 1973 230.6, 1990 Acura Integra, 1991 Volvo 940GLE wagon, 1983 300SD, 1984 300SD, 1995 Subaru Legacy L wagon, 2002 Mountaineer, 1991 300TE wagon, 2008 Murano, 2007 R320CDI 4Matic 52K, some Hyundai, 2008 BMW 535xi wagon, all gone... currently
2007 Honda Odyssey Touring, 2014 E350 4matic