Low oil presssure at idle - PeachParts Mercedes-Benz Forum

#1 01-13-2003, 11:40 AM

I need some help. Bought this 300D with 178K miles. It had low oil pressure at idle with 10W/30 oil in it, so I took a chance. Changing to 20w/50 oil didn't change anything. Changed oil pump, still low oil pressure at idle. At 80 degrees C, idle oil pressure is 0.3. Changed main and rod bearings and oil pressure is the same. Can anyone give me some opinions as to the trouble? Thanks! Dutchman830. P. S. Just found out about this site.

P.E.Haiges · #2 01-13-2003, 11:44 AM

When engine it hot, how fast do you have to rev engine to get full oil pressure?

P E H

Nate Stanley · #3 01-13-2003, 11:49 AM

Are you using the dash gauge or a calibrated one? perhaps the gauge or the sender unit is faulty.

Also, there are two small O rings at the bottom of the long steel tube that is attached to your oil filter housing lid. I don't believe that the condition of these Orings has a direct effect on oil pressure, but they are often neglected and are rock-hard. You might want to check them out as well-

#4 01-13-2003, 12:08 PM

I can reach an oil pressure of 3.0 at about 1200 rpm. The guage I'm using is the dash guage. So far I have unable to find an adapter kit that would allow me to hook up a U. S. guage to the mercedes system. Since many guages now have the metric units on them, it would be easy to check between guages.

Piotr · #5 01-13-2003, 12:44 PM

hate to say this,but I cannot believe you went to all this trouble without actually checking the pressure. had the same problem with my Jeep. Went to a mech and for $20 he confirmed in 15 minutes my pressure was fine, just my gauge was off. his fix-turned the needle on the gauge to where it should be.

Have a mechanic check the pressure. Also, isn't 0.3 at idle normal for this MB???

rwthomas1 · #6 01-13-2003, 01:50 PM

3bar is pegging the gauge, right? This pressure by 1200rpm is fine. I used to have a really tweaked small-block that showed 12psi at idle, jumped right to 45-50psi at 1100rpm, no issues. RT

Elliott Fager · #7 01-13-2003, 02:09 PM

Personally, I don't have a huge amount of faith in the accuracy of the oil pressure guages. Mine gets occasional electrical spikes that cause slight but immediate fluxuations at a traffic light. You may consider cleaning the contacts on the sending unit to see if that helps. If not, perhaps the sending unit is at fault. If not that, the guage is probably the culprit in giving you a false reading. If you don't want to go to that expense, might want to try disconnecting the whole unit and purchasing a mechanical gauge and an auto parts store and mounting it in an appropriate area. The change to a different weight of oil should have had some effect (slight) on your pressure readings

SHARK1007 · #8 01-13-2003, 02:17 PM

I think you're pressure is fine. Mine, 83 300SD does about the same, at 1100 or 1200 it pegs the needle.I've had a few of these cars and they all do the same.

The rubber grommets are les than a dollar a pair here online, or $4.00 a pair at the dealer.

lrg · #9 01-13-2003, 02:28 PM

If pressure is high at startup (with cold oil) I wouldn't worry about it. Most of these gauges are least accurate at the lower end of the range.

#10 01-13-2003, 02:35 PM

Its good to hear from you guys and hope to hear more. The Benz people I talked to said that mechanical oil guages just don't go bad. Did a little shade tree engineering using rubber hose and did a pressure check using a U. S. guage and it read 11 psi. According to the shop manual, the minimum oil pressure at idle, is 0.7 on the dash guage. This equates to about 20-21 psi. This is why I am so concerned. Dutchman830

lrg · #11 01-13-2003, 02:44 PM

If my memory serves me correctly 1.0 bar = about 14psi. If that's right then 0.7bar = about 10 psi. Can anyone else verify that? If I'm wrong I'll just go back the the Alzheimers ward and be quiet.

EDIT:

Actually just checked the conversion table in the Haynes manual. According to them 1 bar = 14.5 psi. If you are getting 0.7 bar at idle it should equate to 10.15 psi.

P.E.Haiges · #12 01-13-2003, 03:10 PM

Dutch, I think you divided 15 PSI per bar by 0.7 instead of multipling by 0.7 which would equal about 10 PSI.

P E H

#13 01-13-2003, 08:00 PM

I stand corrected on the pressure calculations, however I'm still concerned about the pressure being that low.

ctaylor738 · #14 01-13-2003, 10:19 PM

I am a wandering gasser, but we had a 1982 300D for 17 years, so I stll have the engine manual. It says ".5 bar at idle, rising to 3 bar at 3000 RPM." I think your pressure is fine.

There is also an interesting statement in the manual, though, in "Complaints concerning oil filter"

"Excessive force when inserting central bolt during oil filter change may push the sealing ring out of its hold into a position in front of the header pipe. The oil will then be supplied via the overflow valve resulting in a drop in oil pressure."

The "central bolt" is the one that holds the top on. This may not be relevant because I remember ours having three studs, and nuts holding the top in.

The Warden · #15 01-13-2003, 11:05 PM

Maybe they've changed it, but on my '85, I've got two studs/nuts on opposite sides that hold the oil filter cover in place.

BTW, on my car, I get about 1.5 bars at hot idle...maybe down to 1.25 bars after a long run...also, for those who mentioned cleaning the sending unit terminals, as far as I know, all 123 cars use a purely mechanical gauge, although the 126 cars use a sending unit/electrical wire...

Also, my truck shows 10 psi on the aftermarket mechanical gauge, and runs fine...so I wouldn't be worried.