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OM617 dropping oil pressure problem.

Started by Jed, 13 December 2021, 12:51 PM

Jed

Hello all,

I'm looking for a little help diagnosing an oil pressure issue on my OM 617. It's actually a 1984 300 CD. I know this is a W123 (and i am on a w116 forum) but this is definitely my board of choice and I believe it's the same engine in the SD's so hoping to still get a little help?

So, the oil pressure pegs to three from a cold start and as the engine warms up and approaches 80°C the pressure starts to drop. My idle oil pressure remains appropriate cold or hot about 1 3/4 to 2. It's the pressure at throttle that's bothering me. Once the car reaches 80°C the pressure is only 2 1/2 to 2 3/4. I understand it's supposed to be pegged at 3 at throttle (certainly above 2000 RPM) and mine is no longer. Even at highway speed's I'm only getting 2 3/4 tops.(never less than 2) I've had the car a few years and it's always been Pegged at 3 at throttle until recently. In addition if I'm driving at speed and it's measuring 2 1/2 and I downshift with either the kick down button or the shifter the oil pressure actually drops as the RPMs increase. when I come off the throttle alittle or the car up shifts the pressure will increase again to approximately 2 1/2. Very odd. In addition before the car reaches running temperature it will peg at three at full throttle. The problem only starts as the car warms up and reaches 80°C.

Now for the backstory.  I replaced the oil pan because it had been dented up about two years ago and I replaced the oil cooler and oil cooler lines as they were weeping (also years ago). I don't believe the problem started until very recently. Searching for a source of the problem I found that the lower oil cooler line rubber segment was partially collapsing on itself at the bend. It was not made of that hard rubber like the OEM one was but it was not like that when I installed it. I immediately quit driving it when I noticed it and just replaced it now with a new one with a firm (noncollapsible?) rubber. I was sure replacing that hose was going to fix the problem but it did not. Not sure if damage was done with that partially collapsed hose?

Oil level is correct. I have changed my oil using 15 W 40 diesel oil. New oil filter, replaced the O-rings on the filter cover shaft. Blew out the pinhole in the oil filter cover shaft(with low pressure) I've tested the mechanical gauge on the bench with air pressure and appears to be working properly. I Have blown out the oil pressure line to confirm it's not partially blocked.

Not sure what to check next? Could this be an oil pump issue? One of the thermostats built into the oil filter housing? Partially blocked oil cooler (which is only 2 years old)?

By the way the car seems to be running fine no loss of power, no louder than usual. It has 190,000 km on it.I've decided to park it until I figure this out as if the pressure is actually as low as the gauge is reading this certainly cannot be good. I would think that the drop in oil pressure upon accelerating and increasing RPMs is the big hint to the source of the problem? Also the fact that everything is normal until warm up.

Any and all thoughts are as always greatly appreciated!

Thanks in advance!

Jed
1979 Mercedes 6.9 #5206 - restored
1979 Mercedes 6.9 #6424 - ongoing restoration
1976 Mercedes 6.9 #484 - restoration?

Randys01

Disconnect the oil pressure line at the engine and insert a test gauge. This will be a bit of mucking around but lets see what's really going on.

UTn_boy

You're right in that the oil pumps rarely give any trouble, and I'd rule that out. Of course, if the engine was ever starved of oil that would likely affect the oil pump, but since that hasn't happened I wouldn't give that a second thought.  If after you take Randy's advice and confirm the current pressures at the instrument cluster connection, then I'd have to expect the connecting rod and/or main bearings are what's at fault.  In the past when I've encountered this issue it turned out to be the connecting rod or main bearings.  They wear to a point that oil pressure can't maintain due to the excessive clearance.  You won't notice any knocking or different noises most of the time.  But as the wear increases you will start to hear knocking.  On the other hand, many engines have driven many more miles with this condition. It could also be something as simple as having air in the oil line going to the instrument cluster.  Did you bleed that line after you blew it out with air?   
1966 250se coupe`, black/dark green leather
1970 600 midnight blue/parchment leather
1971 300sel 6.3, papyrus white/dark red leather
1971 280sel Black/black leather
1973 300sel 4.5, silver blue metallic/blue leather
1979 450sel 516 red/bamboo

Jed

Thanks guys, I appreciate the thoughts.

I will rig up a connection to the filter housing to connect a gauge and see if the numbers are real (which I believe they will be as the gauge seems to work fine when the engine is cold).

No Aaron, I did not bleed the line after I blew it out.  I just blew it out this morning and when I got nothing but oil (no sludge or blockage) i reconnected it.  I haven't started it since.  I suppose I will be disconnecting it again (at the engine) anyway to connect a gauge but I will bleed it before I put it back in service.

thanks again!
1979 Mercedes 6.9 #5206 - restored
1979 Mercedes 6.9 #6424 - ongoing restoration
1976 Mercedes 6.9 #484 - restoration?

Jed

Well, problem solved.

It is with great humility and shame  that I will now show my ass.
this will likely be my last post ever as I am too utterly embarrassed to ever come back...





1979 Mercedes 6.9 #5206 - restored
1979 Mercedes 6.9 #6424 - ongoing restoration
1976 Mercedes 6.9 #484 - restoration?

Harv

Wait, what?

How did that even get in there?
1980 300SD
1983 240D
2000 E55 AMG
2002 G500

daantjie

Whaaaat...man we need the back story here ;D
No worries, we've all done moronic things with and to our cars, lord knows I have been lucky to not have grenaded mine over the years (so far)...the fact that I still have all my fingers is in itself a bloody miracle  :o
Daniel
1977 450 SEL 6.9 - Astralsilber

Lance

I think he put a sock on the oil pump inlet to keep it clean while the sump was off and forgot to take it off. Run it and see if it has good oil pressure, likely hasn't done any major damage.

Lance

Or more likely left a rag in the sump and it eventually picked it up.

Jan S

One of life's greatest mysteries finally solved - we got the answer to the question where do all the socks go when they get lost in the laundry  ;D
1975-mod W116 450 SE with 6.9 engine

raueda1

OMG.  Sounds like something I would do.  Happily you don't have to rebuild your engine!  Keep on rolling and Happy Holidays,
-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

Jed

 :)

I replaced the oil pan (only because the po had banged it up a little) 2 years ago!
at that time, best I can remember i had the old pan off and was working under the car.  The damn thing dripped forever so I took a blue paper towel/shop towel and stuck it up against the pump to catch the oil while I was working.  Well I imagine days went by before I went to put the oil pan back on and the blue towel was by now soaked black and I didn't see it and forget all about it.  Fast forward 2 years later and the car just started misbehaving (see above). I guess it finally got sucked up against the screen.

Anyway the oil pressure is once again pressurizing as it should.  Pegs at 3 at 900rpm. idles at 2.5 and no longer drops at acceleration.  I don't understand the direction of flow that allowed it to pressurize before it got hot while plugged up (where else could it draw the oil from other than the pan), or how I was getting adequate pressure at all even at idle as that paper towel was really wedged up into the filter screen tight! Super lucky that the towel didn't disintegrate and get sucked in as it would surely have plugged oil passages and bearings everywhere. Hopefully no significant bearing damage done but I guesss time will tell. (it never dropped below 2 at speed while it was plugged but who knows if that's enough to prevent premature wear). 

Best case, I am the forum fool until the next guy out does me....

todays lesson: if it aint broke don't fix it! (I nearly killed the engine over a lightly dented oil pan)
1979 Mercedes 6.9 #5206 - restored
1979 Mercedes 6.9 #6424 - ongoing restoration
1976 Mercedes 6.9 #484 - restoration?

daantjie

Like a surgeon leaving a sponge inside the patient hehehe...
Daniel
1977 450 SEL 6.9 - Astralsilber

Jed

exactly like that!

except in the OR there is someone in charge of counting every sponge prior to closing the patient to ensure this doesn't happen.  Perhaps I need one of them for my garage...
1979 Mercedes 6.9 #5206 - restored
1979 Mercedes 6.9 #6424 - ongoing restoration
1976 Mercedes 6.9 #484 - restoration?