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Old 02-25-2009, 04:38 AM
1990 300se
Join Date: Oct 2005
Location: Honolulu, Hawaii
Posts: 111
m103 with low oil pressure

Hi Guys,

At just a little over 200k miles, I brought my car to the local indy due to plugs fouling in several cylinders. It was diagnosed with worn valve guides and having low compression in a couple cylinders. A leakdown test was also done, but I don't know the results. The head was taken off for machine work and the pistons were soaked in a magic concoction (I don't know what they used), which apparently freed the rings and brought compression levels back up. The timing chain, waterpump, and belt tensioner were also replaced as they were worn out.

The main issue that I'm still having now is with low oil pressure when the engine is idle. At cold startup, I get 3 bars in a couple of seconds. As the coolant temperature rises to 80, pressure drops to less than 1 bar. When coolant temperature is 100, pressure drops to zero or very near zero. The owner's manual specifies that .3bar is acceptable, but I really don't know where .3bar is on the gauge since there's no mark for this. I'm pretty sure that the oil pressure sender isn't the problem, since I've recently swapped it out for a new one (VDO). There are no noises from the lifters, and pressure rises immediately when rpm's go up.

I asked my indy about this, and was told that it's normal. The shop used 10w-30 dino oil, but I've had similar results using 15w-40 in the past. I've made an appointment for them to check the pressure using a mechanical gauge but haven't had a chance to bring the car back to the shop yet.

From what I've read, low oil pressure could be caused by worn bearings. Is there a temporary fix available for this, maybe running thicker oil would help?

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Old 02-25-2009, 05:39 AM
iwrock's Avatar
Join Date: Nov 2003
Location: Hella NorCal
Posts: 3,312
My oil pressure gauge went to 0 at idle before replacing the oil pressure sending unit.

After replacing the sending unit, it will hover around .5 bar at idle.

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Old 02-25-2009, 10:52 AM
Cal Learner's Avatar
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Can't help on the mechanicals, but shouldn't you really be running about 20W-50 oil where you live?
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Old 02-25-2009, 11:14 AM
Join Date: Nov 2005
Location: North Georgia
Posts: 86
I've had the "Zero" oil pressure issue.

If there really was no oil pressure, the engine would be toast in short order. Replacing the sending unit showed the real picture for me. It is on the oil filter housing and is an easy R&R. Cheap too.
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Old 02-25-2009, 11:34 AM
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Location: Phoenix
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The question is not whether there is no oil pressure but whether it's inadequate. He has already replaced the sending unit with a new one. I too would try 20-50 weight oil, no harm where you live, then I'd stop worrying about it.
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Old 02-25-2009, 11:41 AM
Dee8go's Avatar
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Location: Arlington, VA
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I thought the needle should hover around.5 at idle and peg at 3 when the rpms go up as in normal driving. Isn't this pretty much true in all MBs?
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Old 02-25-2009, 02:04 PM
babymog's Avatar
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Location: Northeast Indiana
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Of the 3 gas and 4 diesel Mercedes I've owned, I've always experienced well above 1bar hot & idling oil pressure. 5 of these 7 cars were 124s, one an M103, all were over 100,000miles when sold and most over 200,000miles.

Seems like there's something wrong, whether it is the oil pressure, the sender, or the gauge(voltage) it seems inadequate.

Maybe I should try some non-synthetic oil to see if my pressure drops? Where do you buy the stuff?

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Old 02-25-2009, 02:08 PM
pawoSD's Avatar
Join Date: Oct 2004
Location: Grand Rapids, MI
Posts: 15,176
Our 300E gets down to about 1.1 bar or so at a warm idle (well, in these temps...we'll see in the summer)....running 0w40 Mobil 1.......soon I am switching it over to Rotella 5w40 though, we'll see if that affects anything at all.
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Old 02-25-2009, 07:42 PM
JimFreeh's Avatar
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Location: Hampton Roads, Virginia
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The first thing you need to do is to temporarily install a mechanical gage and verify the reading.

I've recently seen a M103 engine get toasted because of low oil pressure, it was assumed that the gage was faulty. It was not....

The pump is run by a chain off the crankshaft. It's unlikely that the drive has failed, since you do get oil pressure at startup.

Potential problem areas are the pump itself, clogged pickup screen or a failed pressure relief.

Get it checked out posthaste before any damage is done.

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Old 02-26-2009, 05:06 AM
1990 300se
Join Date: Oct 2005
Location: Honolulu, Hawaii
Posts: 111
Hi guys,

Thanks for the quick replies! I'm going to see if I can borrow a mechanical gauge and check it out myself. Regarding the oil, most shops (not mercedes-specific) here fill with 5w-30. I've rarely seen anyone carry 20w-50, but I'll try it out if I can find some.

Last edited by alienman; 02-27-2009 at 02:17 AM.
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Old 02-27-2009, 09:49 AM
TCG TCG is offline
Registered User
Join Date: Aug 2002
Location: St. Petersburg, Fl
Posts: 18

You said the head was removed and machined. Was the oil pressure problem evident at first start up after the head was replaced? Was the correct head gasket used? Threaded plugs from oil galleries in place?

Just a thought.

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Old 02-27-2009, 08:08 PM
1990 300se
Join Date: Oct 2005
Location: Honolulu, Hawaii
Posts: 111

Thanks for your input! The gauge was displaying low oil pressure prior to the head being done, which is why I replaced the sender recently while I was under the car doing the tranny filter.

Unfortunately, I don't know if the correct head gasket was used or anything about the threaded plugs. I was told that the engine's bottom end was toast and that it would be swapped out for one with much lower miles and known good service history, but for some reason or other they just did the head. As a precaution, I haven't driven the car since my initial posting and am crossing my fingers, hoping for the best.

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