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  #1  
Old 09-09-2008, 11:35 AM
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W123 high oil pressure at idle

My oil pressure at idle (transmission in drive, sitting at a stoplight) has always been around "2" on the gauge, or a little bit below. The past few days I've noticed that the gauge is staying high. Sometimes it stays pegged at "3" sometimes it falls to about 2.5. It falls to zero when the engine's off, and is always at 3 with the accelerator pressed. Oil level is normal (halfway between the two sets of notches on the dipstick).

I'm not sure if I'm imagining things but it seems as if the engine might be a little bit "quieter" than usual, although I can't say the character of the noise has changed. Starting, acceleration and smoke level are as usual.

Motor oil is 15W40 Rotella dino, no additives. It was changed in April or May and I've only got 2,000 miles on it since then. Filter was changed at the same time, it's a Turkish-made Fram.

1. What can cause this? Is there some kind of additional strainer or something somewhere?
2. Is this cause for not driving the vehicle? Low pressure would scare me off driving it but I'm not sure what to make of high pressure at idle. My other car's in the shop right now so this is my only set of wheels.

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  #2  
Old 09-09-2008, 11:39 AM
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Maybe your oil cooler isn't opening up?
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  #3  
Old 09-09-2008, 11:44 AM
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Quote:
Originally Posted by Chad300tdt View Post
Maybe your oil cooler isn't opening up?
Wouldn't that produce unusually high oil temperature? I will keep an eye on it but haven't noticed that.
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  #4  
Old 09-09-2008, 11:48 AM
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Originally Posted by herring View Post
Wouldn't that produce unusually high oil temperature? I will keep an eye on it but haven't noticed that.
What, exactly, will you be looking at?
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  #5  
Old 09-09-2008, 11:50 AM
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when its cold the gauge should be pegged at 3 bar. When hot it should be around 1-2.5 bar.
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Old 09-09-2008, 12:24 PM
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Originally Posted by tangofox007 View Post
What, exactly, will you be looking at?
Well, I was thinking of looking at the temperature gauge. I will admit to not being a mechanic, but I assumed that a sign of a malfunctioning oil cooler would be higher oil temperature. Am I wrong on this?
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Old 09-09-2008, 12:25 PM
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Originally Posted by 79Mercy View Post
when its cold the gauge should be pegged at 3 bar. When hot it should be around 1-2.5 bar.
Ok, so I'm trying to figure out what it means if it sometimes (but not always) stays at 2.5-3 bar at hot idle.
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Old 09-09-2008, 12:30 PM
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You don't mention what part of the country you live in but my bet was cooler weather. Cooler weather = thicker oil = higher oil pressures
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  #9  
Old 09-09-2008, 12:34 PM
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Philadelphia. Yes, the weather's a bit cooler, but no cooler than it was in the spring when I put the oil in. I don't recall it staying pegged at 3 bar back then.
Maybe I'm just being paranoid. I've had more mechanical issues with this car than I had planned on so I'm kind of "on high alert".
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Old 09-09-2008, 12:35 PM
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Quote:
Originally Posted by herring View Post
Well, I was thinking of looking at the temperature gauge. I will admit to not being a mechanic, but I assumed that a sign of a malfunctioning oil cooler would be higher oil temperature. Am I wrong on this?
You would be correct, but there is no gauge on these cars for oil temp.
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Old 09-09-2008, 12:46 PM
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It is possible that the coolant temp might rise a little if the oil temp was way high. Many years ago, an Alfa that I owned would show a slightly higher than normal coolant temp when the oil was low (and therefore hotter than normal). However, this is a secondary effect and on a Benz might not even be noticed. Coincidentally, the Alfa also had an 8-quart sump and used Rotalla oil.
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  #12  
Old 09-09-2008, 12:54 PM
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Quote:
Originally Posted by herring View Post
Ok, so I'm trying to figure out what it means if it sometimes (but not always) stays at 2.5-3 bar at hot idle.
The only way I can think of to test if it is an gauge problem or an oil pressure problem is to hook up a test gauge.
If you have and electrical Oil Pressure Gauge it if most often poor electrical connection to the Oil Pressure Sending Unit or the Oil Pressure Sending unit is going bad. You can also have a poor ground up in the instrument cluster.

If you have a mechanical Oil Pressure Gauge and you want to hook up a test gauge; the easiest way is to go to the junk yard and locate where the Oil Pressure Gauge Feed Tube (plastic) attaches down at the bottom rear of the oil filter. Cut the tubing several inches above the fitting and remove the nut/fitting and tubing end.
You will need a gauge that goes up to aroud 110 psi.
I do not suggest you tap into where I hooked up this gauge but this is my pressure with a cold engine at idle.

This will give you something to hook up a test gauge to on your engine.
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  #13  
Old 09-09-2008, 01:40 PM
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So is a malfunctioning oil cooler the only likely cause of this?
There aren't any appropriate Benzes in junkyards around me, I've called and visited (I did find a gasser coupe and looted what I could).

Questions:
*Could I take a shortcut and test my oil temperature after the engine's been driven? What would be a "normal" oil temperature and what would be "too hot"?

*If the oil cooler is in fact not opening (engine oil getting too hot), what's the standard way of taking care of that? Buy a new one?
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  #14  
Old 09-09-2008, 02:31 PM
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I'm not positive, but I believe there is an oil thermostat near or in the oil filter housing that opens when the temp is up and allows the oil to flow through the cooler lines to the cooler.

Perhaps just that thermostat can be replaced? Anyone else know?

Either that or the cooler itself has a clog in it would be my guesses.
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  #15  
Old 09-09-2008, 02:46 PM
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Quote:
Originally Posted by herring View Post
So is a malfunctioning oil cooler the only likely cause of this?
In my view, the oil cooler is not even a possible cause. As you have no doubt noticed, oil pressure varies inversely with oil temperature.

Did this "problem" start when the oil filter was replaced?

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