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  #1  
Old 04-03-2011, 06:14 PM
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81 300SD - why is the oil gauge pegged out?

While testing the transmission and vac system today for leaks, I decided to bypass the old EGR. Everyone says it is not needed anyway and the only thing that leaked vac was two lines to the EGR. I blanked off the two Y fittings and cut the EGR from the vac path; everything looks good.

Started the car and the oil pressure gauge immediately pegged out at 3+. This is new; in my 30 year ownership this car always idles at about 2 and never gets to 3 without acceleration.

Any idea what could be the problem? Bad gauge and just a coincidence that it went out while I was un-EGR'ing the car? I hooked the EGR back up but the gauge stayed pegged out.

Am I safe driving the car and what do I do about the high oil pressure reading?

Thanks for your help.
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  #2  
Old 04-03-2011, 06:19 PM
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It pegs immediately when cold then does what you say after you warm up.

If it pegs with you just turning the key and not starting the engine, then the sender is toast.

The EGR is pretty far away from all this, I doubt it had anything to do with it (where anything = jostling something).
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Old 04-03-2011, 06:20 PM
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is it idling better/faster with the egr plugged?
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  #4  
Old 04-03-2011, 06:47 PM
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The EGR has nothing to do with the oil pressure. A healthy MB will peg the guage when cold at start-up, but if it does this before the engine is running, as was posted before check the sender AND your ground connection. I have seen some really weird guage stuff when a ground was not properly connected that defied all logic.
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Old 04-03-2011, 07:46 PM
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I got some heady advice from Ashedd; he said go ahead and drive it because I certainly have oil pressure.

That was good advice because it helped me figure out what happened. Apparently the indy idled the car up higher when he installed the new tranny - without telling me. The tach does not work so I really did not hear the difference.

So I drove it and it started backing off at stops like it is supposed to do. Guess it was a false alarm. And thanks for your responses; they are much appreciated.
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  #6  
Old 04-04-2011, 12:08 AM
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It should always peg to 3 until it warms up. Warm idle around 2ish, hot day just got off highway idle around 1-1.5 is normal.

If its pegged at 3 all the time no matter what then the wire to the sensor is grounded somehow, there's an issue with the cluster/ground, or the sensor itself has failed.
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Old 04-04-2011, 04:55 AM
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The oil pump is overbilt.... they just did not make a gauge which will show how high it usually runs... if it gets under one bar then one should worry. If it changes pressure FAST.. one should consider the possibility that the screw on oil pressure valve on the back of some of our engines has fallen off.... or given way... before assuming the oil pump is toast..
There are several updates on that oil pressure valve ( suggested rating ,perhaps design )... be sure to check for the latest recommendations before replacing...
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Old 04-05-2011, 02:02 PM
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Quote:
Originally Posted by leathermang View Post
The oil pump is overbilt.... they just did not make a gauge which will show how high it usually runs... if it gets under one bar then one should worry. If it changes pressure FAST.. one should consider the possibility that the screw on oil pressure valve on the back of some of our engines has fallen off.... or given way... before assuming the oil pump is toast..
There are several updates on that oil pressure valve ( suggested rating ,perhaps design )... be sure to check for the latest recommendations before replacing...
Fluctuating oil pressure can also mean you need to replace the O-rings on the oil filter stem.

I have a VW with a 5 bar oil pressure gauge. That one pegs out on cold starts, too. Cold, thick oil makes a lot of pressure. That's why worn oil seals sometimes blow on cold starts.

I actually think Mercedes' truncated 3-bar scale is clever. The gauge doesn't move around while you're driving at speed and distract you, but it shows you the values you care about.
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