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  #1  
Old 03-19-2009, 07:42 PM
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1986 420SEL - low oil pressure

I have done countless searches on this subject to no avail. Would someone knowledgable about this subject please help me understand the possible causes of a gauge that doesn't change much from idle to full gas; about 1 on the 0-3 scale.

Is this a faulty oil pump, or possibly more than that?

Thanks.

Simon Shearer
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  #2  
Old 03-19-2009, 08:17 PM
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oil pressure

You might just need a new oil sending unit.
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  #3  
Old 03-19-2009, 09:03 PM
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Maybe somebody put synthetic oil in the engine. The oil pressure sensor can't 'read' that right. I'd change the oil to mineral (as it should be) and see if the oil pressure gauge doesn't start working again, all of a sudden like......best of luck
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Old 03-19-2009, 09:08 PM
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Quote:
Originally Posted by cliffmac View Post
Maybe somebody put synthetic oil in the engine. The oil pressure sensor can't 'read' that right. I'd change the oil to mineral (as it should be) and see if the oil pressure gauge doesn't start working again, all of a sudden like......best of luck

I put synthetic in my 300E and it did not change the oil gauge.
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  #5  
Old 03-20-2009, 07:03 PM
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I was assuming the oil pump needs to be replaced, no?
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  #6  
Old 03-20-2009, 08:02 PM
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Don't assume the worst until you've eliminated all else. Disconnect the wire on the sender, test the gauge. If that's OK, then you change the sender (cheap compared to what an oil pump would be). I don't remember if its voltage or ground you use to test the gauge, but that's why service manuals and forums exist.

I have not experienced any strange behavior due to syn. oil. They're close enough that I don't think a mechanical sensor would care.
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'79 6.9 Sold (after 27 years of ownership)
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  #7  
Old 03-20-2009, 10:08 PM
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Quote:
Originally Posted by 04 Diesel View Post
You might just need a new oil sending unit.
Would that be an "oil level sending unit;" or an "oil pressure sensor?"
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  #8  
Old 03-20-2009, 10:40 PM
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at any rate, that's what I would do...change the oil and filter to good old Castrol GTX 10w-40 and Fram and see if that oil pressure gauge doesn't come back up. Heck, you should do those two things anyway along the way....
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  #9  
Old 03-21-2009, 08:44 AM
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Since we are talking about the oil pressure gauge, it follows that the oil pressure sending unit would be involved. But oil level could indeed be a cause for concern. Back to the basics......have you checked the oil level, with the dipstick, as per the owner's manual (engine not running)? How long since the last oil/filter change?
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'79 6.9 Sold (after 27 years of ownership)
'83 280SL, 5 spd.
'94 E320 Sdn. 5 spd conversion
'95 E320 sedan
'02 E320 Sdn.(on loan to mom!)
'87 300E (5 spd. conversion) Sold
'05 E500 Wagon
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  #10  
Old 03-21-2009, 02:39 PM
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Quote:
Originally Posted by hanno View Post
Since we are talking about the oil pressure gauge, it follows that the oil pressure sending unit would be involved. But oil level could indeed be a cause for concern. Back to the basics......have you checked the oil level, with the dipstick, as per the owner's manual (engine not running)? How long since the last oil/filter change?
Today. Changed oil and filter. Castrol GTX 10w-40 as recommended. No change on the gauge. In fact it hardly moves off zero now.

Next step a new oil pressure sending unit.

Simon
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  #11  
Old 03-28-2009, 06:11 PM
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that was it...!

I changed the oil pressure sending unit and now I have the gauge leaping to "3" immediately when given gas.

At idle it settles down to about "2" - is that right?

My 300D idles at about "1."

Thanks for everyone's help - especially for stopping me rushing off to purchase an oil pump!

Simon
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  #12  
Old 03-29-2009, 01:13 AM
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On my 500SEC, idle runs around 1.5, 3 at cruise rpms.
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