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  #1  
Old 06-09-2013, 11:09 AM
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Normal Function of Oil Pressure Gauge

What is the normal operation of the oil pressure gauge. On my car when accelerating the needle goes to the top and stays there. When I let of the gas pedal the needle drops to just below the halfway mark. Oil level is at the full mark. The engine is the 4.5 in a 1972 W108.

Should I be worried about this.

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Old 06-09-2013, 11:42 AM
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On my m130, the pressure is always pegged high. Only after the engine is hot and I'm at <1000rpm does it even drop. It's still over 30psi. I run 15-40 oil. And I drive in 100-110F weather.

This doesn't sound right. What weight oil do you use? Is the oil level fine?
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Old 06-09-2013, 12:18 PM
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Not sure about gassers, but on diesels what you describe is the norm. When cold, pegged all the time. When hot, pegged at any RPM above idle. At hot idle, oil pressure can go below 15 psi (1 bar) and still be fine. So when you remove your foot from the pedal, if the engine approaches idle RPMs, the oil pressure will drop. Again, not sure about the gassers, but if you had a diesel I would say you are fine.
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Old 06-09-2013, 01:39 PM
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Quote:
Originally Posted by moby dick View Post
This doesn't sound right. What weight oil do you use? Is the oil level fine?
The oil is 10w-30, not synthetic. I just noticed this issue yesterday. Oil level is at the full mark.
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  #5  
Old 06-09-2013, 10:21 PM
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Sounds like your idle just isn't high. You're fine as long as the engine is turning over smoothly.
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  #6  
Old 06-09-2013, 10:44 PM
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The gauge in this car is a real gauge with a bordrun tube, not an electrical device. The pressure you are reading is the real psi and the gauge is usually measured off in atmospheres where (one A=14.6 psi).

The operation sounds normal to me. Every diesel I have ever owned has been like this, and the gas ones are close to this.

I never noticed any difference in the old 6.3 since I kept it pegged out most of the time. When you see the needle drop down below .5 of one A during idle then you need to figure out what the deal is since your bearings may be worn or your oil pump is not putting up what it should.

Or you could just throw in some Lucas Oil Treatment to bring the pressure up.
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Old 06-10-2013, 12:00 AM
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Read your owners manual for description of where the oil pressure gauge should be. Your description sounds like normal operation to me. Should be pegged as soon as you step on the gas and when hot can idle just above the lowest demarcation on the gauge if I remember correctly. Gassers the same as diesel in this regard.

- Peter.
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Old 06-10-2013, 08:17 AM
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Thank you all for you input. I will read up on the owners manual, I should have read that first! The engine does run rough and it does need a good tune up. However, the old girl turns over the first time when I turn the ignition key.
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  #9  
Old 06-10-2013, 10:43 AM
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That's what my 350SL and 450SLC with M117s do, however I think 10w40 is the correct weight for that motor. 15w40 diesel oil like previously mentionned would work fine, but 10w30 is thinner at temperature so you would get a lower reading.

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