Why does oil pressure go up after changing oil?

[PT1]

Originally Posted By: lazaro
crude oil is thick also when cold and has more friction between molecules.


Oh yeah...when I go out back to the Pennzoil Platinum derrick in my back yard and siphon off a few quarts from the well head it is thick as molasses by gum...

 

[440Magnum]

If your oil pressure sensor is downstream of the oil filter (as all of my vehicles are), then my hunch is that the clean free-flowing filter causes less pressure drop than the old filter.

I doubt that it has anything to do with the oil itself.

 

[JoeWeinstein]

If there is a lot of blow-by at the rings,
oil can be significantly diluted by unburnt
gasoline. Other than that, being a layman,
it sounds plausible that shear in the oil
(working it through the motor against the
forces of it's own viscosity) might break
it down. However, especially with dyno-based
oils, there is a wide range of thicknesses
of oil in the base stock, and some component
of this is volatile, and can evaporate over
time and heat, contributing to thickening
the oil (that fraction that remains).

 

[caprice_2nv]

My 74 350 Rocket Oldsmobile engine shows the exact same oil pressure before and after oil change as long as I use the same oil. I have to make a drastic change in weight like 0w30 to 20w50 to notice a change/gain in oil pressure on the mechanical gauge. Maybe I'm not leaving the oil in there long enough? lol

 

[bruce381]

presure is related to vis thats all

 

[G-MAN]

Originally Posted By: chevrofreak
I believe it has to do with the new oil being thicker, and thus not flowing as freely through the passages and bearings, which causes it to exert more force on the oil pressure sensor.


If he means he has higher oil pressure immediately after he changes it, I'd say it's because the fresh oil is still cool, hence, thicker. Thus, higher oil pressure.

 

[caprice_2nv]

Mine always shows higher pressure until driven for at least 20 minutes at highway speed. I attribute it to the oil temps getting higher. The lower oil pressure i notice when hot is mostly at idle. However my oil pressures all around are the same with the old oil as after i change it.

 

[chevrofreak]

I always see higher than normal pressure for the first 200 miles or so, and it's there for several run cycles, then the pressure goes down to the normal level again and rarely if ever goes back up to the peak of what it was when the oil was new.

 

[440Magnum]

Originally Posted By: Black_Thunder
also remember, less oil....less oil pressure


As long as there's enough oil in the pan to keep the oil pickup submerged, the oil pressure doesn't depend on the level of oil. If the level gets low enough to suck air into the pickup, THEN you'll see low or unstable oil pressure, but just being 1 qt low makes no difference at all. That's why you have a dipstick under the hood- because the gauge is NOT an indicator of oil level.

 

[Jim 5]

Originally Posted By: javacontour
My old 66 Nova suffered this. It sat for years before I got it and I suspect the rings were not good. After an oil change, it would have great oil pressure. Then as time went on, the warm oil pressure dropped. I suspect that the oil was diluted by blow-by and couldn't keep pressure. While it was being burnt off, that didn't happen fast enough to allow fresh oil to counter the effects of blow by.

I'd have close to ZERO according to the self-installed mechanical gauge at idle when the oil was used. That was my indication it was time to change the oil.

While that car used oil, it was practically bulletproof. Well not really, it was rusty

But it survived two teen drivers. My sister would run it out of oil and it would need 3+ quarts to get it going again when she failed to check it. Add the oil and it would just keep going.


My roomate in college had one of these with the iron duke inline 6. It was a really great and reliable car. I like those years little novas before they grew bigger.

 

[RF Overlord]

Originally Posted By: Jim 5
...iron duke inline 6.
Actually, the "Iron Duke" was the 2.5L I4. I think you meant the "Stovebolt"...