Why does oil pressure go up after changing oil?

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JDD

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On my vehicles with a guage or bars, oil pressure is always higher right after I change it and it stays higer for a while while driving. What's the reason--?

Sorry if this is a topic addressed before, I just can't find a thread on it.
 
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.
 
what rig and engine? driving style and how used?

get a UOA run on your next oil change

your engine may have fuel dilution, not a good thing
 
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viscosity is higher with fresh oil, many complain of less MPG until after a few thousands miles then it settles back to regular MPG.
crude oil is thick also when cold and has more friction between molecules. with so many molecules in dino oil it is easy to understand why synthetics are easier to pump due to only a few requiered molecules for lubrication.
 
I have a '01 4.0 SOHC ford and a 93 4.3 GMC--both with guages and the GMC with the electronic dash with the bar indicator. After I changed the oil Monday on the GMC the indicator lights were two one bars above the normal 40psi at operating temp and also 2 bars higher than usual at cold idle. It has always done this after every OC on both vehicles. I'm not worried in any way, just curious. I have done a UOA on the ford once and it was great. I am going to send the sample I took out of the GMC and do a UOA, since I have never done one and the oil was in there 6K miles and 9 months (Maxlife 5-30 synthetic)
 
New motor oil shows better oil pressure because old used motor oil sheered down from a viscosity standpoint.
 
Originally Posted By: Hirev
New motor oil shows better oil pressure because old used motor oil sheered down from a viscosity standpoint.


That plus any possible fuel or water dilution will lower viscosity and oil pressure as a result.
 
Originally Posted By: OVERK1LL
Dude, you did NOT just compare conventional oil to raw crude..... Dear Jesus.

LOL!
 
Originally Posted By: lazaro
viscosity is higher with fresh oil, many complain of less MPG until after a few thousands miles then it settles back to regular MPG.
crude oil is thick also when cold and has more friction between molecules. with so many molecules in dino oil it is easy to understand why synthetics are easier to pump due to only a few requiered molecules for lubrication.
We are talking 2008 oils not 1960 GP1 oils here.
 
Weird-usually new oil in my Mercedes has LOWER oil pressure-probably due to all that chunky soot coming out!!! Seriously, you need to look for leaky injectors/pump-unless you waited way too long, you've got something bad happening to your oil.
 
Maybe I'm mis-reading the OP's post, but...

The fresh oil you just poured in is much cooler (and therefor thicker) than the old oil you drained out, so the pressure will be higher until the new oil heats up and thins out.
 
Didn't think of that...
spankme2.gif
 
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
LOL.gif
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.
 
I seldom let it get below 1 quart low. It needed a quart every other fillup.

So it wasn't an oil level issue in my case. (For my sister, that's a different story.)

But once the oil was worked over by the blow-by, it was time to change it so the car had oil pressure again.
 
Originally Posted By: Black_Thunder
also remember, less oil....less oil pressure


That's not true. Back when my 'stang was leaking oil from the valve covers, even when it was 1L down, pressure was the same.
 
I always thought it was the shearing of the multi-viscosity polymers with time that made later pressure lower.
 
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