High Oil Pressure on Cold Engine Start Up

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1,715
Location
Texas & BWI Area
1998 Camaro w/Mobil 1 5W30 SS (usually car is run on 10W30 Royal Purple, I am experimenting with M1) Today for example at an ambient air temperature of 28 degrees F i for 10mins after cold Engine Start-Up that the Oil Pressure was around 70-75psi. The OE pressure at operating temperature and 2000rpm is 60psi I am running an oversize K&N Filter...and yes at operating temperature 60psi is right on the dot for my engine. Is this 70-75psi reading caused by the bypass valve being shut closing the filter...and hence raising the system pressure reading? Or is it something along the lines that the low outside temperature has essentially made the oil "thicker" resulting in a greater pulse against the pressure sensor. From a Physical Chemistry standpoint is thier a correlation between fluidity-viscosity and Pressure? I thought pressure is based on Force/Area...hence would area be the surface-area of the sensor? Then also I would think a "heavier/cold" oil could produce more force in this same area correct? Sorry for all the question. From reading the posts thier are a good many very wise people on this board from many interesting mechanical backgrounds citing great sources and data. Thanks for the great posts. Sun-
 
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8,937
Location
SC
Any oil, even a synthetic, is going to be more viscous at lower temperatures and produce higher oil pressures than at higher temperatures.
 
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47,630
Location
Duvall WA - Pacific NW USA
quote:
Is this 70-75psi reading caused by the bypass valve being shut closing the filter...and hence raising the system pressure reading?
I think I know what you are asking - but it almost seems like there is a misunderstanding about the oil filter (in filter or engine) bypass valve. I think of the by-pass valve as normally closed. Oil goes through the filter as normal. If the filter media, for whatever reason, gets plugged then the pressure differential forces the valve open and oil can flow. Maybe semantics, but I don't think this has anything to do with a much higher oil pressure on cold starts. As far as any cold oil flowing through any given filter media, more viscous oil (all other things being equal) could cause higher pressure and the by-pass could open and lower the pressure....
 
Messages
450
Location
Louisville, KY
quote:
Originally posted by outrun: Or is it something along the lines that the low outside temperature has essentially made the oil "thicker" resulting in a greater pulse against the pressure sensor. From a Physical Chemistry standpoint is thier a correlation between fluidity-viscosity and Pressure? I thought pressure is based on Force/Area...hence would area be the surface-area of the sensor? Sun-
Oil pressure is not made by the pump the pump just pushes the oil. The pressure comes from the resistance of the engine internals to the flow created by said pump. Hence thicker oil can make for higher presure. Fred.... [Smile]
 
Messages
421
Location
LUBBOCK, TX
Your exactly right Fred. One other point to consider is even though the pump pressure relief valve may be set to relieve at 60 psi, due to thicker oil it is not able to relieve enough of the flow to keep pressures at this setting until the oil warms up.
 
Messages
6,388
Location
Washington St.
quote:
Originally posted by Pablo:
quote:
Is this 70-75psi reading caused by the bypass valve being shut closing the filter...and hence raising the system pressure reading?
I think I know what you are asking - but it almost seems like there is a misunderstanding about the oil filter (in filter or engine) bypass valve. I think of the by-pass valve as normally closed. Oil goes through the filter as normal. If the filter media, for whatever reason, gets plugged then the pressure differential forces the valve open and oil can flow. Maybe semantics, but I don't think this has anything to do with a much higher oil pressure on cold starts. As far as any cold oil flowing through any given filter media, more viscous oil (all other things being equal) could cause higher pressure and the by-pass could open and lower the pressure....

The oil filter's internal bypass valve will open to maintain about the resistance of the nominal valve setting. If the bypass is said to be set for an 8 psi differential pressure, it may begin to open at 7 psid and be fully open at 9 psid, or some similar numbers. This is how much it will lower the oil pressure between the pump and the oil supply to the engine. Of course, if the oil is very thick and the flow rate very high, the bypass size may not be sufficient and you'll get a higher resistance from the filter. If the filter is made to OEM specs, no problem. If the filter is designed for some other engine, but it "fits" there might be a problem. Ken
 
Messages
58
Location
Richmond, VA
What can cause the opposite..low or no pressure on cold mornings...a drop in presure sometimes at highway speeds too lately? I have nuetra 132 in at this time for a cleaning. This started before the nuetra.
 
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