Robert,
I think that the short answer is that as the oil temperature increases, the viscosity
decreases. The oil pressure seen at the guage is a function of several factors,
viscosity being one of those. As the oil temperature goes up, the viscosity goes
down and so does the oil pressure. I bet that if you were to graph the oil pressure
vs rpm cold, and then again hot, you would find that the pressure is consistantly
lower with hot oil then with cold oil (at least until the pressure bypass valve kicks
in).
Doesn't sound to me that you have an engine problem.
-Roger
Robert Wiseman wrote:
>
> Today was the longest time I have had my car running. Up until now
> (other than a few block drive 2 weeks ago) all I have done is idle in
> the driveway. At idle the oil pressure has been a pretty consistant 45
> at 800 RPM, it would go to about 75 at 3500 RPM.
>
> Today I drove it about 1 mile and idled for about 10 minutes. When I
> was done, I noticed that at 800 RPM idle, the OP was just over 25. When
> I go to about 3500 RPM the OP would go to about 65-70..
> Why the change and does this mean I have problems with the engine.
> Rob
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