My V8 is also slower to reach max pressure from cold in freezing temps than
in summer, but not as slow as yours. Possibly the oil in the capiliary is
very thick at low temps and its flow resistance against the tube walls is
enough to stop full pump pressure reaching the gauge. However the amount of
flow in the capilliary is minimal so this seems surprising. When you stop
for a moment freezing air is no longer being rammed into the engine
compartment, possibly allowing engine heat to warm the capilliary so
thinning the oil. I changed my oil yesterday and was intending to
disconnect the union at the gauge and allow some clean oil to be pumped
through the capiliary to see if it made any difference, but got distracted.
I still intend to do it some time soon, though.
----- Original Message -----
From: "Colin Brown" <ccbrown@cwcom.net>
To: <MGS@autox.team.net>
Sent: Monday, January 07, 2002 3:00 PM
Subject: Cold Running Oil Pressure
> Oil pressure never drops below 40psi on tickover, or rises over 60psi at
> 3-5Krpm.
> In Summer the needle stays within this range proportional to the revs as
soon
> as she fires up.
>
> In Winter, at frosty temperatures by the time the temperature gauge reads
> normal, I can see up to 50psi running pressure, but rarely any higher. I
have
> discovered that everything reverts to normal if I later hit a halt sign
and
> sit at tickover for a few moments, if I do not need to stop it can take a
> further 5 miles for normal 60psi to be indicated.
>
> I have rationalised this by guessing that the column of very cold oil in
the
> capillary initially suffers from some 'boundary' effects which inhibits
the
> transfer of pressure from the warm oil below to the gauge, dropping the
revs
> for a moment gives it a chance to warm up & perhaps drain back?
> But this capillary theory does not seem to fit with the Physics I learned
at
> school?
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