Phil,
You described the exact same phenomenon that I have for 1.5 years now.
I was worried too. Replaced the oil pump and all crank bearings. No
difference.
Changing the type of oil (now using an oil especially designed for classic
cars) "improved" the situation a little (hot oil pressure increased by 5 psi)
.
Meanwhile drove 10k miles and no negative effects found.
I support your theory that it is temperature related. I am now using a
slightly adapted BBQ-cooking thermometer sensor as the dipstick. The leads are
just long enough so that the read-out is in the car. My experiences:
Driving the car 45 minutes or less at speeds up to 75 mph never brings the
temperature over 90C (200F). Longer driving slowly increases the temperture to
95-100C (205-215F).
Living in the Netherlands, driving at 80-85 mph is the norm. 30 minutes or
more at this speed also brings the oil temperature to 100C (215F). Driving
90-95 does this in 15 minutes. Extreme driving (rallying at full throttle, or
constant driving at speeds over 80 or more for more than an hour) showed
temperatures up to 115C (240F). Due to high rpm's pressure was not a problem
(except when suddenly forced to slow down).
Slowing down to 75 mph (for about 5 minutes) lowers the temperature to about
95C (205F) with associated rise in oil pressure.
Outside temperature hardly influences the oil temperature. Also changing the
thermostat to a lower temperature type also did not make any difference.
Soooo, I don't worry any longer and just make sure that the temperature
doesn't go much over 100C by slowing down in time and by not hitting traffic
jams. If you don't want to be restricted this way, buy an oil cooler (I
considered doing so, was discouraged by my part-supplier!?, and can live with
the present situation).
Date: Mon, 23 Jul 2001 21:56:53 -0500
From: "Phil Beckman" <phil@opentext.com>
Subject: Chicago-Cincinnati and back
Phil wrote:
RPM was down to 50 PSI at speed and about 10 PSI at idle. I pulled off
and phoned a couple of 6-packers to get advice. Greg Donegan advised me to
press on, saying that my gauge readings were fairly normal for an engine
with 73,000 mile on it. I drove on. By the time I'd been on the road 2
hours, the pressure was 35-40 PSI at 3000 RPM but it never fell below that.
What I found was that the longer I drove at speed, the further the pressure
would fall until it eventually stopped at 35-40 PSI. If I slowed down,
stopped at a traffic light and idled for a bit, the pressure would jump
back
to 50 PSI at speed for a while before settling back to 35-40. Seems odd.
Only thing I can think of is that slowing the car to an idle for a bit
would
allow the oil to cool down some, thus allowing the pressure come back up.
The other oddity is that the the oil pressure at idle never varied from 10
PSI regardless of the pressure at speed. So the difference between
pressure
at idle and pressure at speed varies wildly based on how hot the car is. I
replaced all the crank bearings a couple of years ago so, they're probably
ok. Not sure if I've got a problem or not. Any input people have on my
oil
pressure and whether it should worry me would be greatly appreciated.
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