Miata Mailing List: May 1994, Message #159
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   From: a.mccombs3@genie.geis.com
Subject: Re: Oil Pressure Indication
   Date: Sat, 7 May 1994 06:33:48 -0400

Yet still another for Mike L. (and anyone else who wants to bother).  For
many years now, it's been somewhat of a rarity to find an oil pressure gauge
in a car (including sports cars - my ex-1980 Triumph TR-8 didn't have a
pressure gauge, only an idiot light).  Thus, a lot of people wonder about
what that gauge is saying when they run across a car which has one.  (I
suspect Mazda installed one in the Miata not because of any altruistic sense
that only knowledgeable drivers would buy such a car, but rather because it
fits with the somewhat "retro" and "racey" feel of some aspects of the car.
Notice that they positioned the gauge up high in the center of the dash,
where it is readily visible.  This is hardly necessary on a modern
production car, but I like it anyway and it has historical precedent to back
when sports racers were British and they kept the dust down at the track
with the oil belched from blown MG and Triumph engines.)
 
Anyway, that gauge can tell you a lot, most of which may be bad news, but
with a Miata, such bad news is not likely to occur for many, many years if
you take halfway decent care of the car.  What it is telling you is the oil
pressure AT THE PRESSURE SENDING UNIT in the oil system.  Pressure sending
units can be located about anywhere in the oil system, so readings between
different makes of cars are largely meaningless.  However, you can rest
assured that yours will not move around on your car, and thus under the same
conditions the pressure should read the same.
 
The pressure should be neither too high or too low (refer to another source
for numbers; I'm fresh out.  Yeah, I know, big help dept.)  Too low and you
risk too little oil being pumped to the bearing and wear surfaces and
possible scoring of surfaces - bad news.  However, at idle, with little or
no load on the engine, 10 - 15 psi should be sufficient, although I get
uncomfortable if I see pressure that low.  (remember that many of the "idiot
light" low pressure warning lights are set to trigger at around 3 to 5 psi.
This is generally sufficient, IF the engine is not under load and not
turning fast, and you shut it off at the first indication.)  In other words,
the warning light system is strictly a black/white, no in between system,
whereas with a gauge you have a TREND instrument.  If you're driving along
and the pressure has stabilized at around 60 psi - but then slowly begins to
drop over a period of e. g., a half hour or so, by e. g., 10 psi or so - you
have a problem of some sort which is making itself known.  A warning light
would never tell you about it until it was too late.  By observing the TREND
of the instrument, you can probably determine that things aren't going to
hell too rapidly, and may well be able to drive to the next spot of
civilization to take care of it - then again, maybe not.  (Could be
something as simple as a loose oil filter or drain plug.)
 
Mind you, variations in pressure can be caused by other things too.  Driving
from colder to warmer weather will generally cause pressure to drop a bit
due to higher oil temps. and thus lower viscosity.  (Even so called
multigrades get thinner when they get warmer.)  Anyway, get used to the
trends of your gauge - under the same circumstances, the gauge should
provide the same indication every time.  The precise number indicated isn't
so important, so long as it's reasonably close to a median between minimum
and maximum allowable, and it's not showing signs of changing under the same
circumstances from day to day.
 
Too much oil pressure can be hazardous to the health of your engine too.
You might have noted the msg. about keeping it down below 4000 RPM until the
engine warms up a bit.  The faster the engine is turning, the more pressure
the oil pump puts out.  (This has nothing to do with the load on the engine;
the oil pump doesn't know how much oil the bearings need.  It only knows how
fast the pump drive is turning the pump.)  While there are pressure relief
systems, there is less margin between optimum and maximum when you have to
rely on your pressure relief (either in the engine oil system or the filter,
whichever).  When the pressure gets too high, you run the risk of blowing
out seals and the like - and getting some truly horrendous oil leaks.  This
can be most frustrating (and expensive) when it's the rear main, and you
find yourself with a freshly oiled clutch.
 
Note also that the COOLANT and the OIL will almost certainly warm up at
different rates.  Even after your coolant has stabilized at normal operating
temp. (which doesn't take long on a Miata), your oil is probably still below
normal operating temp - and thus ABOVE normal operating viscosity.  Take it
easy for a bit longer before you accelerate flat out to red line.  Also,
since the oil system on a Miata has no thermal control, it will run cooler
in cold weather, and take longer to stabilize at a given temp - even though
your coolant gauge has long since stabilized.
 
As you may have noticed from other msgs., different filters and different
oils will cause the gauge to read differently, even with the same ambient
temp, same RPM, same coolant temp, etc.  Again, so long as it's somewhere
near the middle of what's permissible, (personal opinion here), I wouldn't
worry about it.  What do I do?  --I try to keep the pressure indication as
close to the center of the gauge as possible.  This means keeping the RPM
down until the engine is thoroughly warmed up, and using multiple grade
viscosity oils (as noted above, they thin when they warm, but not so much as
single weight oils).  This technique has been working well on a '71 Datsun
240Z with oil pressure gauge and close to 200,000 miles on the engine now -
and it still uses a little less than a quart of oil between changes every
3000 + miles.  No new seals, no noise, no internal problems.
 
And now that it's thoroughly warmed up and in the middle of the gauge at
cruise RPM, HAVE A BLAST!
                                                --Jack M.--