> I have a (probably stupid) oil pressure question. What affect does oil
> viscosity and engine temperature have on oil pressure?
Like most engines, the TRactor motor has an oil pressure relief valve that
opens at a particular pressure, around 60-70 psi. As long as your oil
pressure is high enough to open that valve, then viscosity and temperature
will have little effect.
Below that pressure, the pressure is directly related to oil viscosity
(thinner oil = less pressure), which is directly related to oil temperature
(hotter oil = thinner oil).
> Also, what is the
> typical cause for low oil pressure readings?
Assuming the level is adequate, the most common is worn rod and main
bearings. This is the reason for the fascination with oil pressure as being
an indicator of overall engine state of wear. Other causes include :
1) Oil too thin, either an inappropriate weight or due to viscosity improver
breakdown.
2) Stuck relief valve
3) Broken or severely worn oil pump
4) Oil overheated (which can lead to #1 as well).
> My "cold" pressure
> readings at
> idle are decent, but as the engine heats up, my oil pressure
> drops. Also, I
> don't notice a big change (once I reach temperature) between idle and "at
> speed" pressures.
That strikes me as odd, you're saying that it's low at hot idle, and doesn't
increase at speed ?
I think I would start by checking out the pressure relief valve (that big
screw with the locknut, sticking out of the oil filter head), and then get
independent verification of the oil pressure. I keep a cheap ($15)
aftermarket oil pressure gage on hand for just such tasks, ever since a
local garage told my wife that her (non LBC) engine had to be rebuilt, and
the problem turned out to be a bad oil gage sender.
In essence, all the bearing clearances and so on form a fixed size oil leak,
that changes little if any with engine rpm. How much oil flows through
depends only on pressure and viscosity of the oil. However, the oil pump is
a "positive displacement" type, how much oil it delivers is directly related
to engine rpm. Thus without the relief valve, oil pressure would always
depend on rpm. And below 50 psi or so on a TRactor motor, the relief valve
should be completely closed.
Randall
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