>
> Now with the previous mentioned car, the running one, I have a problem
> with oil pressure. When the car is cold the pressure is 50lbs after
> it has warmed up the pressure drops to 25lbs. There is not any noise
> coming from the engine, such as knocking,banging, and klunking but I
> was wondering what else it could be. I don't think that it is the
> check valve but how can you test that without installing a new one?
> I am going to try an oil pump change this weekend and I will also
> check the bearing clearance. Any other ideas? By the way, I'm using
> 20w50 in it.
>
> Tim
>
I started writing this message as "the definitive guide to how oil flows
in your engine", but when I started writing it, I realized that there were
a lot of points that I really didn't know. I'm still posting it, but if
anyone wants to take a stab at filling in the parts that I'm unsure about
or if there are any inaccuracies, please post.
In a modern automobile (actually starting with the Rolls Royce 40/50 HP
"Silver Ghost" model in something like 1909) oil has been distributed through
the engine in a presurized feed system. The oil is picked up from some kind
of reservoir (usually, but not always the sump) by the oil pump, and pumped
under pressure into an oil filter, and from the oil filter into various
passages in the block. These passages lead into the main bearings and some
or all of the bearings on the camshaft (Jaguar camshafts are hollow with a
hole drilled at each bearing, so I assume that some bearings force oil into
the camshaft, and some bearings get their oil from the camshaft. I'm not
sure how other cars' cams work). At the main bearings, the oil is forced
under pressure into holes that lead to the adjacent rod bearing. Here's one
of the parts that I'm not sure about, are the connecting rods hollow, with
oil flowing up them to oil the wrist pin, or does the wrist pin depend on
splash lubrication?
In any case, oil oozes out of the main and rod bearings at a rate that is
determined by the oil pressure and the looseness of the particular bearing.
The higher the pressure the more oil will ooze out of a bearing of a given
tightness, the looser the bearing, the more oil will ooze out at a given
pressure. The oil pressure is determined by the rate at which the oil flows.
If the oil flows at a high rate, you will have low pressure, if it flows at
a low rate, you will have high pressure. And, finally, the rate at which
the oil flows is determined by the amount of oil that is allowed to ooze past
the bearings, which is a function of how tight the bearings are and how viscous
the oil is (the thicker the oil, the slower it can ooze past the bearings).
Let's say that we have an engine where all but one bearing is tight, but that
bearing is pretty much gone. There will be a high flow rate because oil is
easily escaping past this bearing, so there will be low oil pressure. This
bearing will be well oiled, though! All of the oil is escaping past it, so
it is well lubricated. The problem is that there is no oil pressure at all
of the other bearings, and they are being starved (there is no oil escaping
past them, so parts of the bearing have metal to metal contact). I assume
this is the cause of low-oil-pressure knocking.
Now let's say that we have an engine where all of the bearings are kind of
loose. Again we have low oil pressure, but since oil is escaping past all
of the bearings, everthing is well lubricated, and there won't really be
any problems. I assume that this is the case with old engines that have
low oil pressure and no knocking. You know the ones, those that people
drive for 3 years on 15 pounds of oil pressure without any problems.
Of course, the common case is probably somewhere in between these 2 extremes.
Some bearings are going to be tighter than others. So the oil pressure really
gives you an idea of how loose your loosest bearing is, and you have to assume
that at least some of the bearings are as tight as original spec. But, as
long as your loosest bearing is tight enough that enough pressure is generated
that the tightest bearing is still going to have oil oozing out of it, you're
not going to have a problem. I've seen plenty of cars that run fine on 25-30
pounds pressure without any problem, so I assume that that's adequate.
Other possibilities for low oil pressure:
The pump itself seems incredibly reliable (I've never know anyone who had an
oil pump fail), so really isn't a usual cause of low oil pressure (especially
since bearings are supposed to increase clearances as the engine accumulates
miles).
The oil pressure relief valve is a small valve located after the oil pump
that is designed to open when oil pressure is too high (such as on a really
cold day). It is there to keep you from blowing off your oil filter seal.
When it opens, it bleeds off pressure and acts sort of like an oil pressure
regulator, although its regulation is real coarse. They can stick open or
closed, the symptoms being a blown seal on your oil filter on cold days if
its stuck closed (this has happenned to me) or excessively low oil pressure
if its stuck open (emphasis on the word excessive). Trust me, if you had
problems with the oil pressure relief valve you would *really* know it.
Is there anything that I have missed? I think my point here is that bearings
are generally going to be the cause of low oil pressure, but as long as the
oil pressure isn't excessively low and you don't have knocking, I wouldn't
worry about it.
Joe
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