92 Legacy - Where is the oil pressure sensor?

[Vanguard]

1992 Subaru Legacy L wagon AWD non-ABS non-turbo

I think my oil pressure sensor is on the fritz. The oil light came on
so I figured that I was low on oil. Since I was driving, reading the
oil level on the stick wasn't possible so I dumped in a quart of oil.
Turns out now I'm about a quart too high; i.e., when I let the car sit
so I can read the stick, it's over the high mark. So it wasn't that the
oil was low. I haven't burned up the engine yet in the 2 weeks that it
came on. It doesn't light up right away but takes about 1 to 2 miles of
driving whereupon it started to glow dimly and then get stronger. I
don't think there is a problem with the oil level. So the possibilities
are too low pressure from the pump or a bad sensor. I'm targeting the
sensor for now.

So where do I find the oil sensor? Is it just something that I can
unscrew to put in a new one? Cost is $15 so I figure it would be
cheapest to start here and just replace the thing rather than pay for a
diagnosis.

 

[Oscar_Lives]

1992 Subaru Legacy L wagon AWD non-ABS non-turbo

I think my oil pressure sensor is on the fritz. The oil light came on so
I figured that I was low on oil. Since I was driving, reading the oil
level on the stick wasn't possible so I dumped in a quart of oil. Turns
out now I'm about a quart too high; i.e., when I let the car sit so I can
read the stick, it's over the high mark. So it wasn't that the oil was
low. I haven't burned up the engine yet in the 2 weeks that it came on.
It doesn't light up right away but takes about 1 to 2 miles of driving
whereupon it started to glow dimly and then get stronger. I don't think
there is a problem with the oil level. So the possibilities are too low
pressure from the pump or a bad sensor. I'm targeting the sensor for now.

So where do I find the oil sensor? Is it just something that I can
unscrew to put in a new one? Cost is $15 so I figure it would be cheapest
to start here and just replace the thing rather than pay for a diagnosis.

Worn bearings

Leaky oil galley

Malfunctioning oil pressure relief valve

clogged lines

 

[Oscar_Lives]

Worn bearings

Leaky oil galley

Malfunctioning oil pressure relief valve

clogged lines

bent/clogged pickup tube

leaking pickup tube O-ring

improper oil viscosity

 

[Carl 1 Lucky Texan]

bent/clogged pickup tube

leaking pickup tube O-ring

improper oil viscosity

The fact that it begins after the oil warms up leads me to believe you
have worn bearings OR possibly fuel contaminated oil which lowers
viscosity.(both mentioned by Oscar) I'd suspect a bad sender to be bad
at all times. I suppose inspecting the cable harness from it would be a
good idea.

Carl

 

[Vanguard]

The fact that it begins after the oil warms up leads me to believe you
have worn bearings OR possibly fuel contaminated oil which lowers
viscosity.(both mentioned by Oscar) I'd suspect a bad sender to be bad
at all times. I suppose inspecting the cable harness from it would be
a good idea.

I can do an oil & filter change to make sure the problem doesn't appear
with new oil. I don't see anything noticable in the oil on the
dipstick. I use 10W-30 year round and been doing so for decades (well,
not on this car, just 12 years on this one). I guess a plugged oil
filter can also be a problem source, so the oil change would take care
of both (possibly contaminated oil and possibly a plugged filter).

Which bearings? For the oil pump? With the disassembly to check its
bearings, why bother since you have it all apart to put a new one in,
anyway? Not cheap but since you're there with it all apart, and since
it is the original one that is now 14 years old, might as well as go for
it. I just wanted to start with the simplist possibilities first.

How would the pickup tube suddenly get bent? Wouldn't that require
removing the oil pan which has never been done?

Apparently there is a pickup filter screen inside the pan which requires
removing it to check for debris. Oh goody. So does the pan take a
pre-made gasket or do I goop on gasket sealer?

One trick I read was to disconnect the oil pressure sending unit. If
the idiot light comes on then there is a grounding problem or with the
instrumentation (i.e., another screwup in the computer). I had the
engine check light come on repeatedly but no shop ever found any error
codes, even when it finally came on when I was at the car shop about to
leave, started the car, it came on, and they came out to read the codes.
Engine light but no codes. Now the oil light which probably goes
through the computer, too.

I don't know if there is a means of actually checking the oil pressure,
and where to check it. I saw mention of a mechanical oil pressure guage
but have yet to notice one of those in the auto parts stores, but then I
wasn't looking, either. I think you remove the sending unit and screw
in the guage in its place. So what would be the range for good pressure
versus too-low pressure? If I go this route to actually WHAT is the
pressure, it comes back to WHERE is the sending unit. I'm still trying
to find my Chilton's book.

 

[johninKY]

My guess is the sender will be found very near the oil filter/pump.

 

[Kurt]

I can do an oil & filter change to make sure the problem doesn't appear
with new oil. I don't see anything noticable in the oil on the
dipstick. I use 10W-30 year round and been doing so for decades (well,
not on this car, just 12 years on this one). I guess a plugged oil
filter can also be a problem source, so the oil change would take care
of both (possibly contaminated oil and possibly a plugged filter).

If it's not too cold where you live, try 20-50 oil.

If it were my car, I'd get an oil pressure gage and see what's going on.
Normal oil pressure is in the 30-60 psi range. Many of those oil
pressure switches work at about 5 psi. When they say the pressure is
low, it is scary low.

 

[Vanguard]

My guess is the sender will be found very near the oil filter/pump.

So far all I've got from the shops (for other problems with other cars
and poking for info) is that it is somewhere in the engine block. I
found the Chiltons book but haven't gotten to it yet. Late now so I'll
read it at the car shop tomorrow.

I thought the mechanical oil guage was some special tool that you had to
buy. Nope, it's just an oil guage kit that you buy at the auto parts
store. The shop wants $90 to check the oil pressure using their own
guage (and since it was all apart and the sensor was only $13 then I
told them to replace it anyway). However, after slapping my forehead
with not realizing what a "mechanical oil guage" is, duh, and if the
sensor doesn't require a contortionist with skeletal arms to reach it,
I'll just get the $25 oil guage kit (and $13 oil pressure switch) at the
parts store and check the pressure myself (and replace the switch since
it'll be apart, anyway).

I've got an appointment tomorrow morning to have Tires Plus do an oil &
filter change (they use synthetic blend) for only $17 with a coupon that
I found. I figure that's the cheapest possibility so far (that the
filter is plugged and/or the oil is contaminated to reduce its
viscosity). I'll stick in some Seafoam in the engine oil while idling
only in their parking lot before handing it over to them (I don't want
to drive or load the car with the cleaner in the oil but I'd like to
ensure to get it as clean as possible and maybe hit the mesh filter in
the oil pan). I checked with them and they don't see a problem if I
don't drive with the Seafoam in the oil. Gunk has their oil cleaner and
it says on the bottle "DO NOT DRIVE" and to be used as prep for an oil
change.

 

[Vanguard]

If it's not too cold where you live, try 20-50 oil.

Minnesota winter here. Sometimes cold, sometimes miserable, and nearly
always gray skies. Many folks here switch to 5W-30 but I stick with
10W-30 year round. Getting an oil & filter change tomorrow just to be
sure the filter isn't plugged or the oil contaminated. They put in
synthetic blend as their standard oil and it'll only cost $16 (with
coupon), a good deal since 5 quarts synthetic blend and a filter would
cost me more than that just in materials.

If it were my car, I'd get an oil pressure gage and see what's going
on. Normal oil pressure is in the 30-60 psi range. Many of those oil
pressure switches work at about 5 psi. When they say the pressure is
low, it is scary low.

Yeah, I thought it was some special oil guage testing kit that I needed
(some run around $180). Nope, I can just get a $25 oil guage kit to
check the pressure. Since I'll have it apart and be working on it,
anyway, the oil switch is only $13 so I might as well as pick one up
with the oil guage and put it in - but I still want the guage to get a
real reading of the actual pressure value because, as you say, and *if*
the switch is not failing, they trigger on pretty low pressure.

 

[Vanguard]

So far all I've got from the shops (for other problems with other cars
and poking for info) is that it is somewhere in the engine block. I
found the Chiltons book but haven't gotten to it yet. Late now so I'll
read it at the car shop tomorrow.

Well, the manual was of no help. Its instructions are short and simple
but no clue where to find the oil switch. I'm wondering if I can see
if from above by looking down under the hood or if I have to jack up
car and slide underneath and hunt around.

 

[Vanguard]

I'd suspect a bad sender to be bad at all times.

Actually I'm wondering when the oil switch is supposed to start
functioning. That is, when should the idiot light circuit be enabled
to then report low oil pressure. When you start your car, pressure
will be low. When starting, the light with all others comes on as a
check that they will light up (so you know if one is burned out). Then
the oil light goes off. If it was immediately enabled then it would be
on when you first start running your car. How long would depend on how
far away from the oil pump so pressure could build up.

That's why I'm wondering what the bimetal element inside the oil switch
is for. When reading the Chilton's book, it shows a cross-section of
the inside of the switch and notes there is a bimetallic element inside
as part of the switch. Maybe the oil switch does not get enabled until
the engine warms up to heat the bimetallic element to allow the switch
to then operate. That is, when cold and regardless of actual pressure,
the oil switch is not supposed to work. Then when the engine heats up
so does the bimetallic element in the oil switch which enables that
switch to close if pressure is too low (I'm presuming the switch has to
close to complete the circuit so the idiot light comes on).

So if the bimetallic element in the switch were bad, it would explain
why the idiot light doesn't come on until I've driven the car about a
mile. I will test this by getting into the car when it has sat cold
for awhile, like at the end of the workday, start the car, and just let
it idle in the parking lot to see when and if the idiot light comes on.
So, yeah, the switch is probably either always bad or always good but
the bimetallic element delays when that becomes evident. Of course, if
the oil pressure is low then that would also be the expected behavior
of the oil switch: to delay when it reports low pressure until the
bimetallic element heats up.

If I can't find the oil switch after work today (so I could buy and use
my own mechanical pressure guage and replace the switch), it goes in
tomorrow to the shop to have them check the pressure readings at
various RPM (and go ahead with putting in a new switch since it only
costs $13).

 

[Vanguard]

Turned out to be the oil switch (sending unit). Pressures measured with
a guage were:

PSI RPM
---- ----
15 idle
25 1000
55 2000
75 3000
80 4000

So the behavior that I saw was for a bad switch where the pressure was
good but the switch doesn't get enabled until the bimetallic element
inside enables the switch (i.e., the light didn't come on until the
engine got warm but it was a false indication).