92 Legacy - Where is the oil pressure sensor?

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.
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Worn bearings
Leaky oil galley
Malfunctioning oil pressure relief valve
clogged lines
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bent/clogged pickup tube
leaking pickup tube O-ring
improper oil viscosity
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Oscar_Lives wrote:

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

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.
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Vanguard wrote:

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.
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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.

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.
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Carl 1 Lucky Texan wrote:

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).
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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).
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