Oil pressure and temp

Matt13C

Matt13C

Well-Known Member
Few questions about how the oil temp and pressure gages work in a 172P. First an overview of the reasons for the questions.

Yesterday, after doing the complete preflight, everything looked good, including 7 quarts of oil, as the plane should have.

I get in, start it up, 1000 rpms, look at the oil pressure and temp gauges and they are way off. Oil temp has not moved, still full cold reading and pressure is beyond the green, just between the red line and end of the green. I let it run for a few minutes, then I run it up to 1500 rpm. The temp gauge still has not moved, by this point it is normally close to center of the green and the pressure gauge is pegged at full pressure, 115 psi. At this point I shut it down and call an instructor over.

Now, I fly this plane all the time and just flew it on Sunday, nobody has flown it since, I put the oil in last time and everything was perfect for the entire 2.4 I flew it. The temp gauge always comes up a bit then slowly works its way to just to the right of straight up. The pressure gauge always reads a bit high, even has a sticker indicating a 20 high reading above it, but at 1000 rpms, it was always inside the green, even at full power it was just slightly above the green, never spiked.

The instructor, without even starting it up says, its fine, it is just from the heat. I reply that it was just as hot on Sunday and the gauges didnt read near this off. Then he says the sending unit on the airplane is faulty and it always reads high. I dont know anything about how a sending unit works so I cant respond to that, but I say I fly it all the time and I am 100% certain it has never read this high even all through last summer, can you please start it up and tell me what you see. He does, sees the same indications and again says it is fine, says it is just reading high. This time upon shut down the oil pressure gauge is still showing nearly 30 psi.

I didnt fly, I was not comfortable with the indications, the instructor said fine, but I could see he was a bit annoyed.

While I am comfortable in my no go decision I am also curious about what the problem could be. Wouldnt thin oil cause less pressure since it flows easier? And being that it was 90 out yesterday, the temp should have moved at least a little upon start up and certainly after running a few minutes and running it up to 1500 rpms. Also, if the pressure stayed at 30 after shut down, when there is nothing providing pressure, my limited knowledge on the subject might be misleading me here, but doesnt the engine need to be turning to pressurize the system? When it shuts down, should the pressure normalize?

If you have made it this far thanks and sorry for the long winded post. I have never experience this before and while I know low oil pressure is a bad thing I feel that high, while not as bad, if out of the ordinary should be checked before taking the plane flying.

Any input welcome.

Thanks
 
I don't remember much about a 172, but I had to just say kudos on your decision. Nice job sticking to your guns even though the instructor said it was "fine". I would have shut 'er down and called maintenance myself. Hell, I air returned coming out of St. Thomas a few months ago when a CHT gauge pegged on the climb. I was pretty sure it was the gauge, "pretty sure" doesn't cut it in my mind. It's not worth the chance of it being a real indication and intentionally putting yourself in a bad situation.
 
It always takes me about 10 minutes of running an engine until the temperature comes off the peg, maybe only a couple minutes less on hot days, unless something is wrong with the engine. When the engine is cold oil pressure will be on the high side, especially at higher power settings, cold day, straight 100wt. oil, etc. etc.

Reading it again I'm a little confused... you said pressure was "beyond" the green, do you mean above the green or below?
 
trafficinsight said:
It always takes me about 10 minutes of running an engine until the temperature comes off the peg, maybe only a couple minutes less on hot days, unless something is wrong with the engine. When the engine is cold oil pressure will be on the high side, especially at higher power settings, cold day, straight 100wt. oil, etc. etc.

Reading it again I'm a little confused... you said pressure was "beyond" the green, do you mean above the green or below?
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Sorry, it was above the green.

Yeah, the pressure is higher when it is first started, but never this high. Even in the winter, with a quart of fresh cold oil added and a cold block, it never came out of the green, even with a 20psi high reading, until I went to full power and even then it was never close to being pegged on the high side.

The Temp gauge never jumped off the peg to operating temperature instantly, but it always moved over a bit. Normally after start up, getting the ATIS, calling ground for taxi clearance and such it would be about half way to the middle.

It had some time to warm up, after the initial look I got the ATIS and went through everything as if I was going flying. My gf also asked if the plane sounded funny, so we listened for a bit, then I went to 1500 to check the readings before getting a taxi clearance.
 
dasleben said:
I was pretty sure it was the gauge, "pretty sure" doesn't cut it in my mind. It's not worth the chance of it being a real indication and intentionally putting yourself in a bad situation.
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Mine either and being that I have only been flying a year and a half now I don't have a wealth of experience to fall back on. If I see something out of the ordinary I like to have it explained to me before I go blasting off into a potentially dangerous situation.
 
Matt13C said:
Mine either and being that I have only been flying a year and a half now I don't have a wealth of experience to fall back on. If I see something out of the ordinary I like to have it explained to me before I go blasting off into a potentially dangerous situation.
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I haven't been flying anywhere near 50 years as nosehair above has, but I have to say, the more experienced I get the more conservative I've become. I used to do things like fly night IFR over the mountains in single-engine piston airplanes like it was nothing. Now, I'm too much of a pansy to do something like that. Everyone else I've talked to has said the same thing.

Also, remember that it's your ticket if something happens, not anyone else's. It's easy for that CFI to say "it's fine" if he's not taking the plane up. But I guarantee if something happened, you'd be the one on the hook. Always CYA.
 
Well I wont lie, If you asked me I would probably have told you the engine is fine... I probably would have asked a few other questions first, and I would have suggested that you get it all the way up to operating temperature and then see where the oil pressure is, and then decide.

As far as it being 90 degrees out, the oil is designed to work just fine up to 200+ degrees so the difference between 90 and 60 really isn't as much as you think. It's still cold oil. You didn't mention what type of oil you use, it's not that important anyway.

What the instructor told you is, like, the exact opposite of reality, so good on you for not listening to that! You're watching trends, that's good! As far as the instructor's attitude... When someone asks me a question like that I tell them what I think, but I never try to push them to fly or not fly. For one thing, that's not my job. It happens a lot that when someone asks what I think of something their final question is, "Would you fly it?" My answer is always the same, "It's not up to me, PIC." ;)

Feel free to not fly if you aren't comfortable with the situation, you'll probably never regret scrubbing a flight for maintenance reasons, but you might regret flying a broken airplane.
 
I would suggest let the airplane idle at 1000 rpm for about 10 - 15 minutes in this occasion. Just to see what the gauges would read. If the oil temp comes up and the pressure comes back down in the green, she is ready to roll. If not, shop it and see another sunrise.
 
This is pretty normal, but remember, you are the PIC and customer. If you don't like it, don't fly. I suggest going to the run up area to see if the temp comes up. The oil pressure is fine, as it is known to read high.
 
trafficinsight said:
Well I wont lie, If you asked me I would probably have told you the engine is fine... I probably would have asked a few other questions first, and I would have suggested that you get it all the way up to operating temperature and then see where the oil pressure is, and then decide.

As far as it being 90 degrees out, the oil is designed to work just fine up to 200+ degrees so the difference between 90 and 60 really isn't as much as you think. It's still cold oil. You didn't mention what type of oil you use, it's not that important anyway.
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The temperature outside would play a minor factor. It does allow the oil to flow a little better than say when it is 20 outside. Which, when I flew the plane in the winter, even on the coldest morning I flew the pressure never went near that high and the temp would at least move.

JustinA said:
I would suggest let the airplane idle at 1000 rpm for about 10 - 15 minutes in this occasion. Just to see what the gauges would read. If the oil temp comes up and the pressure comes back down in the green, she is ready to roll. If not, shop it and see another sunrise.
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If something like this happens to me again I will try that.


I appreciate everyones input. I am still curious though, what could cause an engine to read so high and not show any temperature indications. As I said, I have flown this plane many many times, I know what is normal. I know how it reacts on startup anywhere from 10 degrees all the way up to 100 and I have never seen these indications. It never pegged, even on the coldest morning and always showed even a slight indication of rising temp. Even if they normalized after a run up or idling for a while, I still would think something was off. I just dont know what it could have been.
 
It is difficult to make a truly informed decision since I was not there, but if the oil pressure sensor was faulty and there was no oil temperature then you were correct in not going. If a situation begins to sound like an NTSB report, you need to stop, and this sounds like one. "PIC reported that on run up that the oil pressure read high and oil temperature did not move. PIC was told by an instructor that the oil pressure relay was faulty (flying with a known faulty gauge required IAW 91.205)..."
I will never take off in an airplane, no matter how cold, until I see an indication on the oil temperature and oil pressure is in the green. On some days this may take a while. If after some time in the run up pad there is no indication, then it's back to the ramp.
 
IMHO, if they are putting a plane on the line where they have a note reading "oil psi reads 20 high," I'd question the whole operation. What else are they putting a band-aid on? This is your life man. Sounds like, and take no offonse to this please, you are a relatively low time pilot. Think about the these things before you go fly, and do a little research on the company you are renting from. I did about 6 instruction flights out of a school located at VNY, before I finally said enough is enough. When the owner tells you to recycle the breaker for turn cord. after in continually fails, and it's certified for IFR, what happens when I'm on an ILS or in IMC, and I have to constantly reset the breaker to make it work? "Why don't you just fix it?" I aksed. "Oh, it's fine, just reset the breaker" he says. Thanks for the opportunity to work here, take me off the schedule, and have a nice day" I say.
 
mshunter said:
IMHO, if they are putting a plane on the line where they have a note reading "oil psi reads 20 high," I'd question the whole operation. What else are they putting a band-aid on? This is your life man. Sounds like, and take no offonse to this please, you are a relatively low time pilot. Think about the these things before you go fly, and do a little research on the company you are renting from. I did about 6 instruction flights out of a school located at VNY, before I finally said enough is enough. When the owner tells you to recycle the breaker for turn cord. after in continually fails, and it's certified for IFR, what happens when I'm on an ILS or in IMC, and I have to constantly reset the breaker to make it work? "Why don't you just fix it?" I aksed. "Oh, it's fine, just reset the breaker" he says. Thanks for the opportunity to work here, take me off the schedule, and have a nice day" I say.
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No offense taken. I am a low timer, around 140 hours total.

I did much research and talked to a lot of people prior to joining this school. Things were great for over a year there, anytime even the smallest thing went wrong it seemed like it was fixed before the next day. However, after this event and a few others I am now looking for a new place to rent/train.

The people there were great and I love my instructor, it is really hard to leave him. However, I need to feel safe when I fly and I simply don't anymore in their planes.

I am still deciding how to handle the situation though. I feel I have to explain the situation to my current instructor, but I also do not want to come off as rude for all he and the school have done for me.
 
Matt13C said:
No offense taken. I am a low timer, around 140 hours total.

I did much research and talked to a lot of people prior to joining this school. Things were great for over a year there, anytime even the smallest thing went wrong it seemed like it was fixed before the next day. However, after this event and a few others I am now looking for a new place to rent/train.

The people there were great and I love my instructor, it is really hard to leave him. However, I need to feel safe when I fly and I simply don't anymore in their planes.

I am still deciding how to handle the situation though. I feel I have to explain the situation to my current instructor, but I also do not want to come off as rude for all he and the school have done for me.
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Maybe after the last oil change they put in a different weight of oil for the summer and you wasnt used to the different readings. One 172 I fly its hard to get the Temp needle up off the peg till you run it for 10 or so minutes, and oil pressure will read high with cold oil.

One of the Cirrus's I fly also has trouble with high Oil pressure even with the Temps in the bottom of the green. Once running it 10-15 mins on the ground everything goes to normal.
 
Blackhawk said:
It is difficult to make a truly informed decision since I was not there, but if the oil pressure sensor was faulty and there was no oil temperature then you were correct in not going. If a situation begins to sound like an NTSB report, you need to stop, and this sounds like one. "PIC reported that on run up that the oil pressure read high and oil temperature did not move. PIC was told by an instructor that the oil pressure relay was faulty (flying with a known faulty gauge required IAW 91.205)..."
I will never take off in an airplane, no matter how cold, until I see an indication on the oil temperature and oil pressure is in the green. On some days this may take a while. If after some time in the run up pad there is no indication, then it's back to the ramp.
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Bingo, I've flown in -30 or so in the sled at startup, and I still don't even think about moving until the needle un-pegs. Period.
 
well, as a proud owner of about 400 hours of 172 time I would have to say that the oil pressure readings overall are regularly high. I don't really think that the oem pressure gauges are that great, but it is always a good idea to look into it. In the end, maybe it was fine, but like everyone else said, its your ticket. also, suppose the gauge was bad and in flight you lost all pressure from a failure... would be nice to know about it a couple seconds before the engine told you.:D
 
mshunter said:
IMHO, if they are putting a plane on the line where they have a note reading "oil psi reads 20 high," I'd question the whole operation. What else are they putting a band-aid on? This is your life man. Sounds like, and take no offonse to this please, you are a relatively low time pilot. Think about the these things before you go fly, and do a little research on the company you are renting from. I did about 6 instruction flights out of a school located at VNY, before I finally said enough is enough. When the owner tells you to recycle the breaker for turn cord. after in continually fails, and it's certified for IFR, what happens when I'm on an ILS or in IMC, and I have to constantly reset the breaker to make it work? "Why don't you just fix it?" I aksed. "Oh, it's fine, just reset the breaker" he says. Thanks for the opportunity to work here, take me off the schedule, and have a nice day" I say.
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Sorry, but it's perfectly ok, normal, and legal to placard the error on a gauge as long as the error is consistent and the error doesn't cause a problem with the operation of the airplane.

The other stuff you mentioned is definitely a problem and shouldn't be ignored.
 
Blackhawk said:
It is difficult to make a truly informed decision since I was not there, but if the oil pressure sensor was faulty and there was no oil temperature then you were correct in not going. If a situation begins to sound like an NTSB report, you need to stop, and this sounds like one. "PIC reported that on run up that the oil pressure read high and oil temperature did not move. PIC was told by an instructor that the oil pressure relay was faulty (flying with a known faulty gauge required IAW 91.205)..."
I will never take off in an airplane, no matter how cold, until I see an indication on the oil temperature and oil pressure is in the green. On some days this may take a while. If after some time in the run up pad there is no indication, then it's back to the ramp.
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There is no oil pressure "sensor" as such. The gauge in the 172P is direct reading, which means the oil pressure line runs directly to the back of the gauge and moves the needle, no devil wires involved, just a bourdon tube ;).

Low temperature = high pressure. I wouldn't get too worked up over it, just be cautious and let it warm up a little more and see what it looks like then.

I wouldn't exceed 1000rpm until I have an oil temp indication anyway.
 
trafficinsight said:
Sorry, but it's perfectly ok, normal, and legal to placard the error on a gauge as long as the error is consistent and the error doesn't cause a problem with the operation of the airplane.

The other stuff you mentioned is definitely a problem and shouldn't be ignored.
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Legal and safe are two different things. As far as normal and o.k., keep telling yourself that. In california, it's perfectly legal to hold your car together with duct tape and bailing wire. But it's not legal to change the exhaust system, as it would no longer pass smog. I would rather drive the noisey car than the p.o.s. car.

I've been around aviation for a long time. Nearly 25 years. And I have never seen a gauge labeled that it reads wrong. If it reads wrong, IMO, IT'S BROKEN. Try explaning that to the FAA when the slap you with 91.13 because you flew with a defective oil pressure gauge and put the airplane down on a road because the engine pushed a rod through the case.

BTW, show me a reg that says you can label a required instrument not only from the Equipment list that is in the POH, but also in 91.205, stateing that you can fly with it broken(after all, if it reads wrong, it is broken/malfunctioning).
 
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