Low oil temps

[LBPilot82]

OK, so I've recently been doing some research and investigation..... It hasn't gotten me very far!!!

Here's the story. I while back I removed the original analog oil temp guage which always ran from center of the green arc to slightly above center. I installed an EI digital oil temp/press guage. When first installed, oil temps were in the mid 130's. I once saw 161 after a very loooooong full power climb out. After about 30 hours or so, I noticed that each flight produced a slightly lower stabilized oil temp. Soon after, I was seeing my peak temps around 110. Assuming there was a problem with either the guage or the temp probe, I decided to test the system. I removed the probe from the engine which required unplugging two spade connectors in order to spin it off. After plugging the connectors back in with no extensions, I tested the probe in hot oil against a Fluke meter with temp probe. From 65 degrees all the way up to 230, I never saw a difference of more that 5 degrees. A little puzzled, I re-installed the probe and went flying. Immediately, I saw temps back in the 130's. I assume my problem was a poor connection at one of the spades. On to test #2. After each of the next several flights, just prior to shut down, I noted the oil temp. Once shut down, I pulled the dip stick, sent down the Fluke temp sensor, and compared. I never saw a difference of more than a few degrees. HMMMM.

At this point, I am quite certain that I am accurately reading the actual oil tempurature. The problem is, I should be seeing numbers closer to 180 or higher. Over the winter, with OAT's at altitude close to or below freezing, I was able to get the temps up to the low 160's after installing a cover over the oil pan air inlet. Now that the cover is removed, and OAT's in the 40's and 50's, I am seeing temps in the high 130's. I should mention a few things, the engine is a C-145, I have the early style cowing with the internal pressure cowel, I have a F/M spin on filter, I do not have a modified lower cowel lip, and I do not have an oil cooler installed. The new guage was installed just prior to a new engine so all of the data is from the new engine. I am concerned about premature engine damage if things aren't getting up to normal operating temps. I don't have a CHT guage which would also be a good indicator of abnormally low engine temps. After each flight, I have water/oil dripping from the crank case vent tube, and have water drops under the oil fill cap. Both should be signs that the oil temps are high enough to evaporate any water collected in the oil. I recently flew formation with another member to an airport about an hour away. Same airplane, same engine, same cowling, he has an oil cooler installed: He showed oil temps just under 180, I showed low 130's.

If I am actually reading accurate oil temps, the only way to change it would be airflow right? When was the last 170 you saw with too much airflow through the cowling? I am a little stumped and looking for suggestions as to what is going on here.

Thanks in advance.

[GAHorn]

Your oil temp readings, most assuredly are NOT that low. Your new gauge is defective. Do NOT try to raise oil temp readings by blocking air flow using this defective gauge or you risk damaging your engine.

[Bruce Fenstermacher]

Actually Richard, until I bought into my current 170A I'd have thought there is no way your plane could be running that cold. My 170A at about 2000ft on an 80 degree day will run between 140 and 160 degrees on a new gauge. The same temperature it ran on the old gauge I thought was bad.

This in when I started taking note of the larger lower cowl lips on the '48 and early A models. A lip of 3/4 inch would be standard on a B model, I don't recall what your A had but twice that or about 1 1/2" would be normal for an A. And I'm learning this makes a huge difference.

You are doing all the correct things. It would seem that you have tested and insured you gauge is reading pretty close. I would inspect your engine baffles and make sure they are all installed. Compare it to the IPC. I'd be curious to see if the small inner cylinder baffles are installed.

If all baffles are in place and once satisfied the gauge is telling the truth I'd probably temporarily block one and then both of the cooling tubes running out the back of the inner engine baffle and directing air in the area of the filter and see what effect that has on oil temperatures. Be conservative, be prepared to pull the power back and land immediately if oil temps start to rise to high. Next I'd block off or restrict the sump cooling hole above the air filter.

I also wouldn't be to worried about your current temperatures though conventional wisdom has 180 degree oil as the minimum for boiling off water.

[GAHorn]

I previously read that "Fluke" meter to be a VOM ...and after reading Bruce's comments thought (Whaa??? that can't BE!)... then I re-read it and see it was a "Fluke meter with temp probe" (I was thinking you were merely reading millivolts and using a comparison chart.)

I am still incredulous about that temperature, however.

Keep in mind that just because the oil temperature may read 130 at the probe ... that undoubtedly it is much hotter at other areas as it circulates inside the engine.
Also keep in mind that airflow thru a cowling is not as intuitive as one might expect... Blocking off airflow can have unexpected results.

Additionally, I'm not certain I understand what you meant by "testing" your probe in hot oil. Exactly how did you accomplish that?
For instance, what control-device did you compare your Fluke meter against? By that, I mean, how did you determine your Fluke meter accurately measured oil at a particular temperature? What comparative system was used to validate that?

[Bruce Fenstermacher]

George it seems by Richards description he checked his Fluke against the gauge and saw little difference. Which tells me his gauge and Fluke are probably working pretty well OR neither of them are working and they've both failed the same, an unlikely scenario. Still it could happen.

Richard, Doug's cowl and baffles should be the same as yours other than the cowl lip. Have you compared them side by side and noted any differences?

[jrenwick]

I believe it's normal for two "identical" 170s to have quite different oil temperatures in typical operations. I often fly in formation with a friend who has a '53 170B, and he always has higher oil temperatures while flying along in the same air, at the same power setting, same airspeed, etc. Our engines and cowling arrangements are the same (stock configurations). His temps are high enough that he has found it necessary to install an oil cooler. He has inspected both engine compartments very thoroughly to try to see something that could account for the difference, and found nothing. I have the feeling that there are things that affect cooling profoundly, which could only be measured with a micrometer.

My oil seldom gets up to 180 degrees, although I've seen as high as 195 degrees while climbing above 7000' on a hot July day in Colorado. I always lean per the owner's manual instructions. The mixture control knob can have quite a significant effect on oil temperature, don't forget.

George, while it's true that oil might be hotter in parts of the engine where there are no probes, you also know that the only temperature we care about is at the inlet to the oil pump -- in other words, the oil that the engine galleys are supplied with before it does its cooling job.

[ghostflyer]

I have had a similar problem with a o-360 superior fitted to my 170a. I removed the oil sender unit but kept it connected to the aircraft system and "borrowed' my wifes electric hot water jug from the kitchen and set it up inside the cowl and boiled some water [water boiles at 212 degs F or 100 deg C at sea level] . So with the water boiling and sender unit in the water I was able to check all systems. Wife went ballistic [no sense of humor]

[GAHorn]

George it seems by Richards description he checked his Fluke against the gauge and saw little difference. Which tells me his gauge and Fluke are probably working pretty well OR neither of them are working and they've both failed the same, an unlikely scenario. Still it could happen.

Richard, Doug's cowl and baffles should be the same as yours other than the cowl lip. Have you compared them side by side and noted any differences?

Which is why I wanted to know exactly HOW he validated his Fluke's temp readings. (Hint: His message does not indicate use of a true "scientific method" in deriving his conclusions.)

The EI instruments are voltage-sensitive. A poor electrical ground will cause them to read low, as will high resistance in the power-circuit. I do not believe that possibility has been eliminated in the description.

I believe it's normal for two "identical" 170s to have quite different oil temperatures in typical operations. I often fly in formation with a friend who has a '53 170B, and he always has higher oil temperatures while flying along in the same air, at the same power setting, same airspeed, etc. Our engines and cowling arrangements are the same (stock configurations). . .

Which is ..again...why I feel his validation method is important. I am not convinced that either his Fluke...or Bruce's similar experience is a valid comparison.

...His temps are high enough that he has found it necessary to install an oil cooler. He has inspected both engine compartments very thoroughly to try to see something that could account for the difference, and found nothing. I have the feeling that there are things that affect cooling profoundly, which could only be measured with a micrometer.

My oil seldom gets up to 180 degrees, although I've seen as high as 195 degrees while climbing above 7000' on a hot July day in Colorado. I always lean per the owner's manual instructions. The mixture control knob can have quite a significant effect on oil temperature, don't forget.

George, while it's true that oil might be hotter in parts of the engine where there are no probes, you also know that the only temperature we care about is at the inlet to the oil pump -- in other words, the oil that the engine galleys are supplied with before it does its cooling job.

Oil coolers are almost never actually required in healthy C145/O300 installations....but are sometimes installed due to incorrect instrument readings.
As for the hotter oil in places other than the pump inlet... YES, I agree that is the location to sample for purposes of meeting the engine mfr's limiations. However, my comment was intended to reflect that inaccurately-low readings in that location are not necessarily of any particular concern because higher oil temps in other locations still vaporize water sufficiently to get it out of the system.

Except for very frigid operations, this engine rarely has a low oil temperature problem. It almost always has the opposite problem. (Except in aircraft such as the Swift where updraft-cooling is used.)

[LBPilot82]

Being that I am reading temps as low as I am, I am VERY suspicious of a faulty guage indication. Whether that is from a bad probe, bad guage, bad ground, whatever.... that is what I am trying to eliminate here. The instrument was installed during a complete panel overhaul and airframe rewire. Although I will hesitate to rule anything out, I doubt the problem lies in faulty grounding or supply voltage. I arranged wiring so as to include a common (new) ground buss for the entire airframe which leads directly back to battery ground.

To answer George's question, I really didn't check the Fluke meter for accuracy, rather, I thought if the meter and guage read similar, both were most likely reading good. To further verify the low end of the temp readings, I also have a new single cylinder EGT guage. Prior to every start up, it's temp reads within 1 or 2 degrees of oil temp.

As for how I actually tested the probe. I submerged the temp probe into a coffee can 3/4 full of vegatable oil. I would have used water, but chose oil since it may have some wierd properties that may influence the data. I also ensured I used only the wiring and connections permanently installed in the system, ie: no wire extensions. I submerged the probe and the meter lead (not touching the sides) into the oil and heated it slowly with a small single burner camping stove.

As I said, I find it hard to believe my readings are accurate, yet I can't seem to prove it wrong. George, do you have any suggestions as far as how to further test the accuracy of the guage? I am open to any and all suggestions.... except for the ones I don't like!!

[LBPilot82]

Richard, Doug's cowl and baffles should be the same as yours other than the cowl lip. Have you compared them side by side and noted any differences?

To be honest, no, I have not. But that seems like a good suggestion. Next time I'm at the airport, I'll take a look. If I had to guess, my lower cowel lip is probably about an inch tall. I am hesitant to start blocking airways through the cowling just in case I am NOT accurately reading oil temps. If I start doing this, and my readings are far lower than actual, I could run into trouble.

I realize no two "identical" airplanes are alike, but I find it interesting that two of the same planes, one with an oil cooler, one without, could still have readings differing by 50 degrees. I would be writing this on Doug's behalf, but I think I've got the problem.

[ghostflyer]

Water only boils at 100 deg C OR 212deg F . This is a known physical fact. The boiling point only changes with pressure. So if your gauge shows 212 deg F when the water boils it must be spot on. However if you live in the mountains you must adjust your boiling point down to compensate for the altitude.There are graphs available to work out the boiling of water at different pressures.Also you can check the resistance value of the sender unit. I am sure EI. has this in the paper work supplied with the gauge.

[GAHorn]

Do you have a dedicated ground from your new instrument panel to airframe?

[cessna170bdriver]

Richard,

The boiling point at your elevation of about 2500 feet is about 208F. That boiling point is also only for pure deionized water. Any minerals in the water will affect it some, but not very much. I'm assuming you are using a thermocouple plugged in to your Fluke, not a thermistor or resistance temperature device (RTD). The accuracy of a standard thermocouple probe is only about +/- 4F. If you put the Fluke probe in boiling water and see anything between about 200 and 215 you are probably good. On the other end of the scale, test it in ice water. If the water is fairly clean (drinkable), you'll see something around 30-35 F. I've used Fluke thermocouple meters my whole career (instrumentation engineer) and have yet to see one that was working at all be off by more than 5 degrees.

The test you did with your airplane probe and the Fluke probe seems to validate that the airplane probe is working correctly, but if in fact the airplane probe is a resistance device, then a poor connection anywhere in the circuit will affect the reading. You might try repeating the oil submersion test with the probe using the aircraft wiring exactly as normally installed, if possible.

[FredMa]

did you say you stuck the meter leads into the oil? what is the resistance of the oil? I would try to find a way to do that test without sticking the leads into the oil and use a calibrated meter.

[cessna170bdriver]

did you say you stuck the meter leads into the oil? what is the resistance of the oil? I would try to find a way to do that test without sticking the leads into the oil and use a calibrated meter.

On a Fluke meter that reads temperature, you use a thermocouple probe instead of meter leads. Nothing about the oil other than its temperature will affect the reading.