An oil temp observation

[BDPetersen]

On recent flights my factory oil temp gauge would occasionally take excursions to the high side. I didn’t take it seriously based on the other indications and the early phase of flight. Fiddled with wiring but found nothing. But I used this as a good excuse to stick a oil temp probe on the JPI. On today’s test flights, the factory gauge performed flawlessly, of course, reading a nice steady 160F while the JPI settled down at 198. Because the probes are on opposite ends of the 0-360 I expected some spread, but I was surprised it was that much. I suppose, among other things, it verifies the effectiveness of the oil cooler. Don’t see any problem here, just an interesting observation.

 

[ArtVandelay]

FWIW, I had the oil temperature on the front of the engine when I had a 830, and I see no difference between then and now.

[PT20J]

I have an IO-360 in my M20J and the JPI gauge in the front reads close to the factory gauge in the rear. Actually 160F sounds really low and 198 seems about right. I generally get between 195 and 205 depending on outside temperature. The hottest part of the oil is maybe about 40 degrees higher that the gauge. You want it to be above 212 to boil off any water that collects in the oil.

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[A64Pilot]

Most of it is attributable to the relative inaccuracy of the factory gauge,even when new they were never meant to indicate actual temps, just to warn you of the end points, many aren’t linear. Then add into how old is it?

By linear I mean if the green is from 100 to 200, then exactly half way isn’t 150 like you would expect.

Piper on their turbines several years ago upon going to digital instrumentation have the instruments have a “snap” function, that is they will snap to the expected reading and it won’t deviate from that until a set difference exists

Edited September 4, 2021 by A64Pilot

[BDPetersen]

Interesting. So my nice middle of the green factory gauge reading may have been wrong all this time.

[ArtVandelay]

You want it to be above 212 to boil off any water that collects in the oil.

Only if you fly at sea level, if you fly at 9000’ the boiling point of water is 195°.
I always pull dipstick after a flight, I let water vapor escape for about 10-15 minutes. Don’t know if it helps but figure it can’t hurt.

[Cruiser]

I do the same but leave it open until the next flight. 

[A64Pilot]

11 hours ago, BDPetersen said:

Interesting. So my nice middle of the green factory gauge reading may have been wrong all this time.

No, it’s it wrong, it’s still middle of the green and that’s still good.

There is a reason why usually factory gauges back in the day only had the numbers for the limits screen printed on.

‘Sometimes when given digital accuracy we have a tendency to become overly concerned about readings and numbers that really aren’t all that important.

‘My mentor as a Civilian test pilot had a few sayings I liked, one was “perfection is the enemy of good enough”

Airspeed calibrations for example, you can waste a lot of time and money trying to work it down to less than a kt of error, but is not required and in the end doesn’t matter.

 

Main thing to take away about factory instrumentation is not absolute values, but trending, it is where it normally is? then good, if it’s not, then maybe some investigation is in order, usually can be explained by much higher OAT’s or something, but it could indicate a problem.

Then realize most of us are flying around in decades old aircraft with factory instrumentation that has never been calibrated, so expect some inaccuracies.

Realizing that a 40 or 50 yr old gauge may be inaccurate is a very good reason to never use that gauge to adjust the engine to, whether it be oil pressure or RPM, use a known good gauge.

[BDPetersen]

Although I’m pretty sure if my factory gauge started rocking readings of 200ish I would have freaked. The probe for that gauge is near the vernatherm-oil filter complex. The installation notes for the JPI probe caution not to install it near the vernatherm ( although they clearly illustrate the place to put it in the front of the engine).

[A64Pilot]

200 isn’t high, 180 is considered the min. 200 is actually quite a nice oil temp. Autos run 220 and over normally, remember a high performance car with an oil cooler uses 195 degree engine coolant to cool the oil. so 200 would be quite low. Not that we fly cars, just a data point is all.

‘Lycoming in several of their literature recommends an oil temp between 165 and 225

[BDPetersen]

I agree. It’s just that mindlessly seeing a middle of the green indication is comforting. I’m glad to see the JPI giving the rest of the story.