Diagnosing high oil temps

[Raptor05121]

So I have noticed this since I bought the plane, but my oil temp gauge seems to *almost* peg out in-flight. On a cold winter day, the needle is below the 80, and it warms up like you would expect. I takeoff around 120*F and it quickly climbs past 180, and settles around 220-240. I notice red line is 260. I have thought this entire time the scale wasn't linear but upon seeing other cluster gauges with oil temps indicating 180, I'm beginning to wonder if I have a problem. The fact the needle moves slowly and isn't totally pegging out leads me to believe the temp probe and gauge are working. I replaced the oil cooler last year due to a leak. I've noticed after flights its mildly warm, should it be scolding hot? I'm wondering if I've got a sticking vernatherm now

Stock cluster gauge
New oil cooler last year
Aeroshell 100W year round
Burns 1at every 6-7hrs

[Guest]

I would start with placing the oil temp probe in a can of boiling water while still connected to the plane.  This should at least confirm 210-212 degrees.

If this is correct,  then likely your oil vernatherm valve or it’s seat located in the oil filter adapter is suspect.

Clarence

[MIm20c]

Mine stays consistently in the 190 ish range. I normally take off when it reaches 130.  I would pull the probe and boil it in water to see how accurate it is. 

[RLCarter]

Like the 2 M's above I would verify the accuracy by heating water and using a digital thermometer to monitor the water temp. Keep both of the temp probes close together and suspended in the water and not touching the actual pan, 2 person job and 20° increments things will move quickly unless you have a way to slowly raise the temp

[bonal]

Not too often I am able to add comment on MS with direct experience but with ours we used to see real nice oil temps in the middle of the green arc.  Then had Garwin cluster redone and calibrated and found that the oil temp needle was sticking at the mid point and not even close to accurate and after the work on the gauge was getting close to red line on hot days.  I did the hot water with a sep thermometer and found oil temp was reading 19 degrees too high on the gauge.  installed a JPI slimline oil temp gauge and it was consistant we were in the 230 degree range on the worst days. checking the vernatherm we found the plunger was worn allowing oil to bypass the cooler and go straight back to the motor.  It would travel from open to closed but because the plug was worn was not making a good seal. When you have it out you need to check the condition on the case side because that could be worn as well.  It works opposite of an auto thermostat in that it closes when hot not opens.  When its closed it forces the oil to the cooler.  After replacing the vernatherm we saw a big improvement as much as 15 to 30 degrees cooler depending on outside conditions.

[Raptor05121]

Thanks guys, will give it a try.

[MIm20c]

@Raptor05121 Today I paid attention to the JPI oil tem in my C model.  Stayed between 183 and 186 degrees in cruise with an OAT of 50. 

[AlexLev]

Is it bad if your oil temp is that high without crossing redline if your CHTs are okay? It seems like it would be good to have it hot to burn off any condensation inside the engine and keep it corrosion free.

[carusoam]

Such a simple system...

• temp sensitive control valve... vernatherm... closes more with higher temp...The more closed, the more oil heads to the radiator...
• Hot oil temp diverts more oil to this cooler...

not sure how often vernatherms fail or if they have a direction they like to fail in...

I would expect that they have a tendency to fail closed sending most of the oil to the oil cooler. Better to be cold than losing its ability lubricate...

There is a temperature that the oil decomposes and breaks down.  Broken molecular chains lose the viscosity required to lubricate parts.

If the oil is cold it has difficulty getting distributed and causes some higher oil pressure issues... There is a large yellow zone on the oilT gauge for that...

The hottest spot for engine oil is the oil lubricating the valves in the guide...  it is briefly getting as close as possible to the EGT, cooling the valve that is also sitting in the exhaust stream...

other pieces of similar data include...

French fries cook in 375°F oil... thermal break-down of engine oil can occur near and above 400°F Depending on environmental conditions...

We have an MSer who has a ton of experience in machine maintenance and oils... see if he is around for this thread...

PP thoughts,

-a-

[cbarnes403]

What was your oil pressure when you notices the high temp.  High oil temp would also effect the oil pressure by reducing it.  Could just be a bad gauge, time to upgrade to a JPI.

[tony]

Are you getting temps like that in the winter?  I would be extremely bothered by that.  

[1967 427]

I feel your pain in trying to determine the real reason for your gauge reading high.  Several months ago on a quick little flight, and I noticed the temp gauge almost pegged.  On landing I checked the oil cooler by touching it and found it dead cold.  Being methodical I pulled the TC and placed it in boiling water along with a multimeter that has a thermocouple.  Both multimeter and the aircraft gauge correlated.  Next the vernatherm was pulled and placed in the same water, it didn't open, problem solved.  It was replaced with a new 85C or 185F vernatherm.  After the vernatherm was replaced, life was good for about 2 months.  Flying up to Tahoe I noticed AGAIN an elevated temperature, not quite pegged but real close to the red line.  On landing I checked the oil cooler, and observed some temperature there, I felt like all was working well, and the reading was false.  A couple of days later I fired up and watched the gauge very closely.  On a 60 degree day and at start up, it was already indicating 80+ degrees and quickly rising.  My first thought was the thermocouple.  I spent a lot of time checking resistance reading from the TC, and they looked good.  (I'll shorten this way too long post).  The gauge was removed from the 6 pack, contacts cleaned, and now it works perfect.  Poor contact increased the resistance and gave a high false reading.  Now in cruise it sits solid 185 to 190.

Edited January 29, 2018 by 1967 427