Rapid Oil temperature fluctuations

W

wpbarnar

New member
I have been experiencing rapid oil temp excursions of 30- 40 degrees from cruise temperature upward and quickly settling back down for some time now in my 8KCAB. The temp excursions are so rapid (30 degree swing in less than 5 seconds) that it is difficult to believe the actual oil temp can change that fast.

I have gone through all the easy stuff: connections, probes and gauges. The electrical system is stable in voltage.

Now that I am on my 3rd gauge and the excursions still occur, I am starting to believe the oil temp indications may be true.

Any ideas?

Bill
 
Bill, it might be the thermostatic bypass valve (Vernatherm) not working correctly. The valve starts to open at about 150 degrees and should be fully open at 185 degrees. Perhaps the spring has gone bad and it is opening and closing at the wrong temps and cycling quickly. You can remove it and check it.

What is the oil temp actual temperature reading?
What is the oil pressure doing at the same time?
Which inverted oil system do you have?

Larry
 
Larry. I think you have pointed me in the right direction.

The gauge does not start fluctuating until the temperature reaches 180. In cruise it swings from 200 to 240 with a few brief excursions up to 260 plus. The oil pressure stays steady between 85-90 psi during the fluctuations. The is no change in pressure that correlates with the sudden temp change.

Your response prompted me to fly this am and try something different. Once the fluctuation pattern was established, I pulled the power to idle, trimmed for 70 mph and descended slowly. It would take a couple of minutes but it would slowly stop. Once the CHT had cooled to 230' the fluctuation had completely gone away. I repeated this twice with identical results.

It definitely looks to be related to how much actual cooling that the oil is trying to do hence your suggestion about the thermostatic valve is probably correct.

I will post my findings.
Bill
 
The vernatherm was replaced without success. I still experience rapid temperature fluctuations of 20-40 degrees once the engine has reached full operating temperature. The fluctuations disappear with time at very low power settings.

As the gauge and sensor have been replaced, I believe the fluctuations to be real. This is also supported by the fact they disappear with low power settings. They appear to be the result of a bypass or valve cycling allowing different temperature steams of oil across the sensor, hence thinking the vernatherm was bad.

My 8KCAB is equipped with an Airwolf remote oil filter. Their installation incorporates the oil temp sensor and vernatherm valve in the same housing which is installed where the oil screen normally is located. The lines to the remote filter are plumbed off of this housing.

I have removed the oil cooler but did not see anything obvious. It has not been sent out for cleaning.

Does anyone know if there is a oil cooler bypass within the accessory case? My Lycoming parts manual does not show one, but the drawing is not the best. I can not find a schematic of the oil flow internal the engine and accessory case. I can only trace the flow external: cooler, inverted system, filter.

Am I chasing a red herring or does anyone have other suggestions?

Bill
 
Bill, which inverted oil system does your 8KCAB have? Early Decathlons have the IO-320 engine with the Bellanca STC inverted oil system. Later airplanes have the AEIO-320 engine with the Christen 801 inverted oil system.

Larry
 
Bill, I spent some time looking at the Lycoming and Christen 801 oil flow diagrams, plus the Airwolf STC installation drawings. I can't see anything that could cause what you have described. One other possibility is the Champion oil filter which has a bypass valve internal, but this is a long shot. If this was my problem, I would call Airwolf and talk with them about your oil temperature fluctuations. Do you know if this indication was occurring before the external oil filter STC was installed?
Good luck and let us know what you find out.
Larry
 
Larry, thanks for digging into this. The problem started long after the installation of the remote AirWolfe filter. I have also changed oil filters after the problem started so I don't think it is related to that area.

I did find a useful Lycoming document that I have been referencing. The URL for it is:
http://www.yeeles.com/Reference/Hi-Temps.pdf

I did flush the cooler with solvent. Difficult to quantify if there was any sludge or trash that could be causing a blockage or restriction. Also inspected the internal condition of the oil cooler hoses and found no abnormalities. Based on lack of findings else where, I am now leaning toward the possibility that there is something going on internally in the cooler which may be making the Vernatherm open due to over pressure which results in a momentary bypass of the cooler. I have replacement oil cooler coming and hope to have it installed soon.

Stay tuned and I will post the results of a new oil cooler.

Bill
 
Replaced the oil cooler yesterday and had a successful test flight. The rapid temperature fluctuations have stopped. Climbed to 9000' and the oil temp was steady at 200 degrees.

I am thinking about performing a postmortem on the old cooler instead of returning it for its core.

Bill
 
let the oil cooler rebuilder do the post mortem.. and ask them for the results !
just an idea.. :)
 
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