oil temp variation.....what is normal??

[flyboy1963]

Flyin yesterday, air temps were mild, (+15c) & after about 10 minutes airborne, I reduced power ( 2100 to 1700-ish) to start my descent and happened to catch the oil temp swing from 185 down to 155 in just a few seconds!
Could this be the vernatherm redirecting oil from the cooler past the temp sender? I can't recall the entire flow schematic for my engine....but seems the change would be more gradual!
I'm still running a partial cover behind my oil cooler, so it would not be ice cold!
call me curious.

[DanH]

Just a few seconds? Sounds like an instrument problem.

[Andy Hill]

May well be poor earthing of the sender. Keep an eye on it, and consider turning Alternator off and seeing whether it still fluctuates?

If it is a single wire sender, consider the GRT 2 wire sender and earth that to the Instrument, not via all the engine / alternator earthing routes.

[flyboy1963]

....I've seen this a few flights ago when banking from downwind to base...likely also including the same power reduction.
Is it possible I'm taking the sender out of the hot oil into the crankcase air momentarily? Frankly I don't recall where the temp sender is...I think on the rear of the dome over the oil screen ( no filter) so that's not likely...it is a single wire sender.

Will check wiring and grounds as suggested.
thanks gentlemen!

[lr172]

sender on a Lycoming 4 cyl is on the oil screen cover or oil filter adapter. It sits in the flow to engine after the oil cooler route.

If the vernatherm is open, oil is coming straight from the sump and will take a good amount of time to cool (lots of minutes and certainly not seconds). Theoretically, If the vernatherm just happened to close, the cooler will drop the temp quickly, as it is reading after the cooler. I would expect a noticeable drop when it closes. However, the vernatherm closes slowly over the temp drop of the input oil (see above), so I would expect the temp to drop gradually, not suddenly. It is possible that your vernatherm is sticking and snapping closed, but that seems unlikely and the timing is highly suspicious.

The fact that you are getting this during throttle reduction makes me think it is not related to the actual oil temp. I would be watching my voltage as I reduced throttle. A defective voltage regulator could cause the voltage to drop or increase as the RPM changes. This could potentially cause a variation in the reading.