Oiltemp around 205F - Check bypass valve (M20K)

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For a longer period of time our oil-temperature is higher than expected on a M20K / TSIO-360-GB.

We see constantly temperatures between 200 and 215F. Oil pressure is always normal and we have never seen oiltemps above limits or even near.

But, all our flight analysed by Savvy come back, saying that our oiltemp is far higher than the group and we should keep an eye on it.

Also, we took a closer look on how the temperature rises: We see a nearly constant increase of the temperature up to 200°, then  the curve flattens and stays more or less constant at 205-215F.

Temperature fluctuates a few degress but is most of the time around 205-210F.

My research tells me, that  normally you can expect the temperature rising quickly to about 180F and then see the curve flattening to about 190F.

A constant rise with no flattening could be a sign of a malfunctioning oil-bypass-valve or a blocked oil-cooler.


So , I have a few more questions:

1. Where is the bypass-valve located exactly. According to the maintenance manual it should be in or near the oil cooler, but all drawings or photos I have seen are of real bad quality.

2. What else can we do to track down the problem? One tip I found says, fly and compare directly after landing the measured oil temp against the oilcooler itself. If it is only handwarm than the oilcooler does not have any effect - may be blocked or the bypass valve does not open

3. Why the oil cooler can become blocked? We never had any metal particles in our oil.

4. Can we test the bypass valve?

5. How much is a new one?

6. And - the most important question .... Do we have to worry?

Edited by NicoN

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Where is the thermocouple located on the engine?  The oil cooler can become blocked internally, restricting oil flow and cooling efficiency.



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At higher power settings it is normal to regulate oil temp by operating cowl flaps in the mid/half open position. 

This is referenced in the POH.  When I bought my GB I made the mistake of adjusting the cowl flaps to a fully closed condition.

Oil temp immediately increased.  

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I already  adjusted the left cowl flap to stay more open because of our #2 cylinder being the hottest.


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Make sure your data is coming from the expected sensor location...

Often adding an engine monitor adds another OilT sensor... which may reside in a hotter location...

Just a technical issue that may come up...

When using a JPI 900 it is acceptable to use in place of the ship’s gauges...

Lesser JPIs are not qualified as primary gauges, and can cause the sensor location challenge stated above...

PP thoughts only, not a mechanic... 

Best regards,


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When I purchased my 231 I picked it us at a well known MSC. I  asked why the cowl flaps did not close completely. The service manager explained that a slight 1/2” opening will improve cooling and won’t affect airspeed. 

I've left it alone for 12 years. I can’t see any loss of speed. My oil temp in cruise is always around 170F. #2 cylinder which is the hottest is typically 365F. 

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... and our #2 is also the hottest. In climb it easily limits the performance to stay under 380F. We have to climb with at least 120kts when coming higher.

In cruise #2 has CHT ~360 to 370F.

We adjusted already the left cowl flap to stay more open, hoping  this directs more air along #2 cyl and oil cooler - but no change.

I think as a next step e have to do the flashlight test. We may have imporved our baffling but I am still not convinced.


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