Decathlon Oil Temperature Seems High

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pfyust

New member
I recently purchased a 1992 Super Decathlon 1000 TT and 80 SMOH. On short 30 minutes flights I have notices that the oil temperature is running about 220 degrees even when I have throttled back to 22 inches and 2400 RPM. Cylinder head tempertures are 300-340 degrees. It is hot outside with temperatures running about 100 degrees F. However, the oil termperature seems high.

You comments and suggestions on how to lower the oil termperture would be appreciated.
 
Just a guess, but are you running an MT prop in place of the original Hartzel?

Either way be sure all your engine baffles are in good shape and bend up or forward when the cowlings are installed. I have never had a mechanic get it right when the put the cowl back on. If the baffles the other way then air is free to flow around the cylinders and oil cooler instead of doing its job by flowing through the fins on the cylinders and cooler. The tough spot for me is just above and right of the cooler where he engine aft baffle jogs back. I always seem to have some sort of a hole there where the baffling folds together.

The MT prop made a big difference for me and I'm guessing it was from differences in the blade shapes near the prop hub. The MT fatter and less airfoil shapped there than he Hartzel and it seems to result in a bit less flow into the engine. I had 20F increase in oil temps right after we changed to the MT.
 
I will speak out in defense of the MT prop. I have an MT on my 2004 Super D and have never seen oil temps above 180-190 or so, even on hot days (over 90 degrees OAT). My guess is it's something else. Baffles are certainly worth a look - making sure they are all properly positioned when reinstalling the top cowl is a dark art, requiring someone with at least 3 hands, but it's worth the effort. How about type of oil? Are you using a multigrade? I use Phillips X/C 20-50, and it not only runs cool, but stays pretty clean, with very good consumption (don't have to add any between oil changes, 25 hours and it uses less than a quart). It also could be your oil temp gage. Try having your mechanic switch gages with one that is known to be accurate. Last suggestion - limit ground runup times to what's really necessary, so you're not starting out the flight with a hot engine, and keep some speed up during climb so that the airfow across the cylinders is sufficient. Good luck!
 
I do not own a S Decathlon and have not had high oil temperatures on my Decathlon, but seem to remember many years back replacing an item called a "thermoderm". This was on an 0320 an I believe a 7GCAA. If I remember correctly this item acted as a thermostat and determined at what temperature the oil was sent to the oil cooler. I do not see any mention of this so I thought I would throw it into the information pot, it may or may not apply.

Ken
 
I think you mean vernatherm. Its basically like the thermostat in your car that open when the engine gets hot, but instead of letting coolant to the radiator, it lets oil to the oil cooler. Yeah, that could also be a source of the problem but it’s easy to pull out and check in with a good thermometer and hot water.

I was not complaining about my MT prop, I think its far better than the Hartzel, but it has a couple small quirks. But I can safely say it was/is the cause of what I'm seeing in oil temperature changes. We initially went down the path of looking at the vernatherm, baffles and overhauling the oil cooler but none of that helped. Swapping back to the Hartzel did the trick. Put the MT on again and the oil temp went back up about 20F. Mines a 1997 and the newer ones (not sure when the changes happened) have a few minor differences to the cowling shape. Not sure if any of that helps with the oil temps, but they do seem to have a larger outlet on the bottom when compared to earlier cowls and that would pull more air through. In theory at least.
 
Thanks for the ideas. I will check the baffles and the oil cooler for a winterization plate. I bought the plane from a individual in the northern part of USA. It was near freezing when I flew it down to Texas earlier this year. The plane has a Lycoming AIE 360 engine that was overahauled a couple of years ago by Popular Grove. It has a Hartzell propeller.
 
Wow! I may be learning stuff here. Go over that dark art one more time - the rubber portions all point up and forward? Does that include the ones at the front of the engine - seems like they would show through the nose holes. And the ones along the side, where the two halves go together - Up, you say? Thanks for an informative thread. I just figured 220 was normal for this installation.
 
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