12.99 – Oil temperature and heat challenges

This entry is part 47 of 48 in the series 12 - Engine / Propeller

I’ve been having an ongoing issue with oil temperature from day one. But unlike most builders, my problem is the oil temps are too low.  Around 150 is about as high as it gets during flight.  For a long time I didn’t notice it because I was only keyed in on if it got too hot.  So it didn’t register that it was running too cold. The Continental operations manual says between 160 and 180 is normal in cruise oil temp.

Another problem is my idea of using a servo to control the damper to block the outside air from the nose oil cooler and recirculate cabin air through the oil cooler failed miserably. The problem is that the NACA duct for the nose oil cooler is useless.  Because the surface of the fuselage at the nose is angled in, the oncoming air goes directly into the oil cooler.  The NACA duct does not reduce the pressure or speed of the air.  As a result, the servo can not overcome the force of the air and remains retracted. So on our recent trip from Chicago, the “heater” was putting out about 40F air into the cabin.

I could try a stronger servo, but because of how the damper attaches to the pivot, I’m worried that it will break the damper off the pivot.

These two problems are somewhat related.

I thought that maybe the vernatherm (thermostat which controls whether oil goes to the cooler) for the nose oil cooler was stuck and was always allowing oil to go through the oil cooler. To test this I blocked the air inlet for the nose oil cooler. That helped some. But it only went up about 10 degrees. But it also allowed the damper to move which permitted cabin air to run through the nose oil cooler.

So I decided that the easy fix was to build a winterizing kit. This would be an easily installed (or removed) cover for the nose oil cooler. Since the nose oil cooler is only for cabin heat and the engine mounted oil cooler is more than capable of keeping the oil cool, closing off the nose NACA duct isn’t a problem.

Here’s the Mark 1, Johnston Velocity Winterizing Kit.

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If needed, I can close off the holes in the baffle to further reduce outside air.

Installed (I’ve started prepping it for paint so there’s some filler on it).

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When I did a test flight with the Mark 1 JVWK, this was temperature of the air coming out of the heat duct.


That takes care of the cabin heat issue.

For the low oil temps, I should replace the vernatherm that controls the oil to the nose oil cooler, but I can’t see a part number on it and replacing it requires removing the oil filter, oil filter adapter plate and maybe disconnecting the oil lines. At some point, I’ll get the part number and swap that out, but for now I’ll start with the engine oil cooler vernatherm.

I picked up a new vernatherm (used, actually) and tested it.  This is done by putting it in oil on the cooktop, heating it while verifying that it opens at correct temperature. The vernatherm in the engine is stamped “77C” which converts to 170F.  It tested correctly.

Getting to it is not the easiest thing in the world.  It’s located almost directly above the engine mount.  But an hour later, the replacement was in and I was doing an engine run-up leak check. Inspection of the area showed no leaks. So I replaced the cowling and cranked up the engine for a test flight.

This graph is from a flight about on month ago.  The OAT was 60F, altitude of the flight was 4,500′ and the engine was running at 25″ MAP and 2,500RPM.


Max oil temp was around 150 at the beginning of the flight and it drops down to 140 in cruise.

This flight is after the new vernatherm was installed. The only difference was this flight was at 3,500′ instead of 4,500′.


Max temp in cruise was 171.  Yay!  RIght in the middle of the allowable range. I’ll test the old vernatherm soon and see how it behaves.

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