Hey folks! I've been battling overheating issues with my Rotax 914 Rans S-19 in Phoenix, AZ when OAT is 85F+. During initial climb out, I can see upwards of 255F on coolant and 265F CHT (245F/255F if I'm careful about leveling out and step climbing). Cruise is fine, but still more toasty than I'd like: 220F on coolant and 240F CHT. Ground is also fine. Other temperatures appear to be fine as well.

It's the old style cylinder heads, but there's a coolant probe installed on a part of the tubing.

The original owner/builder looks to have installed a coolant/oil heat exchanger from mocal in addition to the flat radiators for coolant and oil. I've never used a heat exchanger since I've been mainly in warm climates, but does that stick out to anyone here as problematic for hot climates?

  • Re: Coolant/oil heat exchangers

    by » 11 months ago

    I am assuming the oil/coolant heat exchanger is your only means for cooling the oil? If so, the oil could be contributing to high coolant temps and vise versa. What temps does your oil run at?


  • Re: Coolant/oil heat exchangers

    by » 11 months ago

    There is a separate radiator for coolant and another for oil as well that are at the end of the air cooling path!

    From what I'm reading on how heat exchangers work, an added note that my plane was originally based in Salt Lake City, UT where the unit would've probably helped a good deal in getting everything warmed up.

    I don't have too much data written down, but some data I took at 6500 in a cruise config:

    75% power config (31inHG, 5000rpm)

    Oil temp: 218F

    Coolant: 234F

    CHT: 246F

    EGT: 1406-1477F

  • Re: Coolant/oil heat exchangers

    by » 11 months ago

    Technical point:  Heat Exchanger - A device fort transferring heat from one medium to another.

    In this context, Rotax engine cooling systems;

    Your Coolant Radiator and Oil Cooler are both "heat exchangers" of the liquid to air type.  Not so common in small aircraft, you can also have a liquid (oil) to liquid (coolant) "heat exchanger"

    To minimise confusion, I would suggest using the term Radiator to indicate you are referring to the water/glycol coolant system and Oil Cooler when referring to the oil cooling system.

  • Re: Coolant/oil heat exchangers

    by » 11 months ago

    Your coolant and oil temperatures appear to be just within Rotax max.

    I would be uncomfortable with this, preferring to see cruse temperatures, both oil & coolant, in the low to mid 90C (194-203F) range 

  • Re: Coolant/oil heat exchangers

    by » 11 months ago

    Ah, thank you for the terminology and the heads up to minimize confusion!

    So for folks reading, the installed equipment are:

    1x Oil Cooler (liquid-air)

    1x Coolant Radiator (liquid-air)

    1x Oil/Coolant Heat Exchanger (liquid-liquid)

    That's pretty interesting that the liquid-liquid heat exchanger isn't something you typically see on small aircraft. Perhaps it's keeping oil and coolant temperature floors elevated to the point that it's almost like a feedback loop in high OAT conditions. Or perhaps my cowling needs more efficient ducting to ensure the Coolant Radiator and Oil Cooler are getting sufficient air flowing over their fins. Or maybe the fins need some compressed air to clean everything out. Maybe all of the above!

    One of these colder mornings, I'll have to get a camera rolling on my displays to capture all the data while doing some pattern work since that's where we really get close or exceed limits (no data logging/export on my TruTrak displays from what I can find). I know CHT and coolant gets high, but I'm unsure where oil temperatures were at. Then this weekend, pull off the top and bottom cowlings and do a deeper dive into the setup and do a little inspecting/cleaning.

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