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Thread: High Oil Temp

  1. #1

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    High Oil Temp

    Have stock Super Cub 150. Engine overhauled 200 hours ago. Before overhaul, oil temperature always on low side - less than 180F. Since overhaul, oil temperature has become a problem. It has become very sensitive to the ambient temperature. Figures would roughly be the following:
    Ambient 0C/32F Oil Temp settles 180F. Ambient 10C/50F Oil Temp settles 190F. Ambient 20C/68F Oil Temp about 210F. Ambient 30C/86F (unusually hot for where I live) Oil Temp can get to 235F!!!
    I only have CHT reading on cylinder 4 - also seems hot 200 to 220C - although not sure how accurate the CHT instrument is.
    Have tried various things - Oil Cooler gets hot so presumably Vernatherm working OK - Oil Temp gauge calibrated for accuracy - Baffles carefully reworked.
    The Cowling setup on the Cub seems very inefficient with a large gap around the oil cooler allowing ram air into bottom of cowling - also large air gaps in air intake area around the starter ring and so on - but this doesn't seem to be a problem for most Super Cubs.
    Any ideas?
    Would a baffle fabricated around the oil cooler to only allow ram air through the cooler and not around it be of any use.
    Any thoughts gratefully received.

  2. #2
    stewartb's Avatar
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    Tape the gaps around the cooler with duct tape and see if it helps. Air leaks pressurizing the bottom are usually associated with high CHT, and your CHT is unusually low. I’d be checking that instrument to make sure you aren’t running hot there, too.

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    Do this and report back. Click image for larger version. 

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  4. #4

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    Thanks stewartb and AKaaron for replies. My post has a mixture of temperatures in Fahrenheit and Centigrade which is a bit confusing. The CHT gauge is calibrated in centigrade so I think the CHT at 200 to 225 is also high (400 - 420F or so).
    I will do as you both suggest and report back.

  5. #5
    stewartb's Avatar
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    If you don’t have a cooling lip on aft bottom cowl that might be the next thing to try. Sometimes that simple add helps both CHT and OT.
    Thanks ardmjd thanked for this post

  6. #6
    skywagon8a's Avatar
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    https://www.calculateme.com/temperat...fahrenheit/200

    200C is not too high though it is near the "useful" high limit which is generally assumed to be 400F or 204C.

    Maximum oil temperature limit is 245F.
    N1PA
    Thanks ardmjd thanked for this post

  7. #7

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    Quote Originally Posted by stewartb View Post
    If you don’t have a cooling lip on aft bottom cowl that might be the next thing to try. Sometimes that simple add helps both CHT and OT.
    Unfortunately, I was one of those people that a lower cowl lip did not help. I bought the Charlie center fiberglass lower cowl lip, installed, and had no change in temps. On the other hand, I know others who gained 10 degrees of cooling.

    It’s truly a cat and mouse game when you go chasing cooling.
    Likes RaisedByWolves liked this post

  8. #8

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    Several things can cause high CHT and oil temps. What kind of mags do you have? Slicks are notorious for having issues with points and internal timing. Check the timing advanced timing can cause temp issues, electronic mags will/can also cause this. Was the carb rebuilt/replaced running lean will also cause high CHT's, Make sure the carb is the right one for the sump. If you have EGT gauge you should be able to lean 150 degrees before lean of peak on any cylinder, if not carb heat should give you around 100-150 RPM drop at 2400 RPM. Usually not a carb issue but if someone stuck a pepperbox jet from Cub Crafters in they run very lean and have to be drilled. Check closely for an intake leak. Have someone operate the primer system while you inspect it a pinhole will allow air to be sucked in when and cause a lean condition. I think your high oil temp is the result of the high CHT'S, fix the CHT'S and the temp should go down. DENNY
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  9. #9
    courierguy's Avatar
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    Quote Originally Posted by AKaaron View Post
    Do this and report back. Click image for larger version. 

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    There is also the school of thought that leaving gaps around the cooler, so that some air goes into it and also around it, can create a low pressure zone on the backside, like the rear of a semi trailer. I have been playing with this on the RANS, and I have noticed no difference whatsoever in closing up the gaps I have around my cooler, if anything it ran hotter. I think it was RANS grand poobah Randy Schlitter who came up with the theory....low pressure on the backside is the best way to increase the airflow thru a cooler, other then flying faster. I am in the process of installing a 20% larger cooler, and also putting in lower down in the bottom cowling, Like a Cubs airfilter, so the waste air/pre heated air from it doesn't go over my cylinders but the muffler instead. Right now it sits in a upper cowl nostril

  10. #10

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    Thanks courier guy.
    So you think duct taping (as an experiment) around oil cooler to seal the edges of the oil cooler chin intake will make the cooler less efficient - obviously counterproductive.
    But I think the objective is to increase the pressure diff between the box around the top half of the cylinder cooling fins and the cowling below - which is more relevant?

  11. #11
    RaisedByWolves's Avatar
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    What was changed during the overhaul? Lines oil cooler gauge?


    Sent from my iPhone using Tapatalk

  12. #12
    courierguy's Avatar
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    Quote Originally Posted by ardmjd View Post
    Thanks courier guy.
    So you think duct taping (as an experiment) around oil cooler to seal the edges of the oil cooler chin intake will make the cooler less efficient - obviously counterproductive.
    But I think the objective is to increase the pressure diff between the box around the top half of the cylinder cooling fins and the cowling below - which is more relevant?
    No no! Try it! Just a thought, both concepts, tight and sloppy, make sense, though I think only a given amount of air will flow thru a certain size of cooler, whether or not it's standing apart (somehow) or is in a cowl with tightly sealed edges, only so much air can cram in there, and the only way to increase it is to fly faster or MAYBE kind of accidentally create a lower pressure zone on the backside by allowing air to flow around it. If nothing else this theory is a good excuse for a cooler mounted like mine with gaps around it..... another RANS pilot who went to a lot of trouble to tightly seal his up, seemed to think it didn't help, he eventually went to dual coolers, plumbed in series.

  13. #13

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    Thanks for post Denny.
    Have Bendix S4LSC-21 mags - timing meant to be 25 deg btdc I think - would advanced ignition cause overheating without other symptoms?
    The carb was factory exchange - engine shop reported they had a choice of a couple carbs and fitted the richer burn carb.
    With respect to EGT leaning and carb heat, not sure what you're getting at - do you mean if you can do these two things the carb is probably working OK?
    With respect to the primer, do you mean there could be a pinhole where primer line enters air intake?

  14. #14

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    [QUOTE=RaisedByWolves;806746]What was changed during the overhaul? Lines oil cooler gauge?

    Engine was zero-timed. 4 new cylinder assemblies, camshaft, mags, harness, carburettor etc.
    Subsequently also fitted new oil cooler.

  15. #15
    skywagon8a's Avatar
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    Quote Originally Posted by ardmjd View Post
    Thanks courier guy.
    So you think duct taping (as an experiment) around oil cooler to seal the edges of the oil cooler chin intake will make the cooler less efficient - obviously counterproductive.
    But I think the objective is to increase the pressure diff between the box around the top half of the cylinder cooling fins and the cowling below - which is more relevant?
    Leaving the gap open around the edges opens up the ends of the cooler to exposure of the air flowing by the sides. This is increasing the cooling capabilities of the cooler. If you block the openings as in Akaaron's picture, you are actually closing off some of the cooler's capability to cool.
    N1PA
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