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High CHT's on extended climb, but low coolant temp

in 912 / 914 Technical Questions 11 years ago

  • Re: High CHT's on extended climb, but low coolant temp

    by » 11 years ago

    We had tape over part of the radiator and over part of the oil cooler, but removed it in Spring--which is about the same time we started having high CHT's. But I was referencing some aluminum foil tape wrapped around the silicone baffle material on the edges to keep the wind from pushing the baffles outward. This tape did not cover any of the fin area of the radiator.

    Yes, something has changed, but it runs great and is smooth. The only thing I detect is that it seems a bit harder to start the last 2 times. We thought something smelled hot in out last test flight, and we had not smelled that before.

    Gary
      
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  • Re: High CHT's on extended climb, but low coolant temp

    by » 10 years ago

    A year ago we put in a new radiator cap, sealed the air gap around the radiator, changed the spark plug gap to about.027, moved the alum angle bracket stiffiner from inside the bottom of the cowl to outside (making a lip on the bottom),and cleaned the corrosion off the ground cable. Of those things, something worked and we could climb for 20 minutes in 90 degree heat and stay in the green on CHT's. Life was great and that worked for a long time. Now we are back to high CHT's, especially on climb, but even in level flight at 5000 RPM. Today it was only 80 OAT and we were at 230 F by 1500 feet and 240 by 2000 feet, and 260 by 4000 feet at 5400 RPM using best climb speed (60 mph). When we throttled back to 5000 RPM and slowly lost altitude by about 200 FPM it still held at 240 f CHT. But the whole time coolant temps barely got over 180 f and oil never got over 160 f. Even after shutdown it still showed 220 while we were exiting the airplane.

    The only things that have changed are:
    1. The carbs drip fuel while sitting not running if we leave the fuel valve open. I assume this means a needle valve problem. It runs fine and the exhaust pipes are normal color (brown).
    2. We removed the alum angle in the bottom of the cowl and replaced it w a piece of alum sheet but still with a lip on the bottom of the cowl.
    3. We re pitched the prop from about 9.5 deg to about 7.5 deg to give it less bite. The prop spins faster but flys slower and did not affect CHT temps.
    4. We changed the coolant (still using dex w distilled water mix) FROM 50/50 TO 20 DEX/80 WATER. aDDED "wATER wETTER".
    5. re checked the radiator cap and found the gasket was cocked a bit, so put it back in place. This might be the cause, but the cap is only a year old.

    nONE OF THESE HELPED AND IT SEEMS TO BE GETTING WORSE. What should we try next? A new ($100) radiator cap? Rebuilding the carbs? ps, even though the CHT gauge showed 220 after landing the heads did not feel all that hot to the touch. The coolant hoses were hot indicating good flow.
      
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  • Re: High CHT's on extended climb, but low coolant temp

    by » 10 years ago

    Hi Gary,
    I have one thing to check and have seen this several times. Reduced radius hose, either coolant or oil hose. I just posted a video of how to take care of this on another forum. I actually had the problem myself after 1 year from a hose change. I was seeing before that oil temps around 220F-235F (depending on the day) on hot days in Tucson, AZ.. Then the oil temps started to climb after two flights up to 250F in an easy climb. So I went after one of my oil hoses that I suspected would become a reduced radius when the hose was hot. All I did was insert a stainless steel 1/2" OD spring into this hose. Temps fell back to 220F-230F and has stayed there ever since. Some of the light aircraft companies put springs in coolant lines both the 25mm (1") and or the 17mm coolant lines. It's becoming more common place. Sometimes a hose re-route will take care of the problem, but sometimes you can't re-route a hose. The springs in coolant or the oil lines works very well and I have fixed several this way plus I have had many email saying it fixed their over heating. I have found a number of reduced radius 17MM hose from routing or squeezing down from wire ties or safety wire to hold something on or in place.

    Recently a friend of mine started to have over temps. He sent his oil cooler in to get flushed. They flushed out quite a bit of yellow plastic pieces. Now his temps are back to normal.

    Have you done a hose change? If not do one and if so look for a reduced radius hose or a partial blockage from something.

    The carbs may not be causing the over heat, but they shouldn't be leaking. If they are a number of years old or have some real time on them rebuild them and check the floats to see if one is sinking, check the float level or it may just be a leaky bowl gasket.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

      
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  • Re: High CHT's on extended climb, but low coolant temp

    by » 10 years ago

    Thanks Roger. Our oil temps never get above 160-180, in fact we are usually running too cool. Today in our test flights the oil temps never even made it into the green. Likewise the coolant typically runs about 180-185, even when the CHT's are going through the roof. I can't understand how the fluids can be cool while the CHT's are high.

    We have an external spring on our oil hose so it is not kinked, and used hose that would flex well w/o kinking.

    It is totally baffling.
      
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  • Re: High CHT's on extended climb, but low coolant temp

    by » 10 years ago

    Hi Gary,

    Something isn't right. You can't have low oil temps and low coolant temps and have an actual high CHT temp. For a quick check try this; Switch the wires from the #3 CHT and the #2 CHT and see if the problem follows that wire. You can swap the oil temp probe with the CHT probe for a quick check. They are the same. See if the problem follows the probe. Check the wires for a loose or bad connection on CHT probe. Check any of the grounding associated with the CHT's.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

      
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