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Abnormally low CHT indications

R

RV74ME

Well Known Member
Just did first flight, and was awesome! (I?ll save the details for a separate post).

Anyway, I have a feeling my cht?s are not reading correctly (all low 200?s entire flight). IO-360 M1B, Garmin G3X, Alcor (type K) probes.

Thought it might be configuration issue, and it was initially (i had selected type J). Went back to configuration mode and changed it to type K.

Second flight they were slightly higher, but still abnormally low:

Oat 50 deg
MAP 25
RPM 2500
Full rich
Alt 3000?

CHT?s never got above 300.

1- 233
2- 231
3- 280
4- 251

Thought maybe wires were backwards, so i swapped them. Read normal until engine start, then got a red X. So put them back like they were.

After sitting overnight, turned on the master and they indicated ambient air temp (as I would suspect). But did another quick flight and still all below 300.

Tomorrow I plan to put the probe in boiling water to verify the readings. Assuming they are good, I am out of ideas. Anyone seen this before?
 
Do you know anyone that might lone you an infrared thermometer gun ? ( maybe buy one at the aviation department of the local auto parts store there not very expensive) With that you can check each cylinder head temperature against what your engine monitor is telling you. Wile doing the troubleshooting pull the mixture extremely lean but still running smooth to look for an increase in temperature.

Then there's always that chance that you've found the secret to good baffling & too excellent cooling efficiency :eek:
 
Good

Those numbers are ok and similar to mine. Just means you did good job with baffling. Do you have a plenum by chance?
 
Low cht

In the book that Bill Ross wrote for Superior Air Parts, I believe he stated cht?s that low are not good. The temps don?t get high enough to burn off the carbon and other byproducts of combustion. He also states the cht?s that low generally is associated with low oil temps. If oil temps don?t get to 180 degrees, they will not burn off the water vapour.
I am not reading this material as I?m writing this but it does make for an interesting read. Go to Superior Air Parts for a free download of the book.
I am in no way associated with Superior Air Parts, just find the book to be very informative.
 
If you use oil, you can calibrate the probes up to 300F, just be careful at those temps. With calibration curves to those temps, you will know if the inflight temps are accurate.

Only other possibility I can think of is that the probes are not installed correctly and you are not getting a temp reading right at the cylinders.

Aaron
 
Steve T,

A couple of thoughts --

1. Verify that the probes are wired into the GEA 24, J242 are correct - to wit; Red Wire is "Lo", Yellow Wire is "Hi" for the Alcor "K" types -- see figure 31-2 in the G3X Installation Manual.

2. Boot up the G3X in Configuration mode, select the "System Information" icon, then scroll through all the device lists - verify that the "Network Error Rate" for the devices on the CANBus is at 0%

B
 
Ok, i *think* i may have figured it out (this idea popped in my head at 3am)...

I thought maybe the tip of the probe isnt seated all the way into the well, and not actually touching the metal. Makes sense, right because reads ambient temp before starting engine. And then after start they rise, but just not to as high as you would expect.

So i pulled each probe, and guess what? The spring was compressed when it shouldnt be.

There is a spring that goes around the wire (strain relief) that i guess i had threaded it all the way up inside the nut. This was holding the spring (on the probe) in compression, which is not allowing the tip to make contact.

Too windy to fly today to test, but pretty sure this is gonna be the culprit
 
Test

I think the boiling water test is a good idea. Simple and cheap and will tell you if the probes are correct type and calibration is reasonable.
 
sjhurlbut said:
Those numbers are ok and similar to mine. Just means you did good job with baffling. Do you have a plenum by chance?
Click to expand...
Not normal # for a o360 type engine AND first flight. These numbers would be on the low side even for a IO390.

Which avionics are you using? Some of them let you adjust the k factor.
 
Ugh?

RV74ME said:
Yep, doing that now on 2 of them. And they are reading 180 and 161??? Ugh
Click to expand...

Given these readings, I would look at the wiring. Are the thermocouples wires routed nicely? Are there splices or other connections in the path to the sensor that could be causing another “fake” thermocouple? Is the installation per the instructions? Or maybe the water is not really boiling:).

You could also check the cold side by using an ice bath. As long as liquid water and ice coexist, the water is. 32 F. But i think many different types of sensors have similar temp readings in the ice bath.
 
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I also suspect the wiring. Gonna have to wait til next week, but will recheck all the pins are correct. And no, there are no splices anywhere. Only connection is at the probe. Except i used spade connector instead of the one that came on the probe.
 
Mis-wired

Looks like its the wiring. The red and yellow wires were installed backwards at the dsub. What i learned is these two wires are different. One is alumel and the other chromel. So trying to correct this mistake by reversing the connection at the probe does not work.

Anyway, just finished reassembling the connector and will test everything thursday when i get back.

So much for having my harnesses ?professionally? built🙄.
 
RV74ME said:
Looks like its the wiring. The red and yellow wires were installed backwards at the dsub. What i learned is these two wires are different. One is alumel and the other chromel. So trying to correct this mistake by reversing the connection at the probe does not work.

Anyway, just finished reassembling the connector and will test everything thursday when i get back.

So much for having my harnesses “professionally” built��.
Click to expand...

Keep looking. A thermocouple has dissimilar metals that produce electricity (voltage) above absolute zero. This DC voltage is related in a non-linear way to the temperature. The signal is small and, clearly, polarized. ANY dissimilar metal contacts in the circuit will also produce voltages. Typically this is a non issue as they are low temp and do not contribute to an error. The fact that all are off the same amount means they are either correct or both sides are miss wired in the same way. Look at your schematic for how they are connected to the instrumentation. If two thermocouples are wired in series the the voltage would be half . . . but , then, how would you get 4 readings??

A look at the complete picture of the engine performance parameters will help in the diagnosis. Fuel flow in particular. Is it running super rich? Is the timing way off, as in retarded? If retarded, one would expect higher EGT, but that can be masked with super rich mixture. I ran 700F EGT, but it was very cold ambient. Single parameter analysis limits the process.

Honestly, I think it's a high probability that your temps are being measured correctly. The data point is ambient, it would not be correct with a wiring issue.
 
FLY6 said:
. If oil temps don’t get to 180 degrees, they will not burn off the water vapour.
Click to expand...

My IO360 Lycoming manual states a maximum oil temp of 245 F, and a “desired oil temp” of 180 F. There is no minimum, but Lycoming says: “Engine oil temperature should not be below 140 F during continuous operation.”
In other publications Lycoming recommends 165 to 200 F.
I would suggest that the comment about the oil temperature needing to be at 180 F to “burn off the water vapour” is erroneous. Water will evaporate out of the oil at much lower temperatures than 180 F....it just takes longer.
 
RV74ME said:
Yep, doing that now on 2 of them. And they are reading 180 and 161??? Ugh
Click to expand...
Immersed in 212 F water (what altitude are you at?), a Type K thermocouple puts out 4.096mV. If your box was set for Type J rather than K, that 4.096mV would read out as 170.6 F, pretty close to what you saw. I know you checked this setting once, but I would check it again.
 
BillL said:
Keep looking. A thermocouple has dissimilar metals that produce electricity (voltage) above absolute zero. This DC voltage is related in a non-linear way to the temperature. The signal is small and, clearly, polarized. ANY dissimilar metal contacts in the circuit will also produce voltages. Typically this is a non issue as they are low temp and do not contribute to an error. The fact that all are off the same amount means they are either correct or both sides are miss wired in the same way. Look at your schematic for how they are connected to the instrumentation. If two thermocouples are wired in series the the voltage would be half . . . but , then, how would you get 4 readings??

A look at the complete picture of the engine performance parameters will help in the diagnosis. Fuel flow in particular. Is it running super rich? Is the timing way off, as in retarded? If retarded, one would expect higher EGT, but that can be masked with super rich mixture. I ran 700F EGT, but it was very cold ambient. Single parameter analysis limits the process.

Honestly, I think it's a high probability that your temps are being measured correctly. The data point is ambient, it would not be correct with a wiring issue.
Click to expand...

I confirmed that the wires were reversed at dsub (lo was hi, hi was lo)
 
snopercod said:
Immersed in 212 F water (what altitude are you at?), a Type K thermocouple puts out 4.096mV. If your box was set for Type J rather than K, that 4.096mV would read out as 170.6 F, pretty close to what you saw. I know you checked this setting once, but I would check it again.
Click to expand...

I?m at 800 feet. And yes i checked 3 times that in fact i have type K selected. Now that i have fixed the wiring issue, i?m expecting it to read correctly, but wont know until i get back home in a few days
 
RV74ME said:
Looks like its the wiring. The red and yellow wires were installed backwards at the dsub. What i learned is these two wires are different. One is alumel and the other chromel. So trying to correct this mistake by reversing the connection at the probe does not work.

Anyway, just finished reassembling the connector and will test everything thursday when i get back.

So much for having my harnesses ?professionally? built🙄.
Click to expand...

Glad you found that there was an issue in the harness. I know the weather around the Southeast has bee **** for the past 2 days. I'm interested in hearing to see if that fixed your problem. The one thing that is interesting to me is that you stated in your earlier post that when you turned on your EFIS in the hangar, the temps read the same as the outside ambient temperature... why would it read correctly in the lower degrees but not read correctly in the higher temps. I'm not questioning you, I'm just curious.
 
RV74ME said:
I?m at 800 feet.
Click to expand...
Water boils at 210.5 F at that altitude, so no big deal. I'm not convinced that you found the problem, though. If the wires were reversed, you would be sending negative voltage to the box and get the big red X again. Maybe they're reversed at the box and somewhere else, too?
 
snopercod said:
Water boils at 210.5 F at that altitude, so no big deal. I'm not convinced that you found the problem, though. If the wires were reversed, you would be sending negative voltage to the box and get the big red X again. Maybe they're reversed at the box and somewhere else, too?
Click to expand...

Guess i should clarify. I knew back when i did first engine run that the dsub was wired backwards. I got the big red X?s. so instead of pulling all that apart, i took (what i thought) would be path of least resistance. So i hooked up red to yellow, yellow to red to the leads of the probe. This gave me temperature indications, but just not accurate. Didnt know how inaccurate it was until first flight
 
RV74ME said:
Guess i should clarify. I knew back when i did first engine run that the dsub was wired backwards. I got the big red X’s. so instead of pulling all that apart, i took (what i thought) would be path of least resistance. So i hooked up red to yellow, yellow to red to the leads of the probe. This gave me temperature indications, but just not accurate. Didnt know how inaccurate it was until first flight
Click to expand...

"Least resistance"

Given the wire type differences and resistance down the length, probably gave you the most resistance :)
 
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RV74ME said:
So i hooked up red to yellow, yellow to red to the leads of the probe.
Click to expand...
So what you had there was three thermocouples in series. One at the probe in cylinder head, another at the junction between the probe and lead wire, and a third at the D-sub connector. You do the math...
 
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