Abnormally low CHT indications

RV74ME · #1 12-20-2019, 09:20 PM

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?

VA Maule · #2 12-20-2019, 11:14 PM

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

sjhurlbut · #3 12-21-2019, 06:21 AM

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

FLY6 · #4 12-21-2019, 07:47 AM

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.

AaronG · #5 12-21-2019, 08:01 AM

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

bjdecker · #6 12-21-2019, 08:10 AM

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

RV74ME · #7 12-21-2019, 08:16 AM

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

PilotjohnS · #8 12-21-2019, 08:18 AM

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.

RV74ME · #9 12-21-2019, 08:58 AM

Yep, doing that now on 2 of them. And they are reading 180 and 161??? Ugh

Bavafa · #10 12-21-2019, 09:05 AM

Quote:

Originally Posted by sjhurlbut

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

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.