Cessna 172M - Carb temp greater than OAT

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Jon Wilder

Pre-takeoff checklist
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Hi all.

We have a PFD/MFD Garmin G3X Touch system with Garmin GEA 24 engine analyzer installed. We're seeing carb temps about 20°+ greater than OAT all the time. On the ground without the engine running, it reads ambient temp along with the CHT and EGT senders (within a few degrees of them I'd say).

In our minds, we're thinking carb temp should always be LOWER than OAT due to venturi effect. But this does not seem to be the case.

Anyone else out there running a similar setup? What are your carb temps relative to OAT?

Thanks in advance.
 
Where are the sensors actually mounted? Without that information it is going to be pretty hard to determine if the measurement makes sense or not.
 
Lycoming could theoretically be warmer than oat by design. The intake is routed through the oil pan.
 
Daleandee said:
Turn the carb heat off ... :p
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Funny. I'll add that there is a significant RPM difference between heat on and heat off, so that rules out carb heat leak.

mondtster said:
Where are the sensors actually mounted? Without that information it is going to be pretty hard to determine if the measurement makes sense or not.
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The carburetor is a Marvel-Schebler. The plug above the data plate seen in this photo was removed and the sensor installed there. Sensor is a UMA 1B10R (Garmin p/n 494-70005-00).


MA-3SPA.jpg
 
Jon Wilder said:
I'll add that there is a significant RPM difference between heat on and heat off, so that rules out carb heat leak.
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That's where I was going with my comment but also see if there is a way that under cowl air could be getting into the intake system ...
 
Mine is always colder, but I have a Continental. unless I turn on the carb heat, then that sucker heats up quick
 
Salty said:
Lycoming could theoretically be warmer than oat by design. The intake is routed through the oil pan.
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Then what would be the point of the carb temp system being installed? Cessna themselves installed a lot of these things and they worked fine. That carb has so much air flowing through it, and the pressure drop and fuel evaporation is so strong, that carb ice happens in those things, on Lycomings, on nice summer days. I have seen it many times. People that think that carb ice doesn't happen in Lycomings are sometimes caught unaware. That carb is insulated from the sump by a gasket, too. Doesn't sound like much, but it makes a big difference.

The sensor is either messed up or its wiring is in need of cleanup.

upload_2022-12-9_21-11-12.png

The probe is the thing on the left. Two wires run from it to the gauge on the panel. One of them is connected to the C terminal which is then grounded to the airframe. Now, if that ground at the airframe is loose or dirty or oxidized, the gauge misreads. If someone "modified" the system by grounding that probe wire to the engine case, the gauge WILL be way off. Tiny stray currents from the alternator will find their way into it, especially if the engine's ground strap is compromised even a little bit, and that tiny flow messes up the readings. It happens to oil temp gauges, too.
 
Did you cut or alter the sensor harness?
 
Jon Wilder said:
The sensor has two black wires on it. It's an RTD sensor. That is spliced onto Type K thermocouple wire.
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Thermocouple wire isn't for use with an RTD. That may be part of the problem.
 
mondtster said:
Thermocouple wire isn't for use with an RTD. That may be part of the problem.
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Yup. Heat could upset the resistance of the wire and cause false readings.

Thermocouples are typically used for EGT and CHT gauging. They are a junction of two dissimilar metals that create electron flow proportional to their temperature, enough to drive the gauge. There is no external power input.

The carb heat sensor uses a thermistor, a resistor whose resistance changes with temperature. External power is required.
 
Dan Thomas said:
Yup. Heat could upset the resistance of the wire and cause false readings.
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Makes sense.

Dan Thomas said:
The carb heat sensor uses a thermistor, a resistor whose resistance changes with temperature. External power is required.
Click to expand...

According to Garmin's diagrams (and the fact that the sensor has only two wires), the two wires of the sensor go to two pins respectively on the GEA 24. More than likely, power is being supplied via an internal pull up resistor, making the RTD sensor the bottom of a potential divider.
 
Jon Wilder said:
According to Garmin's diagrams (and the fact that the sensor has only two wires), the two wires of the sensor go to two pins respectively on the GEA 24. More than likely, power is being supplied via an internal pull up resistor, making the RTD sensor the bottom of a potential divider.
Click to expand...

The typical RTD circuit is more or less a voltage divider that is housed within the unit.
 
mondtster said:
The typical RTD circuit is more or less a voltage divider that is housed within the unit.
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Not a wheatstone bridge?

upload_2022-12-13_11-5-7.png

R2 would be the probe. This circuit makes small resistance changes in R2 make big differences in the gauge. Often used in multimeters.
 
Salty said:
Lycoming could theoretically be warmer than oat by design. The intake is routed through the oil pan.
Click to expand...
Except the path through the oil pan is downstream of the Carb. I used to think same thing and why Lycoming makes less ice. Someone on this board showed a picture that set me straight.
 
RTD sensors are normally used in a resistive wheatstone bridge circuit like Dan suggested. Thermocouple wiring is not compatible. Standard wires are all that is necessary.

On my Cessna 195, if my connection gets dirty at the carb, the temp shows too high. If I clean up the connection, it goes back to normal.

On Dan's diagram, the gage reads the voltage between B and D.
 
This is a typical electronic temperature measurement in the automotive world. While it’s possible that Garmin is using a Wheatstone bridge, it wouldn’t be my first assumption.

Regardless how the circuit looks inside the box, thermocouple wire is absolutely the wrong wire to be using for this.
 

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The principal is the same..the thermistor changes with temperature.

The change in resistance is read as a change in temperature.

Thermocouple wire not required.
 
Also, do not route the wire along high voltage or high-current cables like sparkplug leads or alternator cables. These can induce unwanted currents in the temperature gauge wire and mess up the reading.
 
Hang 4 said:
Except the path through the oil pan is downstream of the Carb. I used to think same thing and why Lycoming makes less ice. Someone on this board showed a picture that set me straight.
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On the Lycomings in question, the carb bolts onto those tubes routed through the oil pan. Those tubes get HOT and heat transfers into the carb body.
 
Domenick said:
On the Lycomings in question, the carb bolts onto those tubes routed through the oil pan. Those tubes get HOT and heat transfers into the carb body.
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Absolutely correct, and explains less icing, but it does not affect the intake temperature of the air flowing into the carb, which was the misconception I had. (and I suspect a few others)
 
Domenick said:
On the Lycomings in question, the carb bolts onto those tubes routed through the oil pan. Those tubes get HOT and heat transfers into the carb body.
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That carb, like I said elsewhere, has a gasket between it and the sump. That insulates it quite a bit. Lycomings DO get carb ice. I've had it several times, and frequently had students and instructors getting it on the ramp soon after startup on nice summer days.

That carb has a lot of ambient-temp air going through it. As much as 20 pounds per minute of it, in an O-320. 20 pounds of anything is a lot to heat up, and that little carb isn't going to do it. It will get cold instead. Then we add the pressure drop that causes a temp drop of much as 30°F, and the evaporating fuel, good for another 40°F. Add them up and see what that looks like. This is why we can get carb ice at 100°F:

upload_2022-12-14_13-9-23.jpeg


And it's why Lycomings have carb heat systems, too.

The tiresome myth can Lycs don't ice up misleads a lot of pilots into complacency about it, and that causes some accidents.
 
 
I got carb ice coming back from Sun N Fun on a nice warm day. I was flying over the ocean around Daytona and forgot to use carb heat...scared the crap out of me...I run carb heat almost all the time when flying the Cessna 195....Jacobs R755-9 radial engine.
 
CT583 said:
Mine is always colder, but I have a Continental. unless I turn on the carb heat, then that sucker heats up quick
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Yeah, we had an analog carb heat gauge on our O-300, and it operated as yours, as expected.
 
Hang 4 said:
Absolutely correct, and explains less icing, but it does not affect the intake temperature of the air flowing into the carb, which was the misconception I had. (and I suspect a few others)
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The carb body picks up some heat, but not enough to make a difference to the air temperature?
 
Randomskylane said:
The carb body picks up some heat, but not enough to make a difference to the air temperature?
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Key thing is the surface temp of the carb itself. The air at some point will pick up heat, but if the carb throat has some heat it will resist ice formation a bit more than one at ambient temp. As others have pointed out, the Lycoming is not "ice proof", but can be a bit better than some known "ice makers"
Back to the OP's original question. Have you checked your carb heat flapper valve? One explanation for the higher temperature could be that the carb heat is letting some hot air by due to a cable or flapper valve issue.
 
Hang 4 said:
As others have pointed out, the Lycoming is not "ice proof", but can be a bit better than some known "ice makers"
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That there. It's a bit better, but it's not ice-proof, which assumption is what gets some folks into trouble. The Cherokees and their exhaust system design have a bigger impact on ice resistance than that sump. The exhaust system wraps all around that carb, radiating heat at it. The typical carbed Cessna has its exhaust all up front, or one muffler each side, not all around.

Like I said, 20 pounds of ambient air per minute going through it is not going to be easily heated. The biggest effect of the oil sump on the induction system is its warming of the air/fuel mix after it leaves the carb on its way to the cylinders. That helps vaporize the fuel droplets and allows a little more efficient burning of the fuel.
 
Follow up - as the plane is down for annual and engine swap, as I stated above, I had thought the carb temp sensor was wired with type K thermocouple wire. As it turns out, it was actually hooked up to two of the three conductors in a M27500-24TG3T14 wire.

So this rules out thermocouple wire being the cause.

Any other thoughts?
 
Jon Wilder said:
Follow up - as the plane is down for annual and engine swap, as I stated above, I had thought the carb temp sensor was wired with type K thermocouple wire. As it turns out, it was actually hooked up to two of the three conductors in a M27500-24TG3T14 wire.

So this rules out thermocouple wire being the cause.

Any other thoughts?
Click to expand...

Check the accuracy.
 
I tried to look for info online for the carb sensor for the gea 24 . I saw one ad that kind of threw me because it looks like a pic of a thermocouple and k thermocouple wire. The other pics I saw were thermistors rated at 100 ohms @ 0 C. I agree with mondster about checking accuracy. I would use an infrared thermometer and shoot the beam on the carb hot and cold to see how well it agrees with your gage. My 195 carb runs cold, but yours may be fine...good luck.
 
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