Joe Larussa Posted February 26, 2021 Report Share Posted February 26, 2021 Just had my #2 cylinder exhaust probe fail for the third time in about 400 hours. It’s the only one that ever goes bad. Thoughts? Quote Link to comment Share on other sites More sharing options...
carusoam Posted February 26, 2021 Report Share Posted February 26, 2021 Joe, Post the data, and we can have a look... Post a pic of the probe tip... we might see what is going on... Or... what are they dying from? (If you know) -a- 1 Quote Link to comment Share on other sites More sharing options...
Bolter Posted February 26, 2021 Report Share Posted February 26, 2021 I have never seen anyone else on this board lose multiple probes on the same cylinder in a short time. Random idea 1) I would wonder if the diagnosis is correct? Maybe the failure is on the other side of the connector? Maybe a bad connection that gets closed again with each repair, then vibrates open. Was the probe swapped to another cylinder to verify it was bad? Random idea 2) I cannot imagine that the thermocouple is not rated for ample margin over the predictable range of exhaust temperature. But the temperature capability may just be the tip. It may be that it is getting hot in the wrong place because of an exhaust leak around the tip, and hot gas is overheating the part that is intended to be outside of the exhaust flow. Measure the hole, and see if it was overdrilled a small amount in the past. 2 1 Quote Link to comment Share on other sites More sharing options...
carusoam Posted February 26, 2021 Report Share Posted February 26, 2021 EGT sensors typically last thousands of hours.... Unless they are in the combined exhaust streams like a TIT sensor or downstream of a 3in1 exhaust collector... Where the average EGTs are very high over time.... The jacket of the probe erodes at a higher rate in these environments... ROP has more O2 molecules available for corroding the sensor’s jacket... LOP is an O2 free environment... PP thoughts only, not a mechanic... -a- 1 Quote Link to comment Share on other sites More sharing options...
PilotX Posted February 26, 2021 Report Share Posted February 26, 2021 Doubtful this is your problem but I had a pinched wire once. Replaced the probe and got the same fault, somewhat intermittently. The wire from the JPI that connects to the probe was pinched by the cowl (not a Mooney) coming out of the firewall. Splice the wire and wrapped it with that plastic protective stuff and it was good as new. 1 Quote Link to comment Share on other sites More sharing options...
tmo Posted February 26, 2021 Report Share Posted February 26, 2021 8 hours ago, PilotX said: Splice the wire This. Do NOT solder the thermocouple wire, always splice it. 2 Quote Link to comment Share on other sites More sharing options...
Joe Larussa Posted February 26, 2021 Author Report Share Posted February 26, 2021 My 830 was flashing bad probe when I landed. With the engine still running I recycled the avionics master and it restarted the 830. Warning went away, so I did a run up. All okay till I brought it back to idle, bad probe popped back up. Quote Link to comment Share on other sites More sharing options...
Joe Larussa Posted February 26, 2021 Author Report Share Posted February 26, 2021 Can a probe be tested without running the engine? Quote Link to comment Share on other sites More sharing options...
Bunti Posted February 26, 2021 Report Share Posted February 26, 2021 I just learned about a case of a twin airplane which lost parts of an EGT sensor during flight. This was (as far as I remember) not a JPI-system or sensor. For a non turbo airplane this would be no problem: Buy a new sensor and fly again. But for this turbo airplane it was very different: The broken EGT sensor moved through the exhaust into the turbo and destroyed parts of the turbo system. The owner first saw on the display that the probe was going bad. Then he checked the probe and found out that most of it was missing. He was worried about possible damage to the turbo. So he got the turbo checked and found the damage and the missing part. Really bad and expensive day. I have an EDM930 in my Mooney. I never had a probe mechanically broken. Over the years, we had to replace some because they stopped working right. I even bought a new one which did not display right. It was good at outside temps but as soon as the engine started and temps got higher, the probe showed wrong values. JPI checked the probe and gave me a new one. For the JPI it is really important that the unit is grounded to the engine. At initial installation this was not done right at my plane. After that had been corrected, the reliability improved. Nevertheless, I always have a spare EGT and CHT probe in the plane. It looks to me that the CHT probes last longer than the EGTs. Quote Link to comment Share on other sites More sharing options...
N201MKTurbo Posted February 26, 2021 Report Share Posted February 26, 2021 8 minutes ago, Joe Larussa said: Can a probe be tested without running the engine? Yes, use a barbecue lighter. You can use a cigarette lighter, but your thumb will get hot after a while. 1 Quote Link to comment Share on other sites More sharing options...
PT20J Posted February 26, 2021 Report Share Posted February 26, 2021 First, I’d check all connections. Then, I’d swap probes with another cylinder and see if the issue follows the probe. May be an issue with 830 itself. Skip Quote Link to comment Share on other sites More sharing options...
bradp Posted February 26, 2021 Report Share Posted February 26, 2021 Yeah k type thermocouples have a defined resistance can temperature curve. Measure the resistance at know temps and you’ll get a general idea if it’s working or not. The thermocouples are pretty robust. I’m on my same set going on 12 years. Something is off - could the wires be getting singed, vibration excessive, something corroding the tip, etc. Quote Link to comment Share on other sites More sharing options...
N201MKTurbo Posted February 26, 2021 Report Share Posted February 26, 2021 1 hour ago, bradp said: Yeah k type thermocouples have a defined resistance can temperature curve. Measure the resistance at know temps and you’ll get a general idea if it’s working or not. The thermocouples are pretty robust. I’m on my same set going on 12 years. Something is off - could the wires be getting singed, vibration excessive, something corroding the tip, etc. Thermocouples don’t change resistance with temperature, they generate a voltage proportional to the temperature difference between the cold junction and hot junction. All the systems we use have temperature sensors in there amplifiers that form a pseudo cold junction. https://en.wikipedia.org/wiki/Thermocouple You seem to be describing a thermistor. Our stock CHT and oil temp probes are thermistors. All the aftermarket sensor systems use thermocouples. 1 1 Quote Link to comment Share on other sites More sharing options...
bradp Posted February 27, 2021 Report Share Posted February 27, 2021 I got it reversed thanks Rich. I guess an easy way to remember is thermocouple is a couple of metals, therm/istor/ sounds like resistor. 1 Quote Link to comment Share on other sites More sharing options...
N201MKTurbo Posted February 27, 2021 Report Share Posted February 27, 2021 48 minutes ago, bradp said: I got it reversed thanks Rich. I guess an easy way to remember is thermocouple is a couple of metals, therm/istor/ sounds like resistor. Quote Link to comment Share on other sites More sharing options...
takair Posted February 27, 2021 Report Share Posted February 27, 2021 22 hours ago, Joe Larussa said: Just had my #2 cylinder exhaust probe fail for the third time in about 400 hours. It’s the only one that ever goes bad. Thoughts? Do you have pictures of the old probes? Probes can erode, but that typically takes hundreds of hours. They can be damaged if they are mounted near a slip joint, such that expansion actually makes the joint act like a bolt cutter. Then there is the harness, as other suggested. This is often the week link. I suspect the probes are not failing, but a connector somewhere in the harness. Quote Link to comment Share on other sites More sharing options...
N201MKTurbo Posted February 27, 2021 Report Share Posted February 27, 2021 There are only two things a system can tell about thermocouples, they are open or they are shorted to ground. both are wiring issues. It is possible there is vibration shaking your wires apart. Quote Link to comment Share on other sites More sharing options...
carusoam Posted February 27, 2021 Report Share Posted February 27, 2021 Very helpful if you can post JPI data to savvy... the graphs may indicate where errors are occurring during the flight... If the JPI is finding something, the data will probably be showing it... A typical test for an EGT sensor... is to turn on the JPI in the hangar to see if all the EGTs and CHTs are showing about the same room temperature as the OAT... PP thoughts only, not a mechanic... Best regards, -a- Quote Link to comment Share on other sites More sharing options...
bradp Posted February 27, 2021 Report Share Posted February 27, 2021 Simple thing Joe - make sure that the star washer is between the probes. They read funky if not installed this way. Quote Link to comment Share on other sites More sharing options...
Joe Larussa Posted February 28, 2021 Author Report Share Posted February 28, 2021 On 2/27/2021 at 4:45 AM, bradp said: Simple thing Joe - make sure that the star washer is between the probes. They read funky if not installed this way. Hmm.... I’m pretty certain I didn’t put the washer there! Thanks Quote Link to comment Share on other sites More sharing options...
ArtVandelay Posted February 28, 2021 Report Share Posted February 28, 2021 Hmm.... I’m pretty certain I didn’t put the washer there! ThanksMe thinks you had a bad AP doing the installation. While you’re at it, check the probe wires are not cable tied together with the ignition wires, another amateur mistake. And your fuel flow sensor is horizontal with no 90° bends on in/out ports. 2 Quote Link to comment Share on other sites More sharing options...
MikeOH Posted February 28, 2021 Report Share Posted February 28, 2021 The voltages involved with thermocouples are VERY low; a twenty degree F temperature change is around a half of a millivolt! Connections and wire placement are critical. Quote Link to comment Share on other sites More sharing options...
carusoam Posted February 28, 2021 Report Share Posted February 28, 2021 Surprisingly, It doesn’t mention the materials being used... Is that star washer specific to the TC metals? Is it anything special to be aware of? So a different voltage doesn't get generated by the junction... PP wonderment only, not a mechanic... Best regards, -a- Quote Link to comment Share on other sites More sharing options...
MikeOH Posted February 28, 2021 Report Share Posted February 28, 2021 40 minutes ago, carusoam said: Surprisingly, It doesn’t mention the materials being used... Is that star washer specific to the TC metals? Is it anything special to be aware of? So a different voltage doesn't get generated by the junction... PP wonderment only, not a mechanic... Best regards, -a- Not saying there isn't a special washer, but the voltages generated will be cancelled out as long as both sides of the star washer are at the same temperature; which is pretty certain! Additional thermocouple trivia/minutiae: Common misconception is that the thermocouple voltage is generated at the junction between the two different metals; that is NOT true. If you think about it, it can't be because you would have some, albeit small, voltage across close to zero ohms which would generate an enormous current! The voltage is generated along the length of the two wires; since the wires are different metals, the voltages generated are slightly different. That difference is the thermocouple voltage. This is the reason for both aging and why there is 'thermocouple wire' and 'thermocouple extension wire' available. Thermocouple wire is highly purified and homogenous. Extension wire, not so much. The reason for aging is that the wire's remaining impurities migrate over time, in part due to the temperature difference along their length, and thus change the voltage generated. Thermocouple extension wire operates at a much lower temp (usually room temp) over its entire length. 2 Quote Link to comment Share on other sites More sharing options...
RobertGary1 Posted March 1, 2021 Report Share Posted March 1, 2021 20 years on my jpi probes and never changed one Quote Link to comment Share on other sites More sharing options...
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