EGT readings off the top of the sensor range

I've just installed two MGL type-K EGT sensors on cylinders 2 & 4 of my Rotax 914UL. The airplane is a SeaRey (high-mounted pusher prop), so there is a significantly long run (18 feet) of k-type thermocouple wire between the sensor and the back of my AF-5500. I used overlap-type (solderless barrel) connectors to splice from the sensor to the extension wire, then crimped D-Sub pins on the thermocouple wire for the EGT sensor 25-pin connector. When I fired up the engine, both EGTs were off-the-scale high within 20-30 seconds at idle throttle. For fun, I reconnected the sensors using 22-AWG wire for the long run from the sensor to the EFIS - same result.

Do I need to account for the long length of wire somehow? The AD_VALUEs were identical (I believe > 4000, but did not note it before leaving the hangar), leading me to believe that I was somehow maxing out the resistance sensor.

Any advice on how to troubleshoot?

Advanced Flight Systems internal EMS systems use ungrounded thermocouples. To verify this use your multimeter between one of the output leads and the casing of the probe. You should measure in the megohms. If not give us a call to order the proper probes.

Thanks Shawn - measured 21 ohms between the pin at the EFIS and the probe casing, so calling to order new probes now. For my benefit/education, can you explain what the difference between grounded and ungrounded probes are? I assume ungrounded thermocouples isolate the probe case from the engine body? Or something else?

Yes basically. Grounded probes will react faster but ungrounded probes are more accurate and less susceptible to noise.