EGT Gage Problem

I have an RV-9A I bought with standard Van's steam gauges. Recently, the EGT gauge started acting up. It reads correctly at first, then after 15-20 minutes into a flight, the needle drops to the left peg. It occasionally climbs up to realistic numbers (1300-1400 degrees), then will drop all the way back down. I have checked/cleaned all the connections between the thermocouple and the gauge. Has anyone else had this problem? Is the Van's gauge suspect?

How many hours on the probe??

These things do burn out.

350 hours. Is there an easy way to test the probe?

If the engine is equipped with 4 probes, switch the wires around, and see if the problem follows the probe.

If a single pipe is probed, you could remove the probe, and heat it with a high temp heat gun, like used for paint stripping, or if you are careful----use a propane torch, start far enough from the torch flame and bring the probe to the flame to equal the engine temp, then wait a while.

Someone else may have a better idea or two, but that is all I can come up with at this time.

Good luck.

Thermocouples commonly break at the join because (by nature) they are two dissimilar metals. Heating the metals causes a very weak voltage difference, which is read as temperature. Likely what is happening is that the two wires (each of different metal) have come apart. When they are cold, the wires are touching and will read a voltage (temperature) but when they heat up and expand, they separate and break the circuit, thus your voltage goes to zero and the temperature accordingly. Sounds like occasionally they may touch (due to engine vibration) and give reasonable, or not so reasonable numbers. If you have checked all the connections, and swapped them out (assuming multiple probes), then the probable culprit is a broken connection. Swap it out for a new probe.

greg

Check the EGT thermocouple probes

I noticed the same problem on a Lyc IO360 engine with about 250 hours and equipped with a set of Vans EGT hose clamp probes. I removed the probes and the ceramic exterior coating was completely burned off and exposing the two thermocouple wires. In 2 of the probes, the wires were burned down to the nubs. Not very high quality or long lasting for such costly probes. Bought another set of 4 for $100 ea from another vendor. I asked about the previous failures and got a non-reply. The new hose clamp probes have a larger diameter heavy duty design and the probe tips were enclosed in a SS housing, but the output red/yellow wires were less than 1 foot long , so I had to do a little extra TC wire splicing. Worked fine after installation. The new probes are built tougher, and hope they last longer than the old probes.

Mine acts up occasionally and so far all it has needed was a good cleaning of the probe. My field is at 7000', so I always start, taxi and T/O in a leaned condition. You may be getting a buildup of gunk from too rich a mixture.

Try it and see if it clears the problem.

Jim

I only have one EGT probe, so I can't swap them around. I tried cleaning it, no help. It did not look burned up, just discolored. I will try the heat gun test next time the cowl is off, but it is so intermittent, it may be hard to reproduce. I will probably just order a new probe from Van's. From the responses, it sounds like the gauge is not the culprit. Thanks for the responses.

J or K?

Anybody know if the Van's EGT thermocouple is Type J or Type K? ACS sells these probes, but I have to know which type to match to the guage.

Pull the probe. Replaced all of mine last year. 2 needed it and the other 2 not far behind.

Type K Thermocouples and Wire Color

Type K thermocouples are typically used in high temperature applications and are very accurate up to about 2400 deg F (1350 C). The wires are composed of chromel-alumel alloys and the wire color code for type K is yellow(+) and red(-), or green and white. Type J wire colors are black(+) and white(-), or white and red. I have worked on a few Vans EGT probes and all had Type K with yellow-red wires.