Did You Read the Fine Print? 

Jon, I think I could help in this situation you are having; well, maybe not help, but give you some insight into the issue. I build automated black oxide systems, so I deal with thermocouples daily. I also create the ai logic circuits for operating them, so I've got a little experience in these critters. You are using what appears to be a K Coupler for both your water and your EGT sensors. The way the K-thermocouple works is when it's heated, it generates a voltage. So, for an example, in your scenario, your meter is only suitable for a one voltage differential reading. That means your voltage reading cannot deviate more than 1 volt between the two probes. So, if you are measuring your water (coolant) temperature at around 210 Degrees Fahrenheit, you cannot exceed more than 1 volt. At 210 Degrees Fahrenheit on your k-coupler, you are generating about 4.1 volts. That means your other probe cannot exceed 5.1 volts, which would put your maximum temperature reading ability at about 255 degrees. If you exceed the 255 degrees, you will begin to have an exponential deviation on your meter reading. This goes for whether you are above or below the 210 Degrees on your other probe. For clarification, you are subjected to about a +/- 22.5 degrees Fahrenheit range between the two probes, maximum. To do what you are trying to do, you need an isolated reader which does so internally. Or, you can purchase two probe readers and accomplish the same objective. I hope this helps the problem a little better.

I am going to hate to see this series end ! It has been fun. I am the tech. advisor for our EAA chapter and I highly recommend this your our EAA chapter.

I like how the issue and the fix are on the first page of the manual. It might be tough to find a decoupling mount that will work on the heat of the exhaust but as long as you can insulate the one in the water jacket, that’s enough to separate them from each other.

Try putting a ground wire from the exhaust to the engine block

Hey, your running that engine in my camping spot! LOL, glad to see it running and that you're getting things ironed out. Keep up the good work

Now if you had 2 separate meter's with there own individual probe's, could they still Interfere with each other? Food for thought, you never know sometimes until you try.

Did you try a thick copper wire with allegator clips between the probes? Force the ground to be the same.

Maybe an earth strap between a head bolt and the exhausted stud would remove the voltage difference.

Thumbs up for any running engine.

I use the same meter to monitor the CHT and oil temp on a 420cc Predator engine. I have notice this meter is certainly better than other low cost meters, but if you operate the meter too close to the magneto ignition system, the meter will fault out.

Couldn’t a “ground” strap between the 2 equalize the voltage difference?

Good catch. (EDIT) By the way, are you going to be using that meter just for ground tests, or do you plan on using it in flight?

It is called commen mode voltage error.

G,day from Sydney Australia. Just to confirm. No need to insulate the thermocouple wires only the connection of sensors to engine part: wherein the volts could be present?. Could you harness them (wires) together? 🌏🇦🇺

Ahhh.. now I know why I sold instruments with isolated inputs.

of course differential voltage occur in dissimilar metals and galvanic pair too 👍

Does the radiator have to be above the engine?

You weren't sold something that was defective and they offered you a solution. This is not a flaw it is so well known that you even figured it out yourself. The only other solution is to use two separate meters which basically does nothing different than isolate the probes. If you put an outside voltage on any DUT the readings will be incorrect. Electrically isolating circuits can be nearly impossible, just ask the guys that fight with ground loops.

Ahh, the old RTFM issue....

That is the downside of cheap instruments.