In a grounded EGT probe the wires are connected internally to the outer sheath of the probe.  In an ungrounded EGT probe the wires are isolated from the outer sheath.

Q: I am looking for several EGT sensors for tuning a normally aspirated 2.3 L gasoline engine with 1 5/8” exhaust tubing. Are there any available guidelines about what size EGT probe is recommended, 1/8”, 3/16” or 1/4”?

A: We find that, for most applications, the 3/16” EGT probe is suitable.  It’s our most popular probe, with nearly 75% of the probes we sell being this diameter. 

I have a Mercedes turbo diesel and my idea is to monitor both the exhaust temperature and the temperature of the incoming air in the intake manifold. Is there enough range on your EGT probe to measure also lower temperatures (min. about -10 Celsius)?

A quick way to check the EGT probe is to disconnect it from whatever device you are using to read it & put an ohm meter across both output wires (the wire colors are usually red & yellow). If you get an open reading then the probe is probably bad. If you get somewhere between 3 to 12 ohms then it may still be okay.

Typically it is the center of the exhaust stream. This is not necessarily the center of the runner through the exhaust manifold or the header tube. The exhaust stream is dictated by flow rate, bend radius, the angle of the valve in relationship to the exhaust port, port configurations and other factors.

There are many factors to the "accuracy" any sensor. The most important aspect is what accuracy is needed to satisfy the actual requirement and the evaluation process to get there. Typically if your EGT sensor is within 1%  to 5% of the actual temperature will satisfy most requirements. The major factors are probe accuracy and the indicator accuracy, taking these both in account will give you a close estimate of the actual temperature you are reading from the indicator(s).

One EGT probe placed in the collector will measure the leanest (hottest) running cylinder. For a quick test of which one that is you can do the following:

What we suspect is happening is that the EGT probe has developed a “false junction”.

The primary measuring junction of an EGT thermocouple probe is the tip. This is where the 2 wires meet & are welded together. If these two wires touch at any point other than this junction before it plugs in to your gauge or controller then this second junction would be the “false junction”. The result is that the temperature reading becomes an average of the two junctions.