Individual Cylinder EGT Probe Insertion Depth

[jaypat01]

I wanted to get a consensus on what is the normal insertion depth into the exhaust runner that an EGT probe is placed at. I am using the probes for individual cylinder trims and I have all of them about 20mm off of each exhaust port and currently I have the tip extended into the runner so that it resides right in the center of the cross sectional area. I figured this would be the area where the temp reading is least effected by external cooling around the exhaust runner, but I might be over thinking it. Question is...is this a typical depth for these or should a back it out a bit to something like a 5mm protrusion into the runner?

I'm asking because I've been having problems with some thermocouple probes grounding out the thermocouple tip to the probe housing internally (these are enclosed tip thermocouples) when I get temps up to around 1000degF and then the signal becomes erratic. After consulting the manufacturer extensively, they agree with me that this is happening, but they don't know the cause. We were starting to wonder if since the probe is inserted so far into the runner, that the portion inside the runner is warping under extreme heat and causing the thermocouple tip inside to make contact with the housing....although this is a pretty far out theory...this is what made we question if I have them inserted to far into the runner regardless of the issue that I am having.

[Adamw]

1/2" insertion or pipe center are trhe common rules of thumb.

Note thermocouple insertion depth shouldnt affect the probe, something is wrong with it if it does.  At my previous job in some of our furnaces we would have 3M long 1/8" thermocouples with almost the full length exposed to ~1200C.

Have you actually measured resistance between ground and one of the thermocouple pins to confirm there really is an internal short when hot?  I have seen some terrible thermocouple amp designs that cause odd random common mode noise.  

[jaypat01]

17 hours ago, Adamw said:

1/2" insertion or pipe center are trhe common rules of thumb.

Note thermocouple insertion depth shouldnt affect the probe, something is wrong with it if it does.  At my previous job in some of our furnaces we would have 3M long 1/8" thermocouples with almost the full length exposed to ~1200C.

Have you actually measured resistance between ground and one of the thermocouple pins to confirm there really is an internal short when hot?  I have seen some terrible thermocouple amp designs that cause odd random common mode noise.  

Same here @Adamw, but at my job we are using Emerson thermocouples instead of the ones I am using on my motor haha....so a lot higher quality.

And yes, I did measure resistance to ground on both leads, which is how I came to the conclusion of them being grounded out. Resistance is almost identical between the two conductors on the same probe to ground, but does vary by probe to probe that is unintentionally grounding out. The ones that worked fine had no continuity. Whenever they ground out, it does cause the overall temp reading to skew lower than the ones that are not, but it also imposes a jagged wave form on the signal which I am assuming is noise from the ignition system as it stops when the I shut the motor off. I know what you are thinking..."this could be a loose connection" but I have gone over everything. This time around installing the replacement thermocouples, I was very delicate with handling them and the wires and did not run them through a firewall or anything...I just laid them loose running back to my EGT-to-CAN module and I have ensured there are no loose connections on the module terminals. Also, I see no correlation in my logs while driving with engine operation to when the one problem child EGT probe starts to act up. It seems like temp is the only correlation because it won't act up until it is above 1000F and sometimes not at all.

Additionally, I have logged the EGTs directly from the EGT-to-CAN module and I get the same readings there as I do through the CAN bus on the Link ECU. So we can rule out a CAN bus communication issue.

Just for reference, these were the resistance readings I got from the first batch where I had 3 out of 4 that were bad.

Cyl#1 / EGT#1 - 233.6 ohms from black wire to braided shield - 234.7 ohms from red wire to braided shield

Cyl#2 / EGT#3 - No continuity from black wire to braided shield - No continuity from red wire to braided shield

Cyl#3 / EGT#5 - 38.5 ohms from black wire to braided shield - 38.9 ohms from red wire to braided shield

Cyl#4 / EGT#7 - 12.4 ohms from black wire to braided shield - 11.8 ohms from red wire to braided shield

@Adamw you may also notice I have the EGTs placed on every other thermocouple channel by my resistance measurement notes. This was because it is an 8 channel module (ECUMasters EGT-CAN Module) in which every two channels shares a common ground. During troubleshooting, I moved them to every other channel to isolate them from each other incase that was the issue.

[Adamw]

Replace one of the problem ones with a decent one from omega or similar and if the issue dissappears on that probe then that will confirm.  

[jaypat01]

@Adamw I mean I already know it’s the probe that’s bad and at this point I just think they all are of poor quality. It was confirmed by the company that sold them, that they had a bad batch that they tried to pull all of their stock before they went out, but some went out and I was the lucky one that got them (that was the first batch). Then the replacements came from a different supplier to the same company I bought from before because they couldn’t get them from the previous supplier due to Covid circumstances. Well, the new batch had one bad one with the same issue as the last ones. So at this point, I’m cutting my losses and just ordering a higher quality set from somewhere else. I will probably just order the Link ones honestly. 
 

[Adamw]

The Link ones are exposed tip so they definately wont have the same problem...