Well, I did a thing, It was a thing I knew better then to do, too. (Thermocouple connecting) Options

[VaccaRabite]

Continuing to live my life as an object lesson for others...

I recently had to pull my engine for some repairs. For me, part of pulling the engine includes removing the thermocouple from under the #3 spark plug and fishing it out of the engine tins. This runs to the CHT gauge in the cabin of the car.

And we all know you are not supposed to cut the wire. Everybody knows that.

So I figured I'd cut the wire, add a nice weatherproof automotive plug, and see what happens. All in the name of convenience and curiosity.

I drove 300 miles last weekend. Including going fast up a really long couple of hills (like miles long hills) in 5th gear.
So the results.... At cold start, the gauge reads the same as ambient temps.
Under normal driving, the CHT gauge read slightly cooler then before I added the plug, but within 10 degrees of what I expected.

However, when pushing the car, the error rate grew exponentially. Where I would expect the car to be running 340-350 head temps, the gage only showed about 300. And pushing 90 up hill in 5th for over a mile I would expect the gage to read 360+ the gage only ever got to 325. I did not push past what I know this engine required to get to 375 (which is my back off point).

The result of this experiment is that if I did not know and expect bogus returns on the gage, I could easily roast an engine and start dropping valve seats, while a quality CHT gage was reporting I was WELL under the 400 degree danger zone.

I have another cable I can swap in. Kind of a pain, but not a huge deal. I'll add it o the maintenance schedule.

But, yeah. In this case the group think is accurate. The very slight resistance that the plug will add to the cable will be enough to throw off your CHT monitor.

Do not cut the line between the thermocouple and your CHT gage.

Zach

[Literati914]

Getting the spark plug and ring out has always seemed such a major PITA. If you come up with a disconnect solution it will be a service to us all.

I’m confused as to why not leave the ring under the sparkplug and just unscrew the two small connecting screws that are about a ft. down the line (IMG:style_emoticons/default/confused24.gif)
Here’s my DD lead wire:

There are a couple reasons:
1) it’s common to put some heat shrink tube over the screws to protect them and keep them in place when driving. Which just adds another barrier to getting at them.

2) I also have mine routed under the tin (fairly common) so you can’t get to the screws anyway. They are hidden under the cooling tin. There tend to be a lot of wires and hoses on the 3/4 side of the car next to the battery, and this gets the TC wire out of the way.

3) those tiny screws are a PITA to handle and tend to get swallowed up in the engine bay when you inevitably drop one.
Zach

Uh Hu.... (IMG:style_emoticons/default/happy11.gif)

Note to self: route TC wire above the tin and use a magnetic screwdriver, got it, thanks (IMG:style_emoticons/default/poke.gif)

.

[stownsen914]

Is there a way to join a K type thermocouple to a proper extension lead, other than an Omega plug? Are there butt splices made of the correct material?