Water Temp sender

Fawls · Jan 13, 2026

I am after some feedback...

With the gauge mods I needed to add a new water temp sender, I had intended to use the plug in the drivers side of the block, when I looked there was not enough room to put a socket on it to remove and if I did not enough room for the sender unit without touching the headers, so to plan B, tee it into the factory sender location. My understanding is the original is still required to send data to the ECU.
After Teeing it in I am wondering if I will get accurate readings from it / them due to reading air from a probable air pocket due to coolant not reaching the senders?

Does this make sense? or am I over thinking things. I haven't had a chance to take it for a test drive yet.

cobrajeff96 · Jan 14, 2026

If that T-junction is the highest point in the coolant system and it is not burped free of air, there is a good chance the top most sensor will read full hot a lot of the time. The liquid and the air pocket (if one exists) will kind of slosh around against eachother constantly as they always do.

The ECU-fed sensor is mandatory because it controls radiator fan activation/deactivation.

Fawls · Jan 14, 2026

Thanks for the input, good to know, I 'm not sure weather the air pocket would make for a higher or lower reading.

I managed to take it for a quick spin around the block, when I got back I checked the temps on the bottom of the Tee and they were near enough to the gauge reading which is an educated guess anyway, I will keep monitoring it and see what it looks like on a longer run.

I think the gauge was reading a touch higher than the manifold reading which could be explained by the air pocket. A longer run (thunder storms today) might paint a better picture.

Daryl · Jan 14, 2026

Sounds like you’re on the right track. Please forgive my ignorance but by the picture alone I can’t get a great idea what engine that is?

Fawls · Jan 14, 2026

2002 2Valve.

cobrajeff96 · Jan 15, 2026

Water is one of the most efficient conductors of heat. With an air pocket, the temp will skyrocket if said air winds up on the sensor. Then it will go away, then it'll come back, and on, and on.

The way you have it there in the pic, the water flow looks to be dead-headed right there at the two sensors with no path out. If an air pocket does form, I'd suspect that place (especially if it is the highest point in the system as air bubbles eventually find there way to the highest point). I guess you'll know soon enough.

07GtS197 · Jan 15, 2026

Another option is to either tap the crossover on the drivers side where there is a flat spot or you might be able to swap an npi crossover on a pi intake so it’ll have the two ports.

Fawls · Jan 16, 2026

Thanks I will look into that as an option if it proves to be a problem.

Good news Bad news, Looks like coolant is getting to the sender, bad news, small leak at the top sender (already fixed). The only threads I didn't use any sealer on because I thought red stuff on the threads was a pre applied sealer?

96blak54 · Jan 17, 2026

Im thinking both of the sensors need to be in direct flow of the water. Clean setup tho. Good idea

Fawls · Jan 17, 2026

I think the coolant was only getting up there was because the air escaped first, normally it would have no where to go.

cobrajeff96 · Jan 17, 2026

You are correct that a temp sender should be in a flow path and not in a dead end.

Fawls · Jan 17, 2026

I have ordered a 1/8 NPT Tap, the flat spot on the drivers side looks like the best option.

Miltoid · Jan 20, 2026

Crack it open a little and bleed it out.

96blak54 · Jan 20, 2026

Fawls said:
I have ordered a 1/8 NPT Tap, the flat spot on the drivers side looks like the best option.

Yeah, just bite the bullet and do it like factory. I get lazy tho

Fawls · Feb 7, 2026

All done, hole drilled & tapped, had also notch the alternator bracket for clearance, I put some tape around the tap so I wouldn't go too far down with the tapered tap.

I put some cardboard in to cover the holes to stop swarf getting into the engine, easier to vacuum out.

Fawls · Feb 7, 2026

Took it for a test drive and the needle got up to the H on the gauge, after talking to Mark he suggested it was reading an air pocket and wanted me to take a reading of the resistance at the sender at operating temp to see if the gauge was reading right.
After it had cooled after the first run I had to top the tank up quite a bit and was hope full this was the problem.
Second test drive same thing, took the readings and the gauge and sender are working as they should so I suspect I still have an air pocket.

My next step will be to drain the coolant and vacuum fill it again, before I do this is there an easier/quicker way?