Which temp sensor to trust?

[Roadrunner]

My SE has 2x coolant temp sensors.

- The factory sensor at the back of the head.
- A speedhut coolant gauge sensor in a Maruha spacer/adaptor at the thermostat outlet on the front of the head.

I can read the ECU temps (rear of head) live via an ECU display and the Speedhut gauge is mounted in my dash.

During normal driving both temp gauges are basically even. However when I get on the race track....

During hot track days, the 2x sensors climb evenly until 98degrees. From there the factory ECU maintains between 98-100degrees the whole session while the front sensor just keeps climbing to around 110degs+ in a 6 minute session at Wakefield Park.

Did a night track session last night (SMSP GP circuit) where it was still 30degrees at 7pm and by mid session (20min sessions) the back sensor hadn't budged over 100degs but the front sensor at one stage was 114degs and slowly climbing higher until I backed off and temps dropped almost instantly.
When below 100 degrees both sensors show the same temps down to 90 and below.

Since the ECU relies on the rear sensor, should I not be concerned about seeing upwards of 115degs at the front of the head?
I believe the head gasket gives out at 120degrees which is why I'm worried.

So my trackdays lately have been me backing off for fear of overheating...but am I really? If the ECU is only seeing absolute max of 100degrees and is not pulling timing etc, is 10-15 degrees hotter at the front of the head "normal" and nothing to be concerned about?
And since the rear of the head is suppose to be the "hot spot" due to the poor factory coolant flow path, should I look to move the speedhut sensor to the rear of the head somehow and only be concerned if it goes well over 100 degrees from there?

[Lokiel]

See https://drive.google.com/drive/folders/0B3YbfB30DxW0bEdwS0xSNGVXMzA "Engine Control System.pdf" Pages 64..67
Note: This is for an MSM/SE - the calibration curve may be different for other cars.

Note the OEM CLT sensor's Resistance vs Temperature graph on page 67 that the Resistance delta between 100*C and 120*C is very small which explains what you're seeing (ie. the OEM CLT sensor "isn't much chop" above 100*C, anything over that just reads ~100*C).

I'm betting your aftermarket CLT sensor has a much greater delta between 100 and 120*C - you'll need to find its thermistor graph to verify this.

Do verify that your ECU's CLT calibration curve is correct too - it may be "flat" from 100*C onward which would also explain why you never see it go above 100*C.

Your fans should be programmed in the ECU to kick in between 87 and 93 degrees.
If you do want to use a programmed shutdown if the car gets too hot, replace the OEM sensor or use your aftermarket one to control it - just don't assume that because your OEM sensor is reporting 100*C that it's never actually above that.

You might want to test your CLT sensor as shown in in the pages above to test this.
To test temperatures over 100*C, you'll need to use oil so be careful.

[Lokiel]

Just to follow-up on alternate temperature sensors having better resolution than the Mazda OEM CLT sensor over 100*C, here's the GM fluid sensor's resistance vs temperature graph and chart (from https://www.bmotorsports.com/shop/product_info.php/products_id/1669 "Specifications" tab):




You can see in the chart that above 100*C, the resistance range at 5*C intervals is still significant enough to be able to measure, making it a better sensor to measure temperatures over 100*C.

As I said earlier, the NA/NB OEM CLT sensor was intended to ensure that the fans activate at the correct temperature (which is below 100*C); at 100*C or over the coolant temperature has gone critical - the OEM ECU does no safety shutdown procedure.

[manga_blue]

Excellent response from Lokiel. It's so refreshing to see a complete and factual response here.

I might further add that all the blown head gaskets I've seen have been beside #3 cylinder or in the #3/#4 cylinder area. So it's clear that without a re-route there can be significantly higher temps at the back than at the front. In which case both your sensors could be right.

[Roadrunner]

Thanks for the reply Lokiel. That makes complete sense. I have seen the ecu temp go over 100 (102 chasing a wrx around WP).
Sadly I can’t find any info regarding the speedhut sensor resistance but based on your info it’s pretty safe to say it is reading correctly and I am getting stupidly hot at track days

[Ned Loh]

How hot is the coolant when the needle from normal (call it middle) towards Hot on a standard car? Has to enough resolution in that range to do so?

[Roadrunner]

I reckon that would have to be about 110degrees on the ECU/back of head sensor. I've only seen that once and it moved from 11o'clock (normal) to 12o'clock. That was monitoring with an OBD display.