Coolant sensor heatsoak?

[Roadrunner]

Pretty sure this is heatsoak, but thought I'd put to the forum to confirm.

I recently installed a speedhut water gauge with the sensor in the top radiator hose.
Around town etc it reads pretty close to the adaptronic temp readout which is taken from the back of the head (supposedly the hottest part of the engine) so I have something to compare.

After a 2 hour drive on the freeway on a warm day (both gauges pegged at 88degs the whole way) we pulled into the hotel to check in and turned the car off. I jumped in the drivers seat after 2-3 minutes to move the car and the gauge was on 103 degs! After I started, the ECU showed 93degs and the speedhut gauge quickly fell back to the same.

The next day at the track, the speedhut was hitting 105+ after just 3 laps while the ECU was showing 95 (never got over 99degs the whole day). I'd back off for a cool down lap and it would take half a lap for the gauge to start dropping. ECU dropped quickly to 90degs.
Come in to the pits at end of session, turn the car off but leave ignition on to see both displays active and the speed-hut would slowly climb in temp for a few minutes while the ECU display would slowly drop. Then the speedhut gauge would drop after its initial rise.

I'm assuming this is simply the sensor adaptor squished in between the hot as hell alloy radiator, hot engine and intake pipe of a turbo car getting heat soaked and effecting the sensor? The adaptor was blistering hot to the touch.
No other SE's there had heat issues and I don't believe mine was either.

Seeing that the ECU reads from the factory sensor on the back of the head, and that's the hottest part of the engine, it's not possible for the water temp to be significantly hotter leaving the thermostat is it? Simply heat soak?

[hks_kansei]

The top rad hose line is the last one out of the engine back into the radiator, it will usually be where the coolant is hottest (having passed through the engine)

The coolant still needs to pass from the back of the head/block (oem sensor location) to the front to reach the top hose, I wouldn't consider it to be particularly odd that there's a 10degree difference.



As for the issue when stopped at the hotel, even when the car is off it's still heating the coolant in the engine (it's an iron block, there's a LOT of thermal mass there)
when you first crank it the hot coolant is pumped out into the radiator (past the sensor)
and cooled coolant (from radiator) is pulled in and past the OEM sensor.




Basically, i'd say it's normal to see a difference between the two in those positions.

[project.r.racing]

the cooling system raises in temp before lowing when a car is turned off.

[Roadrunner]

Ok that's good to know.

Getting the gauge sensor put in the back of the head soon next to the stock location and wanted to make sure what I was seeing at the moment was normal. I thought the rear sensor would have shown hotter than the front sensor on track with the somewhat dodgy water flow through the head that mx5s are renown for.

[hks_kansei]

Ok that's good to know.

Getting the gauge sensor put in the back of the head soon next to the stock location and wanted to make sure what I was seeing at the moment was normal. I thought the rear sensor would have shown hotter than the front sensor on track with the somewhat dodgy water flow through the head that mx5s are renown for.

The rear sensor area normally is hotter than the front of the engine, but in this case your aftermarket sensor is mounted in the last point before the coolant reaches the radiator, which is where it's at it's hottest (having been passed through essentially half the engine once more before it comes out the top hose)


Personally i'd just leave it where it is and not worry about the difference. (obviously if it shows a massive difference then your thermostat is probably screwed)

[Roadrunner]

So I have checked my ECU data logs and videos from the day and when the engine was showing 99degs (the hottest of the day) the Speedhut gauge was pushing past 110degs.

As most people use a scangauge/android etc to monitor temps on track (this using the rear sensor) is it acceptable/normal for the exiting temps to be 110+?

Before when I was on the factory ECU (runs lean/hot) and having thermostat issues my scangauge on track would show 108degs, Id hate to see what the exit temp was!

For over 12months using the scangauge I had been chasing down temp issues on track and had finally got it sorted. Not going over 100degs on a stinking hot track day was huge for me. Now I install a gauge in the dash to monitor it without relying on an android phone and the gauge is hitting over 100degs within a lap or two flashing warning lights in my face

The reason I got the gauge was I didn't want to have to turn an app on everytime I went out on the track.

So I guess I'm asking, do I still have a temperature problem on track? If I never installed the gauge, the ECU would be saying, no I don't.