Troubleshooting Coolant Temp Issue (FMII x Miata Link ECU)
 

I'm currently dealing with a discrepancy between the coolant temperature reading from my Miata's LINK ECU and the car's dashboard temperature gauge. After extensive driving, the Miata Link reports a temperature of 116°C. However, the car's temperature gauge remains centered. Given that the TEMP LIMIT is set at 110°C, this leads to a power drop, even though there isn't an evident overheating issue.

**Background:**
- 95 M-Edition 1.8 Liter Engine
- The car was tuned by Flyin' Miata back in 2004 in Colorado and has primarily been driven on the East Coast. It was recently relocated to Phoenix, Arizona.
- It has clocked 52,000 miles, and I've just done a timing job along with an upgrade to a new FM intercooler. I own this car for over 20+ years.
- The setup includes an FMII Turbo kit, FMII Radiator, FM Exhaust, Miata Link ECU etc.
- I've replaced the plugs, belts, hoses, and piping, ensuring they're all new.
- The intake air temperature sensor is functioning as expected.
- Boost levels are manually controlled by Turbosmart Boost Tee, capped at 10psi for the summer.

I attached documentation of both the ECU and Turbo Kit I have onboard. This is legacy equipment so figured this might help.

Truly appreciate any insights or troubleshooting steps this forum can offer. Honestly this is the first possibly "ECU related issue" I'm experiencing in the 20+ years I've owned the car.

Hey bud, awesome that your car was tuned by FM in 2004 and is still kicking.

The sensor for the gauge inside the car is different from the one the ECU reads. I'd look there, I bet the sensor went bad. 116*c is 240*F which is definitely hot, what's it read when cold? There could be an actual overheating issue, but I'd expect the stock gauge to start moving. The stock coolant gauge on mine moves by 220-230* on my 1.6, iirc.

The two use different sensors. The gauge is a dummy gauge, meaning it goes dead center from about 160-220, when it'll rapidly climb. There are writeups on how to linearize it, google it if you want it to act more normally. But if it's reading center and the link says 116c, more than likely it's an ECU sensor issue. Either bad ground or bad sensor. If you truly are at 116, you've warped your head.

Thanks guys! By chance do either of you know the possible location of the ECU sensor (and the ground)? I haven't been able to locate it myself (documentation doesn't indicate anything)
The cold reading is 32 C.

Back of the head on an NB, presume so on all BP. It is the location of the thermostat on the FWD source cars for the engine, which changed when engine converted for RWD.

https://www.miataturbo.net/general-m...sensors-45678/

I found this thread which states .. does this make sense? If so how do I swap it out?

"Then there's a two-pin sensor installed in the rear water-outlet that feeds the heater core. This sensor feeds the ECU, and has the same thread as all of the 1.8 sensors, both the two-pin style in the '94-'97, and the three-pin style on the NBs."

Normal city driving operation the car reads 84-94c in Arizona climate. If that helps.

Your sensor doesn't sound that far off to me, but they are cheap enough I'd buy another one and swap it out to see.

I.E. your cold reading is 89*. Does that match the ambient temp when you took that reading? Summer in AZ that sounds about right.

Your "normal city" driving operation is 183 - 202*, which also sounds accurate.

So to me it sounds like the car might actually have a cooling issue. What is the condition of your cooling system? Fans working? Stuck undertray and ducting intact? Stock radiator or aftermarket? Mix of coolant and water?

My cold reading is 32c
My normal driving reading is 84-94c (startups and drive around a few blocks)
My highway driving reads 94-105c and after about 45 minutes of driving a particularly when i drive north its reads 110-116c