Mk1 Temp Gauge rises slowly and stabilises at below halfway

Technical Area Engine, Transmission, Exhaust etc
1
  1. My model of MX-5 is: 1998 mk1
  2. I’m based near: Northampton
  3. I’m looking for technical help or recommendations on: coolant temp gauge

Hi All, I recently realised that my mk1’s temp gauge rises rather slowly (when compared to a mk2 for example), taking about 15min drive before itll stabilise, and after a long ish drive it only reaches slightly below midway point on the gauge. I had another mk1 a while back which I have sold since, and from memory the temp gauge needle would sit midway on the dash and temperature rises much faster than my mk 1 now.
Is this something I need to pay some attention to? I have had no lights or fault code as it stands.

2

This is an indicator that the thermostat is stuck open. Is the heater giving you full heat?
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3

To be very honest I haven’t paid attention to my heater since getting this car, but it seems to be quite warm in the car when I do turn on heater.
Is the way to go to change out the thermostat then?

4

I would first check the heater output. On the hottest setting with the fan on it should be putting out very hot air.
Next I would check the top and bottom radiator hoses for temperature during warm up. If they both warm up at the same rate then it is another indicator that the thermostat is stuck open.
:heart:

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5

Thanks heaps, will check this today on the way home!

6

Hiya Kaili,

If you need to change thermostat it’s a good time to do the rad too.

If colour of the radiator is green/olive like the picture below it might be worth doing too. Mine went at the bottom of the rad a week after this picture was taken.

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20230213_1514561920×2560 329 KB

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7

Thanks All, on the way home yesterday I’ve checked the coolant hoses on top and bottom of radiator while warming up the car and the heater in car while driving. All seemed normal i.e. there’s a temperature difference between top and bottom hoses and heater seems to have pretty constant output of heat (boiling inside the car given it was 15C yesterday and sunny!). I will do the old trick of putting the thermostat in boiling water and see if it opens up well or maybe just dirty and getting stuck opened at a certain position.
Will look into changing the rad at some point too (its been on my to do list for a while now!) as the car climbs to 105k miles, its a good reminder for that :slight_smile:

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8

One hose should get hot but the other should remain relatively cool until the thermostat opens and warm up with a ‘rush’.
:heart:

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9

Thanks for this, turns out it was indeed a failing thermostat.

When I put the old thermostat in hot water to check, it seems to not have failed fully, but was good to change it out - made my mpg much better :smile:

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