Is my thermostat stuck open?

Hi guys,

First post on here but I have been finding a few conflicting answers regarding my query so apologies if this has been answered 1000 times before!

I have had my first MX5 NC since May last year. I have never paid attention to the thermostat needle at any point before this so not sure if the car has always been this way or just recently, either way I think something is up with the thermostat!

The car seems to heat up to temperature within a few miles as expected 6.5 notches up (4 oclockish position) but as soon as you start driving anywhere between 30-70mph+ the engine temperature plummets down to 2.5 notches (just after 5 oclock) before slowly climbing back up once in slow moving/stationary traffic. Is this a sign that the thermostat is stuck open?

This is a second car and I have been lucky enough to only have cars built within the last 5 years up to this point and once they hit half way never move again. Unsure whether this is a characteristic of older cars or not as I have no prior experience.

The actual heat coming out the vents within the car seems hot enough (never have it past 22 anyway) and I had to have a new AC condenser fitted in September so that all seems ok.

If my suspicions are correct and I do need a new thermostat where is the best place to order one of these? I have no technical knowledge so this will be into a local garage to get fixed. Am I best letting them diagnose and stick whatever thermostat they choose or supply my own? They seem to vary a lot in price from the quick google search I have done. I have already checked chassis number and mine is the short type which is in stock at MX5 parts for roughly £75.00 but I have also seen them as low as £15.00!

The car is due for a service and MOT in Feb anyway and I am also having every fluid in the car (including coolant which I understand will need to be done with the thermostat) and having 30mm lowering springs to bring it the correct height :wink: because I want to do a couple of track days this year. I may as well get everything done in one hit whilst the car is in!

Thanks all!


Yep, but same thermostat was shared with Ford @£21.Mine was same issue as yours.New therm with mazda longlife coolant job done.

Get the long or short neck stat, it doesn’t really matter, if long neck then trim the hose a little, that’s the only difference. I fitted the Mahle stat @£14 approx,  now the heat is great and the gauge rock solid at centre point ish.

The long will replace the short one. Try

Paid £12 if I remember

I am getting the low temp gauge so have just ordered from Opie, on offer and i had a 10% discount code so a bargain at under £11 delivered.

My question is only about 6 months and less than 1000 miles ago i had a flush out and the 11 year longlife F22 think its called coolant put in. How much needs draining to change the stat ?  i have about 4.5 ltrs left and if i have to drain the lot will have to do a 40 mile round trip for another 5 ltrs or can i capture some and reuse ? Which would only be about a litre or less as took about 5.5 ltrs to refill.

You will lose everything above the therm and its going to get messy.How much you lose i haven’t got a scooby doo, but you’ll need to burp it as well. Think you may be ok with 4.5 lts  cant see you losing that much to be honest.


Why don’t you find a suitable container and drain the radiator into it .
You will lose very little fluid.

I did the above and saved mine, it was changed around 18 months before. When returning the coolant to the system I strained it through some gauze although there was very little muck in the system.

Thanks all for replies, i was going to drain through the drain plug so think I’ll drain 3-4 litres and  save some and see if that’s below stat level and good idea to strain any if i need to re-use.