Coolant temp fluctuating

  1. My model of MX-5 is: NC 2.0 2005
  2. I’m based near: Highbury & Islington
  3. I’m looking for technical help or recommendations on: Coolant temperature dropping

Hi there folks,

Just looking for some advice. In the last couple of weeks, I’ve noticed that when I start to drive, the coolant temperature rises to normal, and then it will begin to drop intermittently down quite low, and then will rise again back to normal. This happens on all roads, at all speeds, in all weather - it rises to normal, drops, then rises again. Drives absolutely fine too, with no issues so far.

Presumably something to do with the thermostat, but the heater works grand as well, so I’m just wondering if anyone has any ideas.

Cheers all,


The thermostat keeps the radiator isolated until the coolant heats it sufficiently for it to open and allow the coolant to flow through the radiator.
Maybe the cold water entering the circuit from the radiator is knocking the coolant temperature down temporarily until it all gets up to working temperature.


I have had a similar problem, changed the thermostat and no issues since.
Not a difficult job on a MK3 just fiddly, a good small socket set and a steady hand.
If I remember I used a mirror and torch to locate the bolt under the thermostat housing.
The thermostat is a plastic part and comes complete with new rubber gasket.
Search your favorite auction site for “Mondeo” thermostats they are a quarter of the price a certain MX5 specialist will charge. be aware there are 2 types, long neck and short neck easy to see which you need. I did not have to replace much coolant, but make sure you have the correct fluid on hand.
A few video’s on the internet you can use as a guide.

Good luck

1 Like

Thanks for the advice guys, confirmed my thoughts! Will investigate the Mondeo thermostats. Thanks again.