After much searching on the web I realised there weren’t any installation instructions on how to replace a thermostat on an MX-5 NC. I did find and started to use the Moss Motors supercharger installation instructions that include removing and modifying the thermostat but I found they remove more equipment than needed for a thermostat change, so I decided to make notes and take photos to detail the work involved and hopefully help those following me.
First of all I’d better explain the symptoms that alerted me to a problem with the thermostat. My NC is now 6 years old, an October 2005 build with 125,000 kilometres on the clock. I noticed the temperature gauge fluctuated with the speed of the car. On a cool day at a constant speed of 110kph on a flat road, the temperature gauge could drop to show almost zero and then rise again as speed decreased and load increased. Around town the temperature reading moved up and down and was never consistent, so I decided that the thermostat was either stuck partly or fully open and therefore needed to be replaced.
Next question is where is the thermostat located?
In the “old days”, the thermostat on my British sports cars were situated in the head where the top radiator hose connects and was easily accessed by removing two nuts, pulling the housing off and the thermostat out and dropping in the new unit and a gasket and retightening the two nuts, job done in 5 minutes.
Not the NC! The thermostat is located half way down the block, near the air conditioning compressor and under the inlet manifold, so you can’t see it. I quickly realised that this was going to be a time consuming job to get to the thermostat.
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The maze to negotiate!!
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Thermostat location. Clamp slightly to the left and below the white tag is the radiator hose connection.
Let’s get started. I’ll assume you have some mechanical knowledge; otherwise you wouldn’t be attempting this project.
You need a basic set of tools. A ¼” drive socket set with a 8mm socket (I used one of those El Cheapo Chinese sets), Phillips screwdriver, small flat blade screwdrivers, long nose pliers, a light, torch, jack stands, a container to catch coolant fluid that you’ll discard and replace with new coolant.
1. Jack up the car and place on jack stands. Working under the car, through a hole in the under tray, use a Phillip screwdriver to remove the plug from the bottom of the radiator and drain the coolant into the container. There is over 5 litres so make sure your container is big enough.
2. Disconnect a lead off the battery.
3. Remove the strut brace that goes from the top of the shock absorber to the bulkhead.
4. Disconnect the PVC tube going from the valve cover at the air intake hose (squeeze blue clip to release) and swing out of the way.
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5. Remove the intake hose between the air filter box and throttle body.
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6. Disconnect the small coolant pipe from the throttle body to inlet manifold using long nose pliers on the clips.
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7. Remove the four 8mm bolts holding the throttle body to the inlet manifold and set aside the throttle body.
8. You now will be able to access the two hoses (radiator and heater) attached to the thermostat. Make sure you still have the container under the car to catch the fluid when removing these hoses and thermostat in the following steps.
Two hoses to be removed under inlet manifold blue O-ring.
9. Using the long nose pliers squeeze the spring hose clamp on the large radiator hose and slid it along the hose past the end of the thermostat. Pull the hose off the thermostat and swing out of the way to access the heater hose. Do the same with the smaller heater hose.
10. It’s been easy so far, now comes the fiddly bit. Look at your new thermostat body and familiarise yourself where the three bolts are located. You can easily see and get to the top left bolt in the photo below; the two bolts on the right side require the ¼” drive and 8mm socket with extension attached. If your set comes with a universal joint the next part might not be so hard. Loosen the top left bolt.
Top left bolt location
11. Next, using a torch to see, looking through the maze of pipes and brackets you will just see the top half of the top right hand bolt. Feed the 8mm socket and extension through this maze and undo the bolt, being carful not to drop it. Tip: You can use some “Blu-Tack” in the socket to ensure the bolt sticks in the socket when you draw it out.
12. The lower right hand bolt is removed by “feel” as you can’t see it. With your left hand fell under the thermostat and locate the bolt head. Feed the socket and extension through the maze of hoses and guide the socket with your left hand fingers onto the bolt head and remove the bolt.
13. The hard part is now done. Remove the top left bolt and then the thermostat will come loose and can be removed.
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14. Replace everything in the reverse order.
I used “Blu-Tack” again in the holes of the new thermostat to hold the bolts in place while I refitted it to the engine block. Just make sure that there is no “Blu-Tack” between the face of the thermostat and the block or the bolt head.
How long will this take to do? At a guess, as I made numerous mistakes along the way removing more gear than I needed, I’d say you could finish this job in around 3 to 3½ hours.
On a slightly different subject, I also was experiencing coolant fluid leaking from the coolant tank only when I was at the track. I replaced the cap thinking that it was probably faulty only to find that the next track day the same problem arose.
Turns out that the coolant tank was cracked around the neck where the cap locks into place.
Yep, that's not cheap. There a bit of labour in it though.
