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Thermostat change guide - NB / NBFL

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Thermostat change guide - NB / NBFL

Postby njonesn » Sun Feb 16, 2020 10:26 pm

Changing the thermostat on an NB / NBFL is a quick and easy job.
The only tools you need are:
  • 12mm socket
  • 13mm socket
  • Philips screwdriver
  • Pliers
  • Lithium grease
  • New thermostat
  • New thermostat gasket (recommend Mazda OEM)
  • Catch tray and cloths to clean up spilled coolant


When changing the thermostat the following items may show signs of deterioration and should be replaced if they do:
  • Upper radiator hose
  • Cam cover to air intake hose.
  • Thermostat cover
I recommend draining the radiator, or at least part draining, when doing this job.
Some choose not to do this. It is possible to do the job without draining the radiator, but it is much more likely that you will spill coolant. I will highlight in the process below how you can complete this without draining the radiator at the appropriate point.
The job can also be done without raising the front end of the car and removing the front undertray but again you might find it easier if you do.
ALWAYS put the car on axle stands and NEVER work with a car supported only on a jack.

First, we need to drain the radiator. Leave the radiator cap on at this point.
To drain the radiator find the drain plug on the bottom edge of the radiator, towards the right-hand side. The plug is likely plastic so make sure you release the plug with a suitably sized Philips screwdriver. Place a catch tray under the drain before removing the plug. As you have left the radiator cap on the initial flow will be minimal/slow (unless you have a leak somewhere).
When you are ready release the radiator cap. This will immediately increase the flow. If fully draining the system expect to need to capture about 3.7L of coolant.
Once you have the coolant drained you can re-install the drain plug again, again being careful not to over tighten/damage the plastic with your screwdriver, and clean up any spills.

Next, we need to remove the air intake, making it easier to gain access to the thermostat cover.
Pictured below is my aftermarket K&N Typhoon air intake.
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The OEM air intake is simple to remove though the process is of course slightly different (if I recall correctly just two jubilee type clips, one on the airbox and one on the intake manifold). Lift the air pipe out the way (removing the cam cover to air intake hose if necessary; this can be degraded after nearly 20 years and if it splits you should replace it).

We now need to release the upper radiator hose from the thermostat cover. Again, if this is degraded or shows signs of leaking replace it.
First release the pressure clip, squeezing the clasps with pliers and sliding down the hose.
20200215_120149637_iOS.jpg (331.47 KiB) Viewed 598 times

Now gently twist the pipe while pulling it away from the thermostat cover.
20200215_115928012_iOS.jpg (337.02 KiB) Viewed 598 times

You should expect to lose a small dribble of coolant as the pipe comes away so have rags placed directly below (or a catch tray).
Some people suggest that you can forgo draining the radiator and simple release this pipe and catch the coolant that expels from the pipe. There will be a reasonable amount to catch as the top of the pipe and top of the engine drains however if you are quick you can raise the upper radiator hose above the height of the radiator and save loosing much from the radiator. Expect some mess if you take this approach, personally I prefer to minimise mess so recommend draining the radiator. Your choice.

Once the hose is detached you can remove the thermostat cover, again you might get a dribble of coolant as you do. If the cover looks like it has seen better days, especially where it attached to the upper radiator hose, consider replacing it; this is a known leak point.
20200215_115249504_iOS.jpg (327.03 KiB) Viewed 598 times

Mazda use a 13mm nut (bottom left of the cover, not pictured) and 12mm bolt (top right of the cover, pictured). These should be easy to release. It is likely that the thermostat cover will remain stuck to the thermostat housing once the nut and bolt are removed; gently persuade the cover to come away and remove the existing thermostat and gasket.
20200215_113854023_iOS.jpg (438.9 KiB) Viewed 598 times

The next step is to clean up the surfaces. You must remove any of the old gasket that remains stuck to the thermostat housing and cover. You can gently use a long blade to scrape the remanence off both surfaces.
Take care though as they are aluminium and soft, and you do not want to score the surfaces.
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20200215_113734898_iOS.jpg (511.11 KiB) Viewed 598 times

Once both surfaces are cleaned up you are ready to install the new thermostat.
Your new thermostat will likely have been provided with a rubber gasket pre-fitted. Many manufactures use the same thermostats and this rubber gasket is needed by some but not by others. The MX5 does NOT require this rubber gasket and it should be removed.
20200215_113943335_iOS.jpg (461.12 KiB) Viewed 598 times

The thermostat will sit in the reses of the thermostat housing with the small valve at the top, 12 o’clock position.
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M-m recommends using some lithium grease to help with sealing and water protecting the paper gasket.
Smear a small layer of lithium grease around the thermostat housing surface, where it will contact the paper gasket. Try not to get any inside the reses where the thermostat sits.
20200215_114455369_iOS.jpg (458.67 KiB) Viewed 598 times

Add a small layer of lithium grease to the paper gasket where it will contact the thermostat housing.
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20200215_114626118_iOS.jpg (501.73 KiB) Viewed 598 times

Install the thermostat and install the gasket.
If using the recommended Mazda OEM gasket, the green ring pushes against the thermostat.
20200214_184210179_iOS.jpg (346.46 KiB) Viewed 598 times

Add a thin layer of lithium grease on the outer side of the gasket.
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And onto the thermostat cover edge.
20200215_114943755_iOS.jpg (461.39 KiB) Viewed 598 times

Carefully offer up the thermostat cover to the thermostat housing making sure that the thermostat does not slip out of the reses.

Bolt up the cover, Mazda publish the torque as 14-19 foot-pounds / 19-25 newton-meters.
If you do not have a torque wrench then hand tighten and adding a half turn should be about right.
20200215_115249504_iOS.jpg (327.03 KiB) Viewed 598 times

All that is left now is to re-install the upper radiator hose, air intake, replace (don’t reuse) the removed coolant and burp the system.

To burp the system fill your expansion bottle and radiator with new coolant. As you get to the top of the radiator fins start to trickle it in squeezing the top radiator hose to expel air. Keep doing that until the air stops being expelled but do not fill the radiator right to the top.
Start the car with the cap off so any air /air locks can work their way out as the thermostat and pump do their job.
Keep an eye on the water level. If it drops top up the coolant and squeeze to top hose again. If the coolant level raises straight away, quickly switch off and let the air lock find its way out. Top up and start again, continuing to squeeze the upper radiator hose.
You should be able to see the flow in the radiator from the water pump circulating the coolant.
When you are happy no more air is being expelled refit the radiator cap and watch the temperature while waiting for the cooling fan to kick in and turn off again. Let the fan turn on and off at least 3 times.
If the temperature shots right up, or the fans stay on and does not turn off, switch the engine off straight away as there is an air lock that needs to be worked through the system.
Keep an eye on your expansion tank over the next few drives and top up as needed.
Job done. :handgestures-thumbup:
10th Anniversary Edition (1999 NB)

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