Why not Mazda OEM Miata MX-5 Thermostat?

I have three thermostats from different brands and I boiled them all on my stove in the kitchen. You see my friends, you need to understand this about me and my journey and this has been a long time curiosity in my adventures with Shadow (my MX-5) and how the running temperature affects the performance. I don't have fully realized scientific quantitative data to post here, just a simple metric and a comparison of the physical parameters of the three different thermostats. I feel that given a basic understanding of the physical, thermal and various associated other factors involved and knowing how the Miata is engineered when you see the side by side comparison of the three versions of the same part, the difference is stark and the logical conclusion is nearly universal, which this blog post seeks to illustrate in plain fair terms.

First I want to state the design goal here is to run the Miata's engine at the correct temperature range all year in the cold winter and hot summer. The engineers designed the cooling system to be more than adequate for the general customers use. The thermostat is there to block coolant flow during warm up after cold start. It is a variable valve which regulates the size of the opening allowing coolant to flow from the radiator to the engine. In winter it makes sure the engine is hot enough for good combustion. In the summer it does the same thing in reverse, the thermostat prevents the cooling system from over cooling the engine. The important part to note here is what temperature the thermostat starts to open (low flow) and the temperature when it reaches full open (max flow.)

OEM manual for my 2001 says the measured parameters should look like this;

Initial-Opening Temperature  °C {°F} 83.5-86.5 {183-187}

Full-Open temperature           °C {°F} 100 {212}

Full-Open lift                           mm {in} 8.5 {0.33}



OK good, we want it to start opening at 83C {183F} and reach full open by 100C {212F} and we want it to close again if the temps start to drop too low.
The actuation travel should be 8.5mm from closed to full open.

The three versions of replacement thermostats that I have tried running in my 2001 MX-5 are as follows;

OEM {180F} 82C Stant (USA)

OEM {190F} 88C Napa OEM specified temp

OEM {190F} 88C Mazda (Japan)


This first image tells most of the story. I use a little more than 1500 in this post to say the same thing.

The divergence in size is plain to see. 

Stant on the left, Mazda OEM on the right with NAPA fancy OEM in the middle. 

Starting with the STANT. (The one that I am uninstalling.) The smallest and lowest temp rated of the three, and is made entirely of metal. The other two have added material on the actuator, the NAPA has what looks like a type of rubber added as a ring whereas the Mazda OEM has made nearly the entire stopper out of a type of hard plastic. I am simply guessing at the type of material based on visual inspection. The next four images are the STANT.

I chose to install this one last fall. I am writing this at the end of winter so it didn't take long to figure out that it was not the correct temperature rating. The most obvious change is the heater took a long time to get hot. OBD2 data indicated that my running temperature would hold steady at 84C {183F} and this is just below spec for the "initial opening" temperature.

I put the Stant in to replace the Napa OEM that was making the running temps reach 100C {212F} regularly. The flaw in my logic stemmed from a lack in understanding the role the thermostat played in the cooling system. I learned that at highway speed and cool days the rate of cooling is too great so it closes a bit or all the way to keep the engine hot enough to maintain good combustion.  The temperature rating is the low point in the operating range, anything lower is overcooling the engine. 

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Next is NAPA, Japanese made OEM equivalent which was available at my local store. The 'brick and mortar' store is my preferred method of acquiring parts. I have three parts stores that I can walk to so if I can pick it up today without waiting for shipping then I will. Albeit these days most things need to be shipped in order to source exactly the right thing for my baby.

Size is a little larger and construction is better.

Temperature rating is just right! So why did this one not work out? The construction is most all made of metal. The following image attempts to show the rubber type ring mounted around the actuator. This seems to seal the opening or serve some similar purpose. I did not get an image of it, but when I depress the actuator with my thumb and compress the spring I could see that a portion of the material had separated and adhered to the mating surface. To me, I may be wrong but this seems to be as if the material is sticking the stopper shut.

I was seeing higher than normal temperatures during normal driving. This thing was risking my engine overheating so it had to go.  Mistakenly, I overcorrected by adding a different brand, and lower temperature rating. You remember it was the Stant.

When trouble shooting a problem it is best to go one step at a time and make one correction, and see how it goes, followed by the next one, and so on. Shotgunning a problem by firing off a bunch of possible solutions at once you have no possible way of understanding the true nature of your solution what exactly was it that fixed problem?

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Now back to the present with the newly installed Mazda OEM thermostat rated at 88C made in Japan. 

The actuator stopper is made almost entirely of a plastic type material, dark grey, donut shaped, and stamped with a USA Patent no. and is affixed to the longest spring of the three brands. The wax chamber is also the largest of the three models. (if you don't know why there is wax in there, look it up.) More wax means longer duration melt. I imagine the Mazda OEM has the most resolution when controlling the temp due to the longest actuator and the largest wax chamber. I seems to me that if the wax controls the temp then a larger amount is like a bigger dial. This means that the OEM thermostat provides wider range of flow rates as opposed to the Stant which operates much like and on/off switch.

I have no idea why there is that little black ring at the end of the stopper. I have read that this is a two stage type of thermostat but not sure how that affects this design. There is another model (I did not test) on the market that has a second mini actuator, two ports, so that one is obvious. The Mazda OEM model is the best looking build construction of the three I have here.

The next image clearly shows the girth of the plastic stopper and the stamping of the USA Patent number. This gives me a nice warm fuzzy feeling. The forums have long advised the use of Mazda OEM and for good reason. Obviously the engineers figured out something good enough to patent so why not run this patented design for engine temperature control in my MX-5?

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Now let's cook 'em!

OK so taking pictures of things boiling in water is harder than I thought. You see the Stant on the right is open, the Napa at the bottom is opening and so is the Mazda OEM but these two are open much less at this temp. When the water reach full boil 100C {212F} I quickly pulled each one out of the water and photographed it from the side to show relatively how far they each open at full flow.

Above you can see a good opening which measures within spec for the Stant. It opened to almost full open by the time it should be starting to open so that is not in spec. The next step up in temp rating for the Stant is {195F.} {195F} seems to be the USA interpretation of the Mazda specified temp rating, that's what most all the shops offer as OEM replacement.

Above you can see this Napa OEM version is not opening enough. The initial opening temp is within spec but it reached it's full open position much lower than 100C {212F.} This prevented the cooling flow from the radiator to overcome the heat generated at highway speeds. The temp would rise high enough to turn on the radiator fan. So the system was trying to overcool the radiator to make it colder to compensate for less flow to get the same cooling effect. It looks to me like that rubber ring added size to the actuator plunger also reducing the opening.

Above you can see the Mazda OEM model has opened to full flow and reached the specified actuation. The initial opening temp and temp at full opening are at the required spec. That little black ring has been positioned in the center of the opening which I guess may be to control the shape of the flow pattern somehow. But I also have a good imagination...

After installing this Mazda OEM model I drove 120 miles round trip and the engine temperatures are now averaging at 91C-93C and the engine seems to be running smoother and the heater works better! The running temp was like stuck at 84C with the Stant and that was just rough running.

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[the next two statements are speculation]

Another metric of note is the Long Term Fuel Trim. The low temperature cylinder chamber prevented full good combustion. The LTFT historically hovered around the -5 value. Always dialing it back a bit meant all the fuel was never getting burned. Well, it didn't take long for the trim numbers to reflect the change in running temperature with the Mazda OEM thermostat. The trip saw LTFT at +3 - +5, this means the ECU is able to add in a little more fuel because combustion is efficiently burning it all.

The first refuel after the new thermostat showed improved gas mileage by approximately 2 mpg. I already had a half tank of fuel when I put the new thermostat in. The second refuel ...

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Make sure to use a good gasket! The OEM seems to be the best one, again. I have used the Fel-Pro version which also has the little rubber ring on the ID. But this one is a green material and the shape of the ring has a little ridge formed in the ID. The Fel-Pro is more of a bead of a red material around the ID. I don't use gasket maker either, use proper torque on the housing bolts and get the mating surfaces very clean and smooth and there will be no leaks. You will see some drips at first but they stop after the gasket swells up a bit.

In summary, use the thermostat of your choice.

There seem to be many reasons not to use the off brand thermostats in a MX-5.

The question stands; "Why not Mazda OEM Miata MX-5 Thermostat?"