have had some niggles for a while, cold weather has made me more suspicious, I think I have a Thermostat failing !!!
seems ok in the summer weather I had not noticed put it that way.
let car run standing seems to get up to temp top hose not getting hot, until it seems to open water then runs through expansion tank, heater and fan off.
take up the road for a test drive ( has to be done any excuse ) heater still not on no fan, few miles Temp gauge starts to drop, whack fan on and it falls dramatically to nearly 1/4.
back in the driver temp starts to rise again.
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my questions are am I right of is there something else ?
is it a relative easy DIY job ( are there any pit falls if I try myself ?)
Maybe I should wait for a Tech Day so I have some back up ?
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Thanks in advance ( and a Merry Christmas and a happy new year to all MX5 owner )
all the thermostate does is blocks water to the rad too allow the engine too warm up to temp
So by it being stuck open which is the general fail safe of most modern stats the rad is always cooling hence the car getting up to temp stationary and cooling down on the road
If the car were a MK1 or MK2 a thermostat change is easy even if there are issues such as characteristic bolt snapping.
The MK3 looks like a job that should only be done if you are sure that is the fault.
In your position I would use a code reader on the OBD port to ensure that there is nothing else diagnosed by the car ecu as causing your issue.
As I understand it a malfunctioning thermostat usually sticks open so the car will struggle from cold as too much circulation. You would notice the top hose gradually heating up whereas it should only be incorporated into the cooling system when coolant noticeably hot. You need to make sure that is is sticking open as from the original post there is some doubt.
Do not replace the thermostat unless absolutely necessary.
Steve, it’s not a hard job to replace the thermostat. Once the throttle body is out of the way there is quite a lot of room. I’ve known them to fail on NC, but from memory they tended to restrict flow causing overheating concerns rather than over cooling like a stuck open thermostat does. I can’t think of much else that would cause your concern though so worth changing it I reckon. If it is stuck open it’ll be obvious once removed.
Steve, it’s not a hard job to replace the thermostat. Once the throttle body is out of the way there is quite a lot of room. I’ve known them to fail on NC, but from memory they tended to restrict flow causing overheating concerns rather than over cooling like a stuck open thermostat does. I can’t think of much else that would cause your concern though so worth changing it I reckon. If it is stuck open it’ll be obvious once removed.
Thanks Robbie I think the same, have watched on You Tube and read the Haynes manual a few times, its parked up for the winter so not as if I need it at the moment, its just making a start, or wait for the spring Tech day if there is going to be one ?
One more fact, when I went on the Funball rally, up in the mountains Stelvio pass etc, it was dumping water out of the expansion tank, got after market cap on it which was thought to be the problem by other Funballers at the time, it also did it more recently when we did the Hardknott pass, could this be a possible symptom? driving at altitude thermostat stuck open causing pressure to make the expansion box over flow ?
It is certainly sounding more like a thermostat problem.
The ideal scenario is restricted coolant flow on starting until the engine is up to temperature. This allows the engine to heat up relatively quickly without full coolant flow taking the heat away. Once the engine is hot, the coolant temperature rises and causes the thermostat to open allowing full flow through the entire system and better cooling. This is normally enough to stabilise engine/coolant temperature within an acceptable range. If on a hot day or long high speed journey the temperature rises outside of this range with the full coolant circuit in operation, the engine fan should cut in to quickly cool things down.
If the thermostat does not open as Robbie mentioned, the temperature would build up within the restricted flow circuit. Apart from blowing water out through the radiator cap and expansion bottle overflow when the temperature and pressure rise sufficiently, the hot coolant may not circulate to the sensor that turns the radiator fan on.Â
Radiator caps are pressure operated so can cause an issue on their own but my guess is your thermostat is not opening when it should.  Â
Two further points as your car is a 2006 get a Mazda thermostat not an aftermarket one as some of the aftermarket ones do not have the latest improved design.
The expansion tanks at that age are starting to fracture around the filler area so while quite expensive, I would change the tank at that age.
hOk job done so thought I would let you all know how I got on.
Removing the thermostat was what I would call easy, and all the parts before it, replacing the new one also I would say was easy.
so proud of myself, then it started to go belly up, no heat in cabin was first, air lock, Robbie sent me the procedure which worked.
Then I noticed I was getting no heat in top hose from Thermostat even though fan was on try to cool it down, also nothing running through expansion tank, I pumped hoses all the ones I could find got nowhere, eventually decided thermostat must be faulty ( Blue print ) could not get one from MX5 parts no stock, so removed must admit came off very quick, when compared to original, the inside part seemed to be 3mm longer, took it back.
Supplier not confident it was faulty. but exchanged for a Gates Thermostat which was exactly the same size when we compared to the original one.
fitted had a few air traps but they soon cleared, all seems well, runs a lot better and holds its temp, and toasty in the cab.
glad I went for it, more confident to try more in the future.