Low water temp on gauge.

 On the way home yesterday the water temp gauge only read quarter way
all the way home (30 mins drive on A1, no stop start and it was brass
monkeys outside) The heaters weren’t as hot as usual either.

It usually reads a rock steady “just under the mid line” after about 5 mins drive max.

I’m thinking the thermostat at front of engine?

If it is, will it do any harm driving the car in this condition until a new one arrives and is fitted?

Sounds like a stuck open stat

The car should be fine

I’d check that the coolant level isn’t low, as that could give the same symptoms, but otherwise you’re probably right that the thermostat is sticking open.  Probably won’t do any harm in the short term.

 Cheers guys, I’m in my garage tomorrow faffing with the brakes, so I’ll whip the thermostat off and make sure the coolant is topped up afterwards. I’ve got coolant and distilled water to do a coolant change too, perhaps I’ll do that when/if I have to change the stat.

More likely to be the stat or possibly the cooling fan on all the time.
Low water would normally give a slightly higher running temp, no, little or intermittent heater output. 

I don’t think the fan was on, I’ll have a look today. If the fan is on then I guess it is the temp sensor on the top of the stat housing then?
I’m not going to take the stat housing off today as I haven’t got a new gasket to seal it back up afterwards. I’ll order a new Mazda stat from mx5parts and fit it later in the week and I’ll flush out the coolant and throught the bulkhead pipes etc. with a hose at the same time as doing the thermostat.

Agree with others, thermostat stuck open. My fan runs most of the time once up to temp but the gauge reads ok ie just below mid way and I put a post on here about it. Will get round to changing the sensor in the thermostat housing soon to try to cure it. Running cool as yours is for a short time is not a big issue but may increase your MPG slightly. 


My MPG has been poor for ages. I have been systematically trying to eliminate why. The temp gauge has been fine in all that time though. Let’s see what happens when I get the new thermostat.


The car reached normal operating temp this morning and this evening, but it took the full 30 minute journey and only really reached the full operating temp on stopping in traffic and when I parked up. It didn’t raise any higher than the usual operating temp.

I ran the car with the rad cap off and pumped the pipes but no air bubbles came out.

I’ll feel the hose tomorrow and see about getting a new stat to fit at the weekend.

Thanks for all of the help everyone  [Y] I’ll let you know how I get on.

 Stat fitted and new coolant/distilled water put in.
I’ll see what it behaves like tomorrow on the way to school.
New (Mazda) thermostat was very different to the one in place. The one in place must have been a cheap generic one.

The old thermostat is stuck open (unless it’s very hot & you have asbestos fingers) so I reckon you’ve sorted it. The new one is a 2 stage one, the old one could well be a genuine Mazda one, the 2 stage one is not used on all models.

 How can you tell it is open? Is it because the bottom of the spring is higher up?
Thanks for clearing up the mystery of the differences - I had a mini panic when I first saw they were different, but then remembered Pollyana saying something about the cheap ones and the Mazda ones being different. It fitted in the hole and only went in one way around anyway.
I forgot to say, I sanded the lid and base (the way pollyana suggested) to get the old gasket off and my son had a go at polishing the top of the thermostat cover using wet and dry and the dremmel. Not mirror finish but great for his first attempt. I didn’t get a picture of that though.

Robbie is quiet right. Take a close look at the photo of the old stat and you will see that it is wide open.


I wasn’t doubting his expertese for a second. I was asking how to tell (for future reference) that is is open. Where is it wide open and what am I looking for?

The black seal which is more visiable in the top photo is where the water passes when the stat opens. The seal seals again the housing when the stat is shut. With a little manipulation, the old thermostat will probably pop shut again. For what it’s worth (trying to describe the operation of a thermostat without pictures or a real live thermostast)… The the cylinder on the bottom of the stat is full of wax. As the stat heats up the wax expands and pushes against the centre pin. The pin is hard against the top loop part so the cylinder is forced down opening the stat against spring pressure. As the wax cools & contracts the spring returns the stat to its closed position stopping coolant flow until it warms the wax up again. What amazes me is vehicle technology has come on leaps & bounds with engine management, ABS, DSC, CAN etc but the humble thermostat has remained unchanged.

Edited to add… -halli-, the bottom of the spring isn’t higher up, the wax part is lower down.

 Cheers Robbie [Y]

The car now gets up to temperature
much more quickly than it has ever and it stays on the mid line at all
times. The heaters aren’t as hot as they were when I first got the car,
but they are much hotter than they have been for the past year or so.
I think I might have damaged the pipe that goes into the bulkhead (the one on the passenger side) when I tried to get it off. I was planning to flush out the heater matrix with the hose pipe and I couldn’t get it off. The brass pipe moves in and out, but is still attached to something.
There is a grumbling noise in
the dash when I start the car which goes away after the car has warmed
up. I was thinking air lock though.
What is
involved if I have damaged the pipe? Is it a dash out job or is it head
under the passenger footwell?

 My missus came in last saying “Andrew the car’s smoking”

As I had driven to work with the roof down yesterday (not the first time this year I hasten to add) and I was getting no heat from the matrix when the car was stationary, I suspected there was a coolant issue. True enough steam and hot water had been gushing from the expansion bottle - eek. I took the expansion bottle cap off with a welders grove (used for the woodburner not welding) I topped it back up so that it would pull back into the system. The fan wasn’t running until I switched the ignition on - it ran for some time.

I checked again this morning and the bottle had not sucked back, so I took off the radiator cap to see liquid level at or just below the fins. I top it up and pumped the top hose until no bubbles issued forth, topped up again and refitted the cap. Temperature gauge looked fine on the run into work (with roof up - bit drizzly) and the heater was belting out tropical.

Obviously I’ve lost water, running the matrix dry yesterday, and finally was low enough not to cope with keeping the temp below boiling and ‘whoosh’. Had a look for leaks, couldn’t see any noticable ones in the dark. It’s in for MOT next week so I’ll ask them to pressure test (which will probably blow the 16 tear old rad.

Other option being the aforementioned sticky thermostat. Any other possibilities?

 I’d put money on it being the rad. You have all the symptoms I had which all disappeared with a new (copy) rad from MX5PARTS.