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OK, firstly, this is occuring on my Festiva, but that is immaterial really.
Car overheated couple weeks ago, due to stuck thermostat, and so it was replaced with a new one.
Car has run fine since, and temp gauge sits in its normal operating range.

Upon a random check yesterday morning, I noticed the overflow was over "full". Engine hasn't been started for almost a day.
Topped up radiator and it took the same amount of new coolant as there is in the over- flow over the "full" mark. Does that make sense?

Checked again this afternoon when the engine was cold, and the amount I topped up is sitting in the overflow bottle, which is now pretty much filled to capacity.

Faulty radiator cap is ruled out as I bought a new one, and I also swapped it with a used one that I know works fine, but the result is the same.

So the question is, why is the coolant NOT returning into the radiator?
It's like the engine wants to have a big air-pocket in the cooling system.
Any thoughts?

...why is the coolant NOT returning into the radiator?

Pin hole in the core/hose(s) etc. or a crack in the tank(s). You will find the culprit with a pressure test.

Crap in the pipe work between the tank and radiator?

Is the Festiva overflow bottle like the MX-5 in that it has a 'T' connection? The overflow hose attaches to one side of the horizontal T, while the bottom of the vertical T sits in the overflow bottle. I have read that the overflow hose must be attached to the correct side of the horizontal T. If not coolant will not flow to or from the overflow bottle. I believe there is a one way valve in the T connector, attach to the hose to the wrong side and no more flow.

J.

juffa you are correct besides the one way valve. Its just strange that its letting out/not returning a certain amount of coolant and no more.

If it is not this one way valve causing the problem, it can be caused by a pin hole that allows the coolant to overflow on expansion but causes system pressure loss once the engine is off. Then the system can't "suck" the coolant back from the overflow.

I had a similar problem on another car (not an MX5 or a Festiva). The thermostat did not have a by pass hole, pressure was building up before the thermostat opened and forcing water into the over flow. This then created an air pocket in the cooling system. To fix I just quickly drilled a hole in the thermostat, like pic below that I pinched from another forum.


Not sure about the Festiva is supposed to have a by pass or not, but worth checking since it is the thing that was replaced.

Yup, it has that little bypass hole with a little brass thingy inside it.

JBT wrote:If it is not this one way valve causing the problem, it can be caused by a pin hole that allows the coolant to overflow on expansion but causes system pressure loss once the engine is off. Then the system can't "suck" the coolant back from the overflow.

Ok, this theory makes sense, but as far as I can see/feel, there is no coolant loss due to leakage.
I'll check thoroughly on the weekend. Thanks JBT.

1red5 wrote:Yup, it has that little bypass hole with a little brass thingy inside it.

Then the next check would be to see that the thermostat is facing the right way and/or the thermostat itself has been properly manufactured.. If it is then the little brass thingy will not obstruct the hole when the coolant is flowing in the normal direction. If it is not then you might find that the flat head of the brass thingy effectively closes the hole.
I bought one from Supercheap a couple of years ago with the thingy in the wrong way. The problems were similar to yours so i just removed the thingy. As long as the coolant is clean it's not really needed.

I remembered comparing the OEM thermostat with the Tridon one, and they were pretty identical, and the brass thingy, and the thermostat itself, was installed facing the right way.

I'm choosing to ignore this as it's not actually causing any problems, besides the fact it's doing my head in.

Thanks for your help fellas.

oh and .... brass thingy!

I have fitted it in the wrong way once the dome bit should be facing the rad I think...

It does fit as in you can close up the thermostat houseing and bolt it all up backwards so becareful..