Here's one that has got me puzzled. My 1.4 had it's MOT on tuesday which it passed first time, they did however mention it was running cold, something i had noticed as the gauge barely moved and it had no heat.
I changed the thermostat and coolant let it ran, and up shot the gauge and there was heat - brilliant.

I then tested the stat which opened as it should, so no fault with that. It did have some bright green antifreeze in it which was pretty much neat. I am wondering if this was changing the point to which the stat opened, but then the stat would have to be stuck open surely to make the car run cold which it wasn't.

I am wondering if it had some expensive fancy antifreeze in it which wasn't designed for that type of engine. It now has good old cheap bluecol in it now!!!!

I'm sure I read on turbobricks that you can cook up a volvo thermostat and see it opening but this doesn't prove that it works! I think you probably had a dodgy thermostat, regardless of the test. You swapped it, the problem went away

you can do that with any thermo. just take the thermo out and put it in a pan of water, if you gentally warm the water on the hob and see if it opens you will know if it works or not.

if you use a thermomiter (spelling) you will see at what temp the gauge opens too.

but you can run a car with no thermostat. the turbo boys do it but the problem is the car will run with colder water and you will use much more fuel.

the thermo could be wedged open?

i have had loads of cooling problems with my BMW think its fixed now?

I did boil it in a pan, i thought that someone had removed it originally thats why the car was running so cold, i was surprised to see it in the pipe. At the end of the day its sorted but i like to know the reason for things!!!!!.

Here we have only green antifreeze, I was very surprised to see that blue once. Red at least is different stuff again, this you can't mix with others.

But they all do work just fine.

If thermostat was old it could be that it did not shut completely or maybe pressure made it to open, I think these parts should be replaced every now and then.

Yeah, Jani is right, when they go old the resistance to flow gets weak. They do shut but if you push against the valve, even when cold it'll gradually open a bit. Try that with a new one and its firmly shut. They always seem to behave normally when do the boiling water test though.....

Thats great, thank you, it's got to have been the stat, i think its always good to have a reason why. I don't think i'll bother with the boiling water test anymore, it doesn't seem reliable and its really not impressing my wife!!!

Yeah, I can imagine she wouldn't be too keen on cooking food in the same pan that a thermostat covered in anti freeze was boiled in

Funnily enough thats the pan she uses to cook my dinner in, everything does taste nice and sweet though!!!