Is it normal for the coolant level to be slightly lower in the expansion tank (although still above min) in cooler weather.Mine always is.

 

The level in the coolant tank is dependent on the temp of the engine. So, yes, you may notice that the level will be slightly lower in the winter than it is in the summer. But, this difference should not be that drastic, maybe 0.5 cm or so. You should see a 1-2 cm change as the motor goes from cold to hot.

 

Cheers Thermo..I reckon that's about what mine does.Have read somewhere that a very small amount of topping up may be needed every six months or so due to evaporation,but I always thought that a pressurised sealed system should only ever need topping up if there is a leak.
What do you think ?

 

Sealed, is a relative term. Everything leaks to a degree. Why do you need to keep putting air in your tires every few months? It isn't that you have a small hole in the tires (well, I will explain), the tires are perfectly fine but yet you still add air. Kinda like if you moved over to say helium gas, you would find you would need to fill your tires more often than simply putting in normal air. The reason for this is while your tires are not leaking at an appreciable rate, a finite amount of gas still makes its way through all that rubber and leaks out. The smaller the gas molecule or the more porous the material is, the more leakage that you will get. This is why the "fill your tires with nitrogen" phase was so popular. People could fill their tires and they would not leak as much, therefore their tires seemed to be better because a nitrogen molecule is bigger than a lot of other gas molecules. Unfortunately, 70% of the atmosphere is nitrogen. So, you are not gaining all that much. In the case of your coolant system, some water (while very small) does leak out of the rubber tubes, through the plastic fittings, and yes, even out through the block. It just isn't enough that you can see it. You see this effect more when you start dealing with pressures in the thousands of pounds. You will see the pipes "sweat". Sometimes it is due to the fluid inside being cooler than the room it is in, other times it is the pressure pushing out the fluid.

You also get a finite amount of leakage every time you start or stop your car's engine. Things heat up and cool down at different rates. So, what is sealed at one temperature, may not be 100% sealed at a different temp. This is why you sometimes find a very small amount of leakage at say rubber fittings where they meet metal fittings.

 

Now that's what I call an answer.