What engine?

Don't unscrew the coolant cap when the engine is hot but yes, it's normal for it to rise a certain amount if you do. Whats the level like first thing in the morning?

 

This is the level on a hot engine where is the level sposed to be

 

Do I read your post right ?

When the engine is Hot with the cap fully closed the level is at around minimum. When you loosen the cap while hot, water rises in the expansion tank?
Does it actually overflow out of the cap? [Be very careful loosening the cap when hot - nasty scolding water can fly everywhere unexpectedly]

The coolant in the photo doesn't look as clear and clean as I would expect. I would get the coolant tested for hydrocarbons, many garages can do this, or you can buy a kit at a high street motoring store - or buy on-line. That would indicate or eliminate a head gasket leak with a high degree of confidence.

Other than that, I can only think there is an 'air-lock' (a bubble or bubbles) in the coolant system which compress under the pressure heat and a sealed cap, giving a low level. Then expand when the cap is loosened forcing coolant back into the expansion tank. These bubbles could be air somehow introduced during filling - or exhaust gases from a leaking head gasket.

Reviewing what I write, I'm afraid I'm tending towards a head gasket problem, but not 100% sure at all.

 

What engine?

 

Sorry 1.9 petrol

 

I think you mean the 1.8 petrol?

Common for the oil coolers to leak and cause the sludge in the coolant tank. If you have a look inside does it look light brown in colour?

 

Hi yes it does
Checked the engine oil thats fine
Do i top the coolent back up to the line

 

It only takes a small amount to mix in so you wouldn't be able to notice a difference on the dipstick. Top the coolant up to the max line when cold. Try scoop out the worst of the sludge first

 

You really need to have this coolant checked out--as for opening the cap on the header tank when the engine is hot is just asking for trouble[and it will hurt!!!].the reason the coolant will come pouring out is that when the cap in screwed down the coolant system is under pressure [atmospheric] and when you loosen the cap off the coolant expands rapidly and comes pouring out--not burning your bits and piece's if you are lucky.you will need to have the system checked over and then if everything is ok the cooling system needs to be cleaned out and reverse flushed to remove and sludge etc.the heater matrix will need cleaning out too..your header tank looks rotten with sludge etc and these are easy to remove and clean--otherwise replace[not expensive] along with the cap---where abouts are you located-------Dave

 

That coolant is low.

It's probably got a crack on the bottom of the tank. Very common on these. Feel underneath the expansion tank for moisture. They crack on a seam.

I'm on my third expansion tank at 166k miles on my 1.8 insignia. £35 and a 30 minute job.